Surge 1.7.1 User Manual

Table of Contents

Getting Started

Thank you for using Surge!

Surge is a virtual synthesizer released into open source by creator Claes Johanson in September 2018, and maintained by a group of volunteers since then.

This first section is intended to give you a brief overview of some concepts that are specific to this synthesizer and an introduction on how to navigate, manipulate, and use Surge to its full potential.

For detailed information regarding the synthesis engine and other advanced technical specifications and options of this synthesizer, there is a second section dedicated to Technical Reference.

Finally, for more tips and tricks, tutorials, and to download additional content, you can also take a look at Surge’s wiki.

Installing Surge

Audio Units, AU is a trademark of Apple Computer, Inc
VST is a trademark of Steinberg Media Technologies GmbH

Surge’s installer is available at https://surge-synthesizer.github.io.


On the Windows platform, Surge is delivered as both a 32 or 64-bit VST3 plug-in instrument.

The filename for the VST3 is Surge.vst3.

System Requirements:

  • Windows 7 or newer
  • A reasonably fast CPU (Pentium 4/Athlon 64 or above)
  • At least 4GB of RAM
  • VST-compatible host application

In addition, to use the 64-bit version on Windows you need the following:

  • A CPU supporting the x64 (AMD64/EM64T) instruction set
  • A 64-bit version of Windows
  • An application capable of hosting 64-bit VST plug-ins

The VST3 version of the plug-in should be automatically installed in the default VST3 plug-in location and should be found by your host application. However, the Windows version also comes with a portable mode:

  • Portable Mode allows you to store assets in the same directory as your surge.vst3.
  • If Surge.vst3 is installed in a folder and in that same folder there is a directory called SurgeData, Surge will use that for factory data rather than %PROGRAMDATA%\Surge.
  • If in that same folder there is a directory called SurgeUserData, Surge will use that for user data rather than %DOCUMENTS%\Surge.
  • Either none, one, or both of those folders can be there. Surge will fall back to the defaults if they are not present. You can always see your data paths in the about screen.


On Mac, Surge is delivered as a 64-bit Plug-in Instrument for both the Audio Unit (AU) and VST Plug-in interfaces (VST3).

System Requirements:

  • Mac OS X 10.11 or newer
  • A 64-bit Intel CPU
  • At least 4GB of RAM
  • 64-bit AU or VST-compatible host application

To install, run the packaged installer. You will be given the option of automatically installing the AU Surge.component and the VST3 Surge.vst3 to their correct locations. The factory presets and wavetables will also be automatically installed.

Running the packaged installer will install Surge for all of the users of your Mac.


On Linux, Surge is delivered as a 64-bit VST3.

The system requirements can be hard to determine, as there are a lot of distributions out there and other factors. However, the following information might be good to know:

  • The installation package on Surge’s website is in the form of a Debian package
  • The distribution package is built on Ubuntu 16.04
  • The packages required are listed in the source and in the deb file

Building from source

If you choose to build Surge from source, see the instructions on our Github repository.



The preset library and wavetables are at C:ProgramData\Surge. The user presets are at C:\Users\your username\My Documents\Surge


The preset library and wavetables are at /Library/Application Support/Surge. The user presets are at ~/Documents/Surge.


The plugin itself, preset library and wavetables are at /usr/share/Surge with a standard install. The user presets are at ~/Documents/Surge.

Note: These locations can be changed in Surge’s menu (see Data and Patches).

Introduction to the User Interface

The user-interface of Surge is divided into four main sections:

  • Patch/Global
  • Scene controls
  • Modulation/Routing
  • FX

Keeping this structure in mind will make it easier to understand the layout.

Illustration 1: The four sections the user-interface that Surge is divided into.

The four sections of the user-interface that Surge is divided into.

The “Scene” Concept

Every patch in Surge contains two scenes (A & B) and an effect-section. Both scenes and all effect settings are stored in every patch. A scene is similar to a traditional synthesizer patch as it stores all the information used to synthesize a voice. Since there are two scenes in each patch, it’s possible to have layered or split sounds stored within a single patch. (see Scene Select and Scene Mode).

Illustration 2: Both scenes and all effect settings are stored in every patch.

Audio Outputs

When loaded into a DAW, each instance of Surge has 3 audio outputs:

  • Stereo Out
  • Scene A Out
  • Scene B Out

Note: In some hosts, like in FL Studio for example, all 3 outputs might be enabled by default and routed to the same track. In most cases, you should only keep enabled Stereo Out, or Scene A and Scene B Out in case you want each scene routed to its own separate track. Make sure to set this up correctly, as this could result in a change of volume and FX mixing in older patches and projects when updating Surge from 1.6.X. to 1.7 or newer in those hosts.

Sliders and controls

The most common user-interface control in Surge is the slider. They come in both horizontal and vertical orientations but their functionality is otherwise identical.

Sliders are always dragged, there is no jump if you click on the slider tray instead of the slider head, it enters dragging mode nonetheless.

Slider interactions:

  • LMB - Drag slider
  • Shift+LMB - Drag slider (fine)
  • Ctrl+LMB - Drag slider (quantized steps)
  • Scroll Wheel - Move Slider
  • Shift+Scroll Wheel - Move Slider (fine)
  • LMB double-click - Reset parameter to default value
  • RMB - Context menu

Other than sliders, some of Surge’s parameters are also displayed as number and value fields, buttons and button rows.

Parameter Context Menu

Any parameter’s context menu can be brought up with a right-click. This menu has numerous useful functions:

Illustration 5: Slider context menu

Name And Contextualized Help

Clicking on this first option will open this user manual to the correct section explaining the parameter in question.

Edit Value

This option allows you to type in the desired value of a parameter. Once the value popup appears, its text will already be highlighted, and you can start typing the value right away. When you are done, simply press Enter to confirm the change. To cancel and close this popup, simply press the Escape key or move any other parameter.

For discrete parameters (Unison Voices, or a button row for instance), instead of a type-in field, all the possible values will be displayed right in the menu so it can be accessed directly.

If a control is modulated, there will also be edit options for the amount of modulation for each modulation source. The entered value corresponds to the position of the modulation slider (blue slider) for that modulation source (see Routing for more information).

Note that in both cases, the actual unit of the parameter doesn’t need to be typed in.

Extend Range

Some parameters can have their range extended. The option Extend range will appear in the context menu if they do. Pitch, for instance, is one of those parameters.

Tempo Sync

Some parameters can be synchronized to the host tempo. The option Tempo sync will appear in the context menu if they do.

Once tempo-synced, when using the Surge Classic skin, the slider will show a “TS” symbol on their handles to indicate that state, like so:

This indication can vary depending on the skin used.

Activate / Deactivate

Some parameters can be activated or deactivated. If a slider appears transparent or is missing its handle, in some cases, it can be because the parameter is deactivated. To toggle it, simply use this option.

MIDI Learn Parameter…

This is where you assign a MIDI controller to the desired slider. To abort MIDI learning on that parameter, simply right-click again and the option will now become Abort Parameter MIDI Learn.

Clear Modulation

This menu also includes an easily accessible option to clear any or all modulation routings to a slider that is being modulated (those that have a blue tint) (see Routing).

VST3 Options

Finally, the VST3 version of Surge supports VST3 context menu items. Depending on the host, there may be more or less options regarding automation, MIDI, or parameter values.

On the bottom-right corner, there is a small menu button. Left-clicking on it will reveal some interesting options.

Note: Some of these options are also present at the top of the user interface for easier access. (see Status Area).

MPE Options

MPE stands for MIDI Polyphonic Expression. It can be enabled or disabled in its sub-menu. The current and default pitch bend range can be changed here as well.

Tuning Options

Surge features full-keyboard microtuning support, and uses an implementation of the complete Scala SCL and KBM microtuning format.

The Tuning menu option allows you to import and Apply .scl file tuning, or Apply .kbm keyboard mapping files to use different scales than the standard one. Tuning settings are stored in the DAW state and optionally stored in a patch.

There’s also an option to set to standard tuning, and even an option to show current tuning, which will open an HTML file containing all the information of each of the tones in the scale.

Alternatively, Scala SCL and KBM files can also be imported using the Status Area

See Microtonal Tuning in the Technical Reference section for more information.


The Zoom option can be extremely useful on certain monitors and configurations.

In its sub-menu there are various options to change the scale of the whole user-interface to a certain size. Keep in mind that it will not let you change it to any size, as there is an upper limit depending on your screen resolution.

When a new instance of Surge is loaded, its zoom will be set to default size. To change this value, go back in this sub-menu and select the option “Set [zoom %] as default”, or “Set default zoom to …” then enter the desired value.


This is where the UI skin can be chosen, reloaded and scanned. Surge comes with the Classic skin and a Dark Skin, but there are more to come in the future.

Surge Dark skin

If you would like to get on board with the skinning engine, see the documentation on developing Surge skins.

User Settings

In this sub-menu, there are a couple of options regarding the user interface.

Mouse Behavior

This option allows you to change the sensitivity of the mouse when moving sliders. While Classic is used by default, the other 3 options range from Slow (more granular) to Exact (as fast as the mouse pointer). Also, there is an option to show the mouse pointer on screen when dragging a slider.

Middle C

As this option’s name suggests, this option allows you to change Middle C to be either C3, C4 or C5.

Patch Defaults

This is where you can configure what appears by default in the Author and Comment fields when saving a patch.

High Precision Value Readouts

When this option is enabled, value popups when tweaking parameters will show more digits after the decimal point (6 digits). Can be useful in some advanced scenarios.

Modulation Popup Shows Bounds

If this option is enabled, when applying modulation and adjusting its amount to a parameter, the value popup will show more values, such as the relative range in the negative direction, and both absolute minimum and maximum values underneath.

Data Folders

In this sub-menu, there are a couple of options regarding user data and patches.

Open User Data Folder

This opens the location where custom patches saved by the user will be stored.

Open Factory Data Folder

This opens the location where factory patches, wavetables and other configuration files are stored.

Set Custom User Data Folder

As its name suggests, it allows you to change where user patches will be saved.

Rescan All Data Folders

This option can be useful after importing patches created by someone else, after transferring user patches to another computer, or after downloading patches from the internet.

MIDI Settings

This sub-menu contains options for MIDI mappings.

Save MIDI Mapping As…

This allows you to save the current MIDI mapping. The newly created profile will appear in this menu under the two top options.

Show Current MIDI Mapping…

This opens up an HTML file listing the currently loaded MIDI mapping.

Online Options

The following items are for reaching the developers and user feedback information, reading the code on GitHub, downloading additional content, opening Surge’s website, and finally opening this user manual.

About Surge

Finally, there is an option to open the About pane containing various version, configuration and license information.

Developer Menu

When right-clicking on the Menu button, some more options for development and testing purposes appear.

Patch/Global Section

Scene Select and Scene Mode

There are two setups of all controls within the Scene section of the user interface. The Scene Select buttons [A|B] determine which one is selected for editing. Right-clicking on these buttons brings up a context menu that allows you to copy/paste scene content.

Depending on the Scene Mode, these two buttons could also be used to choose which scene will be played. Indeed, whether a scene will generate a voice when a key is pressed is determined by the Scene Mode setting:

  • Single – Notes will be played only by the selected scene.
  • Key Split – Notes below the split key will be played by scene A, notes above and including the split key will be played by scene B.
  • Channel Split Notes from MIDI channels below the split MIDI channel will be played by scene A, notes from MIDI channels above and including the split MIDI channel will be played by scene B.
  • Dual – Both scenes will play all the notes.

In both Key Split and Dual mode, if MPE is disabled, the system also supports MIDI channel routing where Channel 2 plays only Scene A and channel 3 plays only Scene B. MIDI channel 1 and all other channels higher than 3 play the Split/Dual mode.

Poly shows the number of voices currently playing and allows you to set an upper limit to the number of voices allowed to play at the same time by dragging horizontally on the value. The voice-limiter will kill off excess voices gently to avoid audible artifacts, thus it’s not uncommon for the voice count to exceed the limit.

The state of the polyphony limit setting is not currently stored in patches.

Patch Browser

Illustration 3: The patch browser

Finding sounds in Surge is easy: just press the arrow buttons until you find something you like. If you left-click the patch-name field (anywhere in the white area), a menu will list all available patches arranged into categories. A right-click will bring up a menu with just the patches of the current category.

These categories are also grouped into three sections depending on who created them:

  • Factory Patches - Patches created in-house by the Surge authors.

  • 3rd party patches - Patches created by users and 3rd parties. Categorized by creators.

  • User Patches - Your own patches will be stored here. How you categorize them is entirely up to you.

Finally, at the bottom, there is an option to download additional content.

The Store Dialog

Clicking the store button of the patch browser opens the store dialog. It is where you name your new patch and choose which category it should belong in. You can also create a new category manually here as well. The patches you store will end up in the user section at the bottom of the patch menu. The store dialog also provides text fields for the name of the patch creator and comments.

Note: Comments are not currently shown in the main GUI.

Status Area

This area is meant to be a quick access to some of Surge’s features that are also present in the Menu. (see Menu Button)

Right-clicking on one of these buttons will reveal more options which are also present in sub-menus under the Menu button as well.

For instance, if no alternate tuning is used, left-clicking on the tun button will do nothing. This button is meant to engage and disengage alternate tuning after it has been selected by right-clicking on the button and choosing the option Apply .scl file tuning, for example.

Alternatively, .scl and .kbm files can also be dragged and dropped on the tun button to import alternate tuning.

See Microtonal Tuning in the Technical Reference section for more information.

FX-Bypass, Character and Master Volume

FX Bypass lets you quickly hear what a patch sounds like without the effect-units. (see FX Section)

  • Off – All effects are active.
  • Send – The send effects are bypassed.
  • Send + Master - The send and master-effects are bypassed.
  • All – All effects are bypassed.

Character controls the amount of high-frequency content present in any oscillators of the patch that are using the “classic” algorithm. The possible choices are Warm, Neutral and Bright.

Master Volume controls the last gain stage before the output. The VU-meter above it shows the output-level and will become red if it goes above 0 dBFS.

The state of these two settings are not stored with patches. They are however stored by the host application in your project files.

Scene Controls Section

The UI of the scene section can also be further divided into two parts:

  • Sound generation
  • Sound shaping

The sound is generated and mixed in the sound generation section. After that, it goes through the sound shaping section.

Sound Generation

This is where the sound is born. The oscillators generate waveforms according to the notes played. They are then mixed in the oscillator mixer.


1/2/3-buttons – Chooses the active oscillator for editing. You can right-click on one of them and a context menu with the name, Copy and Copy (with modulation) options will show up.

Display – Shows the active waveform. When the Wavetable or Window oscillator is used, it will also work as wavetable selector by clicking on the orange bar or on the arrows to cycle through them.

Type – Oscillator type. Chooses which algorithm is used for the oscillator. Available options are Classic, Wavetable, Window, Sine, FM2, FM3, SH Noise and Audio Input.

See Oscillator algorithms in the Technical Reference section for more information.

Pitch & Octave – Controls the pitch for this particular oscillator. Its context menu can be used to extend its range, or to set the pitch to Absolute mode, which makes the pitch shift in absolute frequency as opposed to relative to the note that is being played.

Keytrack – When disabled, the oscillator will play the same pitch regardless of the key pressed.

Retrigger – If active, the oscillator and all its unison voices will always start immediately at the same phase position. This is useful for snappy sounds where you want the attack to sound exactly the same each note.

Other - The rest of the sliders from the oscillator editor are specific to each oscillator type. See Oscillator algorithms in the Technical Reference section for more information.


Mixer Channels

Excluding the Pre-filter Gain (slider on the right), the Mixer has 6 channels (sources) from left to right:

  • Oscillators 1, 2, 3

  • Ring Modulation of 1x2, 2x3 – The source of these two channels is digital ring modulation from the oscillators. This type of RM is a bit different from the traditional carrier-modulator style ring modulation. Digital ring modulation is simply the result of multiplying the output of oscillators 1 and 2, or 2 and 3.
  • Noise Oscillator

Channel Parameters

Each channel has the following controls:

  • M – Mute

  • S – Solo (only play channels that have solo active)

  • Green Box (Filter routing) – Chooses which filter the channel is routed to. The left position routes the channel output to filter 1, the right position routes it to filter 2, while the middle position, which is selected by default, routes it to both. However, this setting will only route the channel output to filter 1 if a serial filter block configuration is used, since the audio will then go through the second one in the filter block anyways. If any other configuration than serial is used, the audio will then be routed to both filters, as expected.

  • Slider – Gain control for each input.

Other sound generation parameters

Pitch & Octave – Controls the pitch for the entire scene. Affects the filter key-tracking and the keytrack modulation source as well. The range of the slider can be extended using the context menu.

Portamento – Portamento is when a new note will slide in pitch from the pitch of the last played note. This setting determines how long the slide will be. A setting of 0 disables Portamento. This parameter can be tempo-synced.

Portamento has some interesting options accessible in its context menu:

  • Constant rate: if this option is enabled, the time to cover one octave is defined by the Portamento slider value. From there on, gliding between 2 octaves for instance will take twice as long, and so on. By default, this option is disabled, so the glide rate is proportional to the distance between the two keys, making it so that it always takes the same time to glide between any two keys.
  • Glissando: if this option is enabled, the pitch slide will be quantized to the scale degrees.
  • Retrigger at scale degrees: if this option is enabled, the FEG and AEG (see Envelope Generators) will be triggered each time the portamento slide crosses a scale degree.
  • Curve options: you can choose between a Logarithmic, Linear or Exponential portamento curve. By default, the portamento slide follows a linear curve.

Osc Drift – Applies a small amount of instability to the pitch of all oscillators, making them subtly detuned. Although the parameter is shared, the randomness of the instability effect is independent for all oscillators and all the unison voices of each oscillator.

Noise Color – Affects the frequency spectrum of the noise generator. The middle position results in white noise. Moving the slider to the left emphasizes low frequencies while moving it to the right emphasizes high frequencies.

Bend Depth – Pitch Bend Depth Up/Down. Controls the range of the pitch bend wheel, in semitones.

Play Mode – Chooses how multiple notes are handled. Poly will allow multiple notes to be played, while Mono will only let the last note play. Latch will continuously play the last played note (mono).

Mono has two possible modifiers:

  • Single Trigger EG (ST) means that the two envelope generators are not restarted when sliding between two notes (two notes that overlap in time)
  • Fingered Portamento (FP) means that portamento is only applied when sliding between notes and not when there is time between the played notes.

Sound shaping

Filter controls

Filter Block Configuration – Chooses how the filters, waveshaper and the gain stage are connected together. Note that only the Stereo and Wide configurations will output a stereo signal.

Feedback – Controls the amount (and polarity) of output that’s fed back into the input of the filter block. It has no effect when using the Serial 1 filter block configuration (which because of this has a lower CPU load).

Note: Be careful with your monitoring volume when using feedback. It’s easy to make really loud high-pitched noises by mistake if you’re not familiar with how the synth reacts to feedback.

Don’t let this scare you though. There’s a lot to be gained from proper and creative use of feedback. Changing the character of filters, making filters interact together, making basic physical models, making sounds that are just about to break apart. It is these things that make Surge truly special.

Filter Balance – Controls how the two filters are mixed. The behavior depends on the filter block configuration.

Type – Selects the type of the filter. There are 10 choices. Off, 2-pole low-pass, 4-pole low-pass, 4-pole low-pass ladder filter, 2-pole high-pass, 4-pole high-pass, band-pass, notch, comb-filters with both positive and negative polarity and a sample&hold module.

Subtype – Selects variations of each filter type. The difference can vary from subtle to radical depending on how the filter is used. See Filter algorithms in the Technical Reference section for information regarding subtypes of each filter type. It is displayed as a number next to the filter type (when available).

Cutoff – Controls the cutoff frequency of the filter. When tweaked, while its tooltip will show frequency in Hz, it will also show its approximate MIDI note value, very useful when using the filter for melodic and tuning purposes.

Resonance – Controls the amount of resonance of the filter.

Filter 2 Offset Button (small “+” button to the right of the filter parameters) – When active, the cutoff frequency will be set relative to filter 1. This includes any modulations (including the hardwired FEG depth & keytracking). Filter 2’s cutoff frequency slider becomes an offset setting relative to filter 1’s cutoff frequency.

Resonance Link Button (small button, filter 2 only) – Makes the slider follow filter 1’s resonance slider setting.

Keytrack > F1/F2 – Controls how much the pitch of a note affects the cutoff frequency of the filter. A setting of 100% means the filter frequency will follow the pitch harmonically.

Envelope Generators

There are two envelope generators connected to the filter block.

On the left is the Filter Envelope Generator (Filter EG). It is hardwired to the two filters, whose depth is set by the >F1 and >F2 sliders.

On the right is the Amplitude Envelope Generator (Amp EG). This one is hardwired to the gain stage of the filter block.

Illustration 10: ADSR envelope structure

The ADSR envelope structure

The envelope generators are of the 4-stage ADSR type. This is the most common form of EG used in synthesizers and it is named after its four stages Attack, Decay, Sustain and Release. If you’re new to synthesizer programming the illustration should give you a good idea how they work. The thing you need to remember is that after going through the attack & decay stages the envelope will stick in the sustain stage until the key is released.

Above the envelope stage controls is a graphic representation of the ADSR structure.

If the envelope mode is set to Digital, there will be small adjustable orange fields on the graphic. Dragging them horizontally allows you to choose the curvature of the different stages of the envelope.

If the envelope mode is set to Analog, the curvature of the different stages will automatically be set to a shape that tries to emulate analog behavior.

Other sound shaping parameters

Keytrack root note – Sets the root key of the filter keytracking and the keytrack modulation source. At the root key, the keytrack modulation source will have the value zero. Above/below it it will have positive/negative modulation depending on the distance to the root key in octaves. This parameter does not affect the oscillator pitch.

Keytrack amout sliders - Sets the amount of filter keytracking applied to each filter.

HP – Controls the scene high-pass filter. (scene parameter)

FM configuration – Chooses how oscillator FM (frequency modulation) is routed.

FM depth – Sets the depth of the oscillator FM.

Waveshaper type – Chooses type of the non-linear wave-shaping element.

Waveshaper drive – Sets the drive amount of the waveshaper.

Amp Gain – Controls the gain element inside the filter block.

Amp Vel. - Controls how the Amp Gain scales with velocity. This is neutral at the maximum position. Other settings provide attenuation at lower velocities, thus this setting will never increase the Amp Gain parameter by velocity.

Output stage

The output stage is located after the filter block in the audio-path. As it’s outside the filter block-structure changing the gain here doesn’t have any affect on the timbre of the voice (unlike the previous gain-control which may affect how the feedback and wave-shaping acts). It can still change the timbre of the effect section if non-linear effects (like distortion) are used.

Volume – Volume control

Pan – Pan/balance control

Width – The amount of stereo spread (only present for the Stereo and Wide filter block configurations)

Send 1/2 – Send level to Send effect 1/2. (scene parameter)

Modulation/Routing Section


The modulation section is different from the sound generation and shaping sections as no audio data is passed through it. Instead it allows you to control the parameters in the other sections from various sources.

Surge has three main types of internal modulation sources :

  • LFOs
  • Voice and note properties
  • Macros

All of these modulation sources are located in the routing bar (see Routing) :

The three types of modulation sources, separated in categories.


Compared to other synthesizers, Surge does not have dedicated Envelope or Step sequencer modulation sources. Instead, those are integrated with the LFOs, as they are considered LFO waveforms. This enables the flexibility of having up to 12 LFOs, envelopes, or step sequencers, and everything in between.

The LFOs (Low Frequency Oscillator) in Surge are very flexible and come with a built in DAHDSR-envelope which lets the LFO work as a dedicated envelope generator or shape the magnitude of the LFO over time.

LFOs vs. S-LFOs

Surge has a total of 12 LFOs, evenly divided into two categories :

  • 6 Voice LFOs (labeled LFO 1-6)
  • 6 Scene LFOs (labeled S-LFO 1-6)

Although they might seem similar, there is an important factor that distinguishes them.

An LFO has a separate envelope and oscillator for each voice, so it can control voice-level parameters (like oscillator pitch) but cannot control scene level parameters (like FX levels).

An S-LFO has an envelope and oscillator per scene, so it can control scene level parameters, but cannot control voice level parameters.

To demonstrate this distinction, let’s say an LFO with a Sine wave is modulating the cutoff of a filter. Now, if 3 notes are being hit with a small delay between each of them, the phase of the LFO will be delayed between the notes accordingly.

You will indeed clearly hear the cutoff of the filter moving independently for each note, which gives the impression that there are three LFOs. The same principle applies for envelopes.

However, unlike the first demonstration, this time, if an S-LFO is modulating a certain parameter, hitting more notes will not “add” an LFO for each voice, which gives the impression that there is a single LFO modulating the cutoff frequency of the filter instead of many.

Note that there are other modulation sources that act on the whole scene, such as Channel After-Touch (labelled Channel AT), Pitchbend and Modwheel.

See Modulation routing in-depth in the Technical Reference section for more information.


Rate – Controls the rate of the LFO oscillation. When the waveform is set to Step Seq, one step equals the whole cycle. This slider can be tempo-synced and Deactivated in its context menu. Deactivating the rate effectively freezes the LFO to a certain constant value depending on the Phase/Shuffle parameter. This can be useful for manually scrubbing in a waveform cycle of the LFO, and can also be used in the same way in the sequencer. This feature can also be used to make the LFO act as a randomizer. Furthermore, modulation can even be applied to the Phase/Shuffle parameter with another LFO, which opens up a lot of possibilities.

Note: In the LFO editor, when right-clicking parameters that can be tempo-synced, there will also be an option to Tempo sync all the LFO’s parameters at once.

Phase/Shuffle - Controls the starting phase of the LFO waveform. As with any parameter, it can be modulated. However, in this case, its modulated value will not change once the LFO is triggered (for instance, it’s not possible to shift the LFO’s phase while a note is pressed.) Only starting phase is taken into account.

Amplitude – Controls the amplitude of the LFO. This is the parameter you should use if you want to control the depth of an LFO with a controller (like controlling vibrato depth with the modulation wheel, for instance).

Deform – Deform the LFO shape in various ways. The effect varies with the LFO waveform.

Trigger mode – Chooses how the LFO is triggered when a new note is played:

  • Freerun – The LFO’s starting phase is synchronized with the host’s song position to make it continuously running in the background. Freerun acts the same with LFOs or SLFOs.
  • Keytrigger – The LFO’s starting phase is triggered when a new note is pressed. If the synth is set to “Poly”, each new voice gets its own LFO triggered with it when using an LFO. However, when using an SLFO, the first voice sets the LFO’s position, then the other ones will follow it.
  • Random – The LFO’s starting phase is set to a random point in its cycle. If the synth is set to “Poly”, each new voice gets its own LFO triggered with it when using an LFO. However, when using an SLFO, the first voice sets the LFO’s position, then the other ones will follow it.

Unipolar - If active, the LFO-output will be in the [0 .. 1] range (unipolar). If not, it will be in the [-1 .. 1] range (bipolar).

The modulation range on a parameter is represented by a green bar when routing mode is engaged (see Routing).

Modulation on a control from a bipolar source

Modulation on a control from a unipolar source


LFO Shapes:

Sine Sine wave Vertical bend
Triangle Triangle wave Vertical bend
Square Pulse wave Pulse width
Ramp Ramp wave (sawtooth) Vertical bend
Noise Smooth noise Correlation
S&H Step noise Correlation
Envelope The LFO waveform output is one, making the LFO-unit as a whole work as an envelope generator. Envelope shape
Step Seq The ‘Step Seq’ waveform is a special case that has an additional editor. It can be used to draw waveforms or be used like a step sequencer. (see Step Sequencer). Smoothness/Spikyness

On the left, the different shapes and their explanation. On the right, the way that the Deform parameter affects the waveform.

Depending on the selected waveform for a particular modulation source, its name in the routing bar will change. When using the first 6 waveforms, it will be called LFO. However, when using the envelope shape, ENV will be displayed, and SEQ will be displayed when the Step Seq waveform is used. Scene LFOs have their equivalent labels as well:


The LFO Envelope Generators are of the 6-stage DAHDSR type that are multiplied with the waveform generator.

Illustration 12: 6-stage DAHDSR envelope

6-stage DAHDSR envelope

Step Sequencer

The Step Seq waveform is a special case. Instead of the graphical preview, there is an editor that allows you to draw the output waveform with up to 16 steps.

Illustration 13: Step Seq

Step Sequencer editor

The two blue markers define loop-points in which the LFO will repeat once it gets into the loop. The left mouse button is used for drawing while the right one can be used to clear the values to zero.

To quickly reset a step to 0, either double-click on a step, or hold down Ctrl and click or drag with the mouse over the desired step(s).

Right-clicking and dragging over steps allows you to draw a straight line over the desired steps, thus creating a perfectly linear staircase pattern.

Holding down Shift while drawing will quantize the values to the scale degrees (1/12th in case of standard tuning, or possibly other for custom tuning) spanning the range of one octave. Furthermore, holding down Shift + Alt makes two times more values available, hence useful when modulating pitch by two octaves instead.

For more information on microtonal pitch modulation using the step sequencer, you can read this article on Surge’s wiki.

The step sequencers inside Voice LFOs have an extra lane at the top of the step editor allowing to re-trigger the two regular voice envelopes (The Amplitude and Filter Envelope Generators) when the small rectangle is filled at that particular step.

Illustration 14: Envelope retrigger pane of Voice LFO 1

Step Seq of LFO 1 containing the re-trigger pane

However, shift-clicking or right-clicking those rectangles allows the specified step in the sequencer to only trigger one of the two envelopes. When the step is half-filled on the left, only the filter envelope will be triggered. When filled on the right, only the amplitude envelope will be triggered.

The Deform parameter give the Step Seq waveform a lot of flexibility. A value of 0% will output the steps just as they look on the editor. Negative values will give an increasingly spiky waveform while positive values will make the output smoother.

Negative deform
Positive deform

Illustration 15: Effect of the deform parameter on the step Seq waveform

Effect of the deform parameter on the step Seq waveform

Copy/Paste LFO settings

Finally, after setting up an LFO, its settings can be copied and pasted on another LFO simply by right-clicking on any of them in the blue routing bar and using the option Copy and Paste.

For more information on LFO algorithms, see LFO algorithm in the Technical Reference section.

Voice and note properties

Like other synthesizers, Surge receives MIDI data to determine what note(s) to play. However, it can also use MIDI CC data to modulate any routable parameter.

There are 10 of those voice and note properties in the routing bar:

  • Velocity
  • Release Velocity
  • Keytrack
  • Polyphonic Aftertouch (labeled Poly AT)
  • Channel Aftertouch (labeled Channel AT)
  • Pitchbend
  • Modwheel
  • Amp EG
  • Filter EG
  • Timbre

Release Velocity is integrated with the Velocity tab and can be accessed by right-clicking on it, and then choosing Switch to Release Velocity. Switching back to normal Velocity can be done with the same menu option which will then be called Switch to Velocity.

Channel Aftertouch, Pitchbend and Modwheel act on the whole scene, where as all the other ones act on each voice. This means that only those three can be routed to FX sends and parameters, for the same reason as the LFOs vs. S-LFOs logic.

Timbre is a modulator used primarily for MPE controllers. It takes the midi CC#74 (timbre) and applies it as a voice-level as opposed to scene-level modulation source. This allows the MPE convention of one-note-per-channel to take an otherwise scene-level midi parameter (CC#74) and turn it into a voice-level modulator.

In the current version of Surge, Channel Aftertouch is a scene level modulator but in MPE mode acts on each voice independently. We aim to resolve the inconsistenty between these two controls in an future release.


There are 8 macros, and by default, they are blank.

What separates these assignable controllers from the rest is that with a right-click, they can be assigned to a MIDI controller or any MIDI CC signal, and their value can be edited on-screen with the blue digital slider below their names.

By default, the macros are assigned to midi CC 41-48, which is often mapped by default to knobs or slider banks for a lot of midi controllers.

See Continuous Controller information (CC) in the Technical Reference section for more information.

The right-click context menu also allows you to rename the controller. There is also the typical routing and clearing options, (see Routing) and you can choose if their modulation is bipolar (both positive and negative with 0 in the middle) or unipolar (just positive).


Modulation routing in Surge is a bit different than most synthesizers, but it’s actually very intuitive and extremely powerful, thanks to the routing bar.

How to apply modulation to parameters

Here’s how it works:

  1. Select the modulation source you want to use.

  2. Engage routing mode with a second click on the source. It will become bright green, and sliders that can be modulated will display a blue modulation depth slider on top of its normal slider.

  3. Drag the desired modulation slider (blue slider) to the position you want the parameter to be at when fully modulated (at the top peak of a Sine LFO, or after the attack stage of an envelope for example). The modulation’s full range will then be shown with the corresponding green bar on the slider.

  4. Disengage routing mode by clicking again on the modulation source.

Alternatively, routing mode can also be engaged or disengaged by pressing TAB on the keyboard, or clicking with the Mouse3, Mouse4, or Mouse5 button over any parameter.

Note that modulation range is always relative to the base value represented by the gray slider, meaning that moving its position will then shift the whole modulation range up or down. This also means that if a modulation slider’s value is smaller than the base value, the modulation polarity will be inverted.

Cross-modsource routing

When clicking on the main button of one of the LFOs, both the modulation source selection and the LFO editor will be selected. However, the two actions can be separated, as you can choose which button is selected as the modulation routing source, and at the same time edit a different LFO than the source.

To do that, select the source normally, and then click on the mini-button on another LFO (the small orange arrow):

This effectively lets you modulate the parameters of one LFO with another. However, note that logistically, an S-LFO can modulate parameters of an LFO, but an LFO cannot modulate parameters of an S-LFO (see LFOs vs S-LFOs).

Modulated sliders

Once a slider is routed to a modulation source, the shade of blue on its tray indicates whether the parameter is modulated and by which source.

Illustration 7: The shade of blue on a slider tray indicates
whether the parameter is
modulated and by which source.

1) Parameter is not modulated (gray)

2) Parameter is modulated (gray-blue)

3) Parameter is modulated by the currently selected modulation source (bright-blue)

Modulation source buttons

Once routed to any parameter, the modulation source buttons change their appearance depending if they’re selected, and if they’re routed in the current patch or not. (scene dependent)

Illustration 8: Modulation sources look different when

1) Unused modsource

2) Used modsource

3) Selected modsource

Clearing modulation

After right-clicking on a modulated slider, you will see an option to easily clear the modulation and un-link it from its source.

Illustration 5: Slider context menu

Alternatively, you can also reset its modulation slider (blue slider) to 0 by either double-clicking/Ctrl+clicking on it once routing mode is engaged, or typing in 0.00 in the type-in editor (see Edit Value).

By right-clicking on any modulation source, there will be an option to clear a particular parameter, but also all of them at once.

FX Section

The FX Section controls the 8 effect units of the effect block stored in every patch.

FX Section

Effect Unit Selector

On the top, the effect unit selector is also representing the signal path of the effects bloc. Here it is in more detail:

Illustration 18: The effect block

The effect block

A left-click on a particular unit in the effect unit selector brings that unit in the editor. A right-click on a unit disables/enables it. This state is stored within patches, unlike the global FX bypass setting.

Effect and preset picker

Effects can be added or removed from the Effect and preset picker (just below the FX return sliders). You can also cycle through effects and presets using the same arrow buttons as those found in the global Patch Browser.

You can also save your own effect presets which will be stored globally with the synth. Finally, at the bottom of this menu, there are Copy and Paste options, which allows you to copy an effect and its parameters and paste it on another unit.

Effect Editor

This is where every effect parameter can be edited. Like with the oscillator editor, the parameter of each slider will change depending on the loaded effect. See Effect algorithms in the Technical Reference section for more information about each effect.

Note: remember that FX parameters are scene controls. This means that only S-LFOs and other scene modulation sources can modulate them.

Technical Reference

Surge Hierarchy


Illustration 16: Block diagram of the synthesizer engine.

Block diagram of the synthesizer engine.

Illustration shows an overview of the synthesizer engine of Surge.


Illustration 17: Block diagram of a synthesizer voice

Block diagram of a synthesizer voice

Illustration shows most audio and control-paths of a single voice. Not all processing elements of the voice are shown in the diagram.

LFO algorithm

Each voice has 6 modulation sources called LFOs (Low Frequency Oscillator) that can be used for modulation purposes. Each scene has an additional 6 LFOs making each voice capable of receiving modulation from a total of 12 LFOs.

Calling them LFOs is a great understatement as they have an integrated envelope generator and can function as a 16-step waveform-generator as well.

LFO-unit structure

Modulation routing in-depth

How the modulation routing works internally isn’t something you normally have to think about when using Surge. Just activate the modulation mode with the desired source and see which of the sliders that become blue. Nonetheless, it is useful to know which limitations are present and why.

Illustration 19: Modulation routing behind the scenes

Modulation routing behind the scenes

The thing to remember is that the voice modulation sources can’t modulate the scene parameters, global/effect parameters or the scene LFOs. Other that that it should be pretty straightforward.

Oscillator algorithms

Surge provides 8 different oscillator algorithms, each capable of generating sound in different ways with a different set of controls. They’re not just different waveforms.


The classic oscillator algorithm consists of a main oscillator that can generate a pulse wave, a sawtooth wave, a dual-saw wave or anything in between.

A sub-oscillator provides a pulse-wave one octave below the main oscillator. Changing the pulse-width of the sub-oscillator does affect the main oscillator as well, as they will both change levels at the same time except that the main oscillator does it twice as often.

The classic algorithm is also capable of oscillator self-sync. Note that the sub-oscillator will be used as the base-pitch for the sync.

The algorithm provides unison at the oscillator-level with up to 16 instances. Unlike the wavetable-oscillator the cost of unison in terms of CPU usage for the classic oscillator is quite modest. The unison oscillator-instances are affected by the scene-level Osc-Drift parameter independently.

Shape Waveform shape -100% = pulse, 0% = saw, 100% = dual saw -100 .. 100 %
Width Pulse-width (pulse) or relative phase (dual saw) 0 .. 100 %
Sub Osc Width Pulse-width of sub-oscillator 0 .. 100 %
Main<>Sub Mix Sub-oscillator mix, 0% = only main, 100% = only sub 0 .. 100 %
Sync Oscillator self-sync 0..60 semitones
Unison Detune Detuning of unison oscillators. 100% = 1 semitone in both directions.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of oscillators used for unison, 1 = disabled. 1 .. 16


A wavetable in Surge consists of up to 4096 single-cycle waveforms. Using the Morph parameter it is possible to sweep across the waveforms in the wavetable.

The individual waves are equidistant in the table. When the shape setting is between two individual waves, they will be mixed to ensure smooth travel. You can’t edit the wavetable contents directly within Surge, but it is possible to generate custom wavetables with external software.

Surge can also import wavetables containing a clm block to indicate loop size (as used by Serum), a cue block (as used by various products including Native Instruments) and a smpl block. Wavetable files without loop information are loaded as one-shots.

This effectively lets you import various wavetables from other products such as Serum. All those 3rd party wavetables that have been tested in Surge have been reported to work flawlessly.

To import custom wavetables, use the wavetable selection bar at the bottom of the oscillator display. This is where you can also download additional wavetable content.

Alternatively, you can simply drag and drop any compatible wavetable file over the oscillator display itself to load it.

You can even create your own wavetables for Surge using wt-tool or WaveEdit.

Once a wavetable is loaded, you can also export it using the wavetable selection bar.

Then, by modulating the Morph parameter, it is possible to create motion, dynamic response to playing and sonic variation. If you want to select an exact frame, drag the slider with Ctrl-mouse, which allows you to snap to exact values in the table, useful for switching between distinct shapes, for example.

What real-life property, if any, the Morph parameter is supposed to mirror depend on each wavetable. Common cases are:

  • Analyzed from sounds that evolve over time. The behavior can be recreated by letting shape increase over time by modulation. It’s the most common among the analyzed wavetables.
  • Analyzed from static sounds over different pitches to capture the formant shift of a sound. The behavior can be recreated by modulating shape by the keytrack modsource.
  • A parameter of a mathematical equation.

In the end it’s just a set of data and Surge doesn’t care how it was generated, all that matters is how it sounds.

The wave-table oscillator has some interesting sonic characteristics. It outputs the waveform in a stair-stepped fashion, making no attempts to ‘smooth the steps’ in the process, but does so in a manner that is completely band-limited. This makes it similar in sound to 1980s era wave-table synths and samplers which didn’t use resampling but had dedicated D/A-converters for each voice instead and changed the pitch by varying the sample rate of the individual D/As.

The fact that the steps aren’t smoothed causes an artifact known as harmonic aliasing. This is not to be confused with inharmonic aliasing which sounds somewhat similar to an AM-radio being tuned and is generally nasty. Instead, this artifact will cause the harmonics of the waveform to repeat themselves and fill up the entire audible spectra even at low pitches, just like a square-wave would, preventing the waveform from sounding dull. As this artifact is completely harmonic it is also musically pleasing. Nonetheless, it may sound a bit out of place on very smooth waveforms but the effect can be filtered out by a lowpass-filter in the filter block if desired. Some of the wave-tables, such as the regular triangle wave, are large enough for this artifact to never appear in the normally used range for this specific reason.

The important thing is that just like most other oscillators in Surge, it doesn’t output any inharmonic aliasing whatsoever or any audible levels of interpolation-noise, two artifacts which has played a big part in giving digital synthesizers a bad name.

For more information, you can read this article on Surge’s wiki.

For developers and advanced users:
There is a reference for the .wt file-format used by the wavetables. It is located at: surgedata/wavetables/wt fileformat.txt

Morph Waveform shape. 0% = first, 100% = last 0 .. 100 %
Skew Vertical Vertical skew of the waveform -100 .. 100 %
Saturate Soft saturation of the waveform 0 .. 100 %
Formant Compresses the waveform in time but keeps the cycle-time intact 0..60 semitones
Skew Horizontal Horizontal skew of the waveform -100 .. 100 %
Unison Detune Detuning of unison oscillators. 100% = 1 semitone in both directions.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of oscillators used for unison. 1 = disabled 1 .. 16


The window oscillator is another shot at wavetable synthesis that is quite different from the previous wavetable algorithm.

The wave, which can be any waveform included with Surge, is multiplied by a second waveform, the window, which can be one of 9 waveform types that are specifically made for the window oscillator. The formant parameter controls the pitch of the wave independently of the window, but as the wave is always restarted with the window the pitch will remain the same. Instead, the timbre of the sound will change dramatically, much depending on which window is selected.

Unlike the wavetable algorithm, the window oscillator uses a more traditional resampling approach which doesn’t result in harmonic aliasing.

Morph Waveform shape. 0% = first, 100% = last (doesn’t interpolate) 0 .. 100 %
Formant Pitch of the wave independently of the window -60 .. 60 semitones
Window Chooses the window waveform. Triangle, Cosine, Blend 1, Blend 2, Blend 3,
Sawtooth, Sine, Square, Rectangle
Low Cut Integrated oscillator high pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
High Cut Integrated oscillator low pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
Unison Detune Detuning of unison oscillators. 100% = 1 semitone in both directions.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of oscillators used for unison. 1 = disabled 1 .. 16


The sine oscillator algorithm generates a sine wave.

Shape Shaping with quadrant masking, shifting and pitch doubling 1 .. 24
Feedback FM feedback amount 0 .. 100%
FM Behavior Chooses wether FM behaves like and earlier or consistent with FM2/3 Legacy (before v1.6.2),
Consistent with FM2/3
Low Cut Integrated oscillator high pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
High Cut Integrated oscillator low pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
Unison Detune Detuning of unison oscillators. 100% = 1 semitone in both directions.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of oscillators used for unison, 1 = disabled. 1 .. 16


FM2 modulation matrix

FM2 provides a miniature FM-synthesizer voice in an oscillator that is specifically tailored towards making nice and musical FM sounds. A single sine carrier is modulated by two sine modulators, whose ratios to the carrier are always integer thus the resulting waveform is always cyclic. However, “Mx Shift” lets you offset the modulators slightly in an absolute fashion, creating an evolving and pleasing detune effect.

M1 Amount Modulation amount of the first modulator 0 .. 100 %
M1 Ratio Ratio of the first modulator to the carrier 1 .. 32
M2 Amount Modulation amount of the second modulator 0 .. 100 %
M2 Ratio Ratio of the second modulator to the carrier 1 .. 32
M1/2 Offset Absolute detuning of the modulators -10 .. 10 Hz
M1/2 Phase Changes the initial phase of the modulators to give different variations of the waveform. 0 .. 100 %
Feedback Modulation amount of the carrier to itself -100 .. 100%


FM3 modulation matrix

As a contrast to FM2, FM3 is the algorithm of choice for scraping paint off walls. The modulators have a larger range, the ratios can be non-integer and there’s a third modulator which has its rate set as an absolute frequency.

M1 Amount Modulation amount of the first modulator 0 .. 100 %
M1 Ratio Ratio of the first modulator to the carrier 0.0 .. 32.0
M2 Amount Modulation amount of the second modulator 0 .. 100 %
M2 Ratio Ratio of the second modulator to the carrier 0.0 .. 32.0
M3 Amount Modulation amount of the third modulator 0 .. 100 %
M3 Frequency Frequency of the third modulator 14Hz .. 25kHz
Feedback Modulation amount of the carrier to itself -100 .. 100 %


S&H is an abbreviation for ‘Sample and Hold’. The S&H-Noise oscillator algorithm works like a pulse oscillator, but instead of always switching between +1 and -1, the levels used are determined stochastically.

The correlation parameter determine how new levels are calculated. A setting of 0% will have no memory and each new level will effectively be a random number (white noise). A lower setting will favor new values that is closer to the previous level and will provide a noise with a darker spectra. Higher values will favor values as far away from the previous one as possible, with 100% resulting in a harmonic pulse-wave.

Correlation Noise correlation. 0% = white noise, 100% = pulse -100 .. 100 %
Width Pulse-width (pulse) 0 .. 100 %
Low Cut Integrated oscillator high pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
High Cut Integrated oscillator low pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
Sync Oscillator self-sync 0..60 semitones
Unison Detune Detuning of unison oscillators. 100% = 1 semitone in both directions.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of oscillators used for unison. 1 = disabled 1 .. 16

Audio Input

Audio Input lets you route external audio into the voice-architecture of Surge. It also allows you to route the audio output from Scene A into Scene B.

Audio In L/R Channel Chooses which external input is used. -100% = left, 0% = both (stereo), 100% = right. -100 .. 100 %
Audio In Gain External input gain in dB. 48 .. +48 dB
Scene A L/R Channel1 Chooses which input from Scene A is used. -100% = left, 0% = both (stereo), 100% = right. -100 .. 100%
Scene A Gain1 Scene A input gain in dB. 48 .. +48 dB
Audio In<>Scene A Mix1 Blend control between the external audio-in signal and the output of Scene A. -100 .. +100%
Low Cut Integrated oscillator high pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz
High Cut Integrated oscillator low pass filter.
Must be activated in its context menu for it to take effect.
13.75 .. 25087.71 Hz

1 Only available in Scene B

Note: When using the Audio Input oscillator type in Scene B to get audio from Scene A, you would probably want to set Play Mode to latch. That way, Scene B will always be triggered.

For more information and possible applications, you can read this article on Surge’s wiki.

Filter algorithms

  • There are 9 filter algorithms available (+ off) for each of the 2 filter units in the filter block. Each of the algorithms have different subtypes, which alter their sound.

  • Most of the filter-(sub)types have some non-linear elements in them to allow them to self-oscillate in a stable and predictable manner. This means they will sound different depending on how hard they’re driven, which can be conveniently controlled with the Pre-Filter Gain setting. For example, if the resonance peaks of a filter is too loud, increase the Pre-Filter Gain to make the rest of the signal more dominant (and if needed decrease the gain at the output stage of the voice to compensate).

Subtypes for LP12/LP24/HP12/HP24/BP

Depending on the setting of the subtype switch, the characteristics and behavior of these filters will be altered, although their main purpose remains the same.

1 Clean with a strong resonance, capable of self-oscillation. Handles transient behavior extremely well. (default, except for Lowpass 6-24db Ladder)
2 Chesty, somewhat distorted sound with a more held-back resonance. Capable of self-oscillation.
3 The smoothest subtype, capable of lower resonance than the others, which is suitable when you do not want the sound of the filter to be noticed but only to roll-off a part of the spectrum.

Lowpass 12dB

  • 2-Pole Low-Pass filter.

Lowpass 24dB

  • 4-Pole Low-Pass filter.

Lowpass 6-24db Ladder

  • 4-Pole Low-Pass ladder filter. You can select at which stage (1-4) the signal is output using the sub-type control. Has stable self-oscillation.

Highpass 12dB

  • 2-Pole High-Pass filter.

Highpass 24dB

  • 4-Pole High-Pass filter.


  • 2-Pole Band-Pass filter.

  • For this particular algorithm an extra subtype (#4) is provided which is a 4-pole equivalent of subtype 1.


  • 2-Pole Band-Reject filter.
1 Default subtype
2 Included for backwards compatibility (smaller resonance range)


  • Delay-Based Comb filter.
1 Positive feedback, 50% dry/wet mix
2 Positive feedback, 100% wet mix
3 Negative feedback, 50% dry/wet mix
4 Negative feedback, 100% wet mix
  • When the sub-type is set to 2 (or 4) and resonance is 0%, the comb-filter will work purely as a delay-unit (with sub-sample precision). This can be used together with the other filter-unit along with filter block feedback to provide interesting options. The “wind/clarinet” and “pluck (fast)/simple waveguide” presets showcase how this ability can be used for simple physical modeling. They only use the oscillator section to ignite the sound, the rest is in the filter block.

Sample & Hold

  • Sample & Hold module. Will sample the audio at the rate set by the cutoff-frequency. Resonance will emphasize oscillations around the cutoff frequency, not unlike the resonance peak of a lowpass-filter.

Effect algorithms

Surge has 8 effect units which each can run one of the 10 provided algorithms.


The EQ unit provides 3-bands of fully parametric equalizing. This high-quality algorithm has a much better response at high frequencies than digital equalizers usually have.

Band 1/2/3
Band gain -48 .. +48 dB
Band 1/2/3
Band frequency 14Hz .. 25kHz
Band 1/2/3
Band bandwidth 0 .. 5 octaves
Output gain Gain control -48 .. +48 dB
Mix Blend control between the dry and the wet signal. 0 .. 100 %


Distortion algorithm. Provides plenty of EQ options as well as a feedback loop to alter the tonality of the clipping stage.

Illustration 21: Distortion algorithm block diagram

Illustration 21: Distortion algorithm block diagram

Pre-EQ Gain/Freq/BW Parametric EQ band parameters prior to the clipping stage,
Gain can be extended.
Pre-EQ High cut High cut element prior to the clipping stage 14Hz .. 25kHz
Drive Drive of the clipping stage,
Can be extended.
-24 .. +24 dB
-120 .. 120 dB
Feedback Feedback loop around the clipping stage -100 .. 100 %
Waveshaper Wave shape used for distortion Soft, Hard, Asymetric, Sine, Digital
Post-EQ Gain/Freq/BW Parametric EQ band parameters after the clipping stage,
Gain can be extended.
Post-EQ High cut High cut element prior to the clipping stage 14Hz .. 25kHz
Output gain Output gain -24 .. +24 dB


The conditioner is a simple EQ, stereo image control and a limiter built into one unit. The limiter applies make-up gain automatically.


Bass LF boost/cut -12 .. +12 dB
Treble HF boost/cut -12 .. +12 dB
Width Stereo width. 0% = mono, 100% = stereo, -100% = reverse stereo -100 .. 100 %
Balance Stereo balance -100 .. 100 %
Threshold Limiter threshold level. -48 .. 0 dB
Attack Limiter attack rate -100 .. 100 %
Release Limiter release rate -100 .. 100 %
Output Limiter output attenuation -48 .. 0 dB

Frequency Shifter

Frequency shifter effect. Provides a delay unit and a feedback loop to give consecutively shifted repeating delays.

Shift Left Amount of frequency shift (in hertz) for the left channel,
Range can be extended
-10 .. 10 Hz
-1 .. 1 kHz
Shift Right Amount of frequency shift (relative to the left channel) for the right channel. -100 .. 100 %
Delay Delay time for the frequency-shifted signal. Can be tempo-synced. 0 .. 32 s
1/512 .. 16 whole notes
Feedback Feedback around the frequency shifter and delay-unit. -inf .. 0 dB
Mix Blend control between the dry and the wet signal. 0 .. 100 %

Ring Modulator

Flexible ring modulation algorithm.

Shape Shape used for the ring modulation. 1 .. 24
Pitch Pitch (frequency) of the ring modulation. 0 .. 127 semitones
Unison Detune Detuning of the carrier unison voices.
Can be extended.
Can be switched between relative (default) and absolute.
0 .. 100 cents
0 .. 1200 cents
0 .. 16 Hz
0 .. 192 Hz
Unison Voices Number of unison voices used for the carrier. 1 .. 16
Forward Bias Controls the approximate model of the diode 1. 0 .. 100 %
Linear Region Controls the approximate model of the diode 1. 0 .. 100 %
Low Cut Low cut element before the output stage. 13.75 .. 25087.71 Hz
High Cut High cut element before the output stage. 13.75 .. 25087.71 Hz
Mix Blend control between the dry and the wet signal. 0 .. 100 %

1 For more information on the diode model used by the ring modulator, you can read this paper.


The audio-input of Surge is used to modulate the carrier signal at the input stage of this 20-band vocoder algorithm. The carrier channels are in stereo while the modulator use the mono sum of the input channels.

Gain Gain control of the modulator -48 .. +48 dB
Gate Bands below this level will be silenced. -96 .. 0 dB
Env Follow Rate of the envelope followers. 0 .. 100 %
Q Controls the steepness of the filters. -100 .. 100 %
Bands The number of vocoder bands. 4 .. 20
Min Frequency Frequency of the lowest vocoder band applied to the carrier. Bands will be spreaded evenly in pitch between it and the high band. 55 .. 3520 Hz
Max Frequency Frequency of the highest vocoder band applied to the carrier. Bands will be spreaded evenly in pitch between it and the low band. 440 .. 14080 Hz
Input Chooses the input source configuration. Mono Sum, Left Only, Right Only, Stereo
Range Squeezes or expands the range of the modulator bands. -100 .. 100 %
Center The modulator bands default to the carrier bands, but this recenters the modulator while keeping the same low/high distance. -100 .. 100 %


4-stage chorus algorithm.

Time Delay time used as chorus mid-point. 0 .. 0.125 s
Rate Rate of the modulation,
Can be tempo-synced.
0.008 .. 512 Hz
64 .. 1/1024 note
Depth Depth of the modulation. 0 .. 100 %
Feedback Amount fed from the output back into the input. -inf .. 0 dB
Low/High-cut EQ controls of the chorused signal. 14Hz .. 25kHz
Width Gain scaling of the Side-component of the wet signal. -24 .. 24 dB
Mix Blend control between the dry and the wet signal. 0 .. 100 %


Versatile Flanging algorithm.

Mode Mode used for the flanging algorithm Dry Signal + Combs, Combs Only,
Dry Signal + Arpeggiated Combs,
Arpeggiated Combs Only
Waveform Waveform of the modulation Sine, Triangle, Sawtooth, Sample & Hold
Rate Rate of the modulation,
Can be tempo-synced
0.008 .. 512 Hz
64 .. 1/1024 note
Depth Depth of the modulation 0 .. 100%
Count Number of comb filters used for the flanging algorithm. 1.00 .. 4.00
Base Pitch Cutoff frequency/pitch of the first comb filter. 0 .. 127 semitones
Spacing Cutoff frequency offset for the other comb filters. 0 .. 12 semitones
Feedback Increases the flanging resonance. 0 .. 100 %
LF Damping Damping for low frequencies. 0 .. 100%
Width Gain scaling of the Side-component of the wet signal. -24 .. +24 dB
Mix Blend control between the dry and the wet signal. -100 .. +100%


4-stage phaser.

Base freq Base frequency for all the stages. -100 .. 100 %
Feedback Feedback of the phaser. -100 .. 100 %
Q Q setting for the stages. -100 .. 100 %
Rate Rate of modulation LFO,
Can be tempo-synced.
0.008 .. 512 Hz
64 .. 1/1024 note
Depth Depth of modulation LFO. 0 .. 100 %
Stereo LFO Phase relation between stereo channels
0% = 0 degrees, 100% = 180 degrees.
0 .. 100 %
Mix Blend control between the dry and the wet signal. 0 .. 100 %

Rotary Speaker

Rotary speaker simulator algorithm.

Horn rate Rate of HF horn rotation, the LF horn is a lower multiple of this rate,
Can be tempo-synced.
0.008 .. 512 Hz
64 .. 1/1024 note
Rotor Rate Horn rate amount (multiplier). 0 .. 100 %
Doppler The amount of Doppler shift used in the simulation (vibrato). 0 .. 100 %
Tremolo The amount of amplitude modulation used in the simulation. 0 .. 100 %
Width Gain scaling of the Side-component of the wet signal. -24 .. +24 dB
Mix Blend control between the dry and the wet signal. -100 .. +100%


The delay algorithm in Surge is very versatile and can work well both as an echo/delay and chorus.

Illustration 20: Delay algorithm block diagram

Illustration 20: Delay algorithm block diagram

There is an LFO connected to the delay-lines (not shown in diagram) which can provide stereo-widening/detuning of the delay-line.

Pan Routes the two channels to the delay-units by panning.
The gain of the input-channels remain unaffected, it’s only
their stereo location that changes. (a sound only heard in the left channel
will still be heard when pan is set to 100% here, but only in the right channel.)
-100 .. 100 %
Delay time L/R Delay time for the two channels.
Can be tempo-synced.
0 .. 32 s
1/512 .. 16 whole notes
Feedback Amount fed from the channel to its own input. -inf .. 0 dB
Crossfeed Amount fed from the channel to the input of the opposing channel. -inf .. 0 dB
Low/High- cut EQ controls of the delayed signal. 14Hz .. 25kHz
Modulation rate Rate of the modulation LFO (triangle). 0.008 .. 512 Hz
64 .. 1/1024 note
Modulation depth Indirect control of the modulation LFO depth.
The effect adjust the depth to match the detuning in cents set here.
0 .. 200 cents
Mix Blend control between the dry and the wet signal.
0% = 100% dry, 0% wet
100% = 0% dry, 100% wet
0 .. 100 %
Width Gain scaling of the Side-component of the wet signal. -24 .. 24 dB

Reverb 1

The Reverb 1 algorithm is a classic and older sounding digital reverb.

Pre-Delay Amount of delay applied to the signal before it is fed to the
reverberation unit,
Can be tempo-synced
0 .. 32 s
1/512 .. 16 whole notes
Room Shape Selects between 4 room shapes that has different sounds.
(changing this parameter will interrupt the signal)
0 .. 3
Size Changes the apparent size of the simulated room.
(changing this parameter will interrupt the signal)
0 .. 100 %
Decay Time The time it takes for the reverberation to ring-out. (-60 dB) 0 .. 64 s
HF Damping Amount of HF damping applied to the signal inside the reverberator. 0 .. 100 %
Low Cut, Peak Freq/Gain, High Cut Post-reverb equalizer controls.  
Width Gain scaling of the Side-component of the wet signal -24 .. 24 dB
Mix Blend control between the dry and the wet signal. 0 .. 100 %

Reverb 2

The Reverb 2 algorithm is a second version of Surge’s original Reverb effect and has a different algorithm and controls. Reverb 2 is more natural and contains less digital artifacts. For most use cases, Reverb 2 sounds better than Reverb 1.

Pre-Delay Amount of delay applied to the signal before it is fed to the
reverberation unit,
Can be tempo-synced
0 .. 32 s
1/512 .. 16 whole notes
Room Size Changes the apparent size of the simulated room. -100 .. 100 %
Size Changes the apparent size of the simulated room. 0 .. 100 %
Decay time The time it takes for the reverberation to ring-out. (-60 dB) 0 .. 64 s
Diffusion Changes the complexity of the room, thus adjusting diffusion amount 0 .. 100 %
Buildup Controls how long the reverb takes to come to its peak and how
“smeared” in time the effect is.
0 .. 100 %
Modluation Amount of pitch modulation applied to the input for a more lush sound 0 .. 100 %
LF/HF Damping The amount of absorption/redution for Low or High frequencies 0 .. 100 %
Width Gain scaling of the Side-component of the wet signal -24 .. 24 dB
Mix Blend control between the dry and the wet signal. 0 .. 100 %

Microtonal Tuning

Surge features full-keyboard microtuning support, and uses an implementation of the complete Scala SCL and KBM microtuning format from Manuel Op de Coul, the developer of the Scala application. Scala is a freeware utility that can be used for the creation and analysis of historical, ethnic and contemporary musical instrument intonation systems. A powerful capability of Scala is that it enables the user to create and export the proprietary tuning data required for microtuning a wide range of hardware and software synthesizers and samplers.

Here are some external links to Download Scala, information about the Scala format and Keyboard mappings.

The Scala format is comprised of two human-readable text files:

SCL: The scale file containing data for the degrees of an intonation system in either cents or ratios.

KBM: The keyboard mapping file, which specifies the allocation of scale degrees contained in an SCL file to MIDI Notes on a keyboard controller.

Loading Scala SCL and KBM Files

As explained earlier, Surge offers two methods for loading Scala SCL and KBM files for changing the underlying intonation system of the instrument:

  1. Using the Menu Button on the bottom right of the interface.

  2. Right-clicking or Drag-and-Drop Scala SCL and KBM files on the tun button located in the Status Area.

View SCL and KBM Tuning Data

Surge has a useful analysis feature for viewing information about the loaded Scala SCL and KBM files, and how the pitches are mapped to MIDI Notes on the keyboard controller. To open the loaded tuning data from an HTML file in a browser, right-click the tun button, and select, Show current tuning.

The exported HTML page then shows the tuning description contained in the SCL file, the degrees of the scale, and the mapping of pitches to MIDI Notes. Below we can see that the Bohlen-Pierce tuning is mapped with its 1/1 starting note on C.60 @ 261.626 Hz.

To change the 1/1 mapping to another MIDI Note, drag-and-drop a different KBM file onto the TUN button, then open the HTML page again with the Show current tuning option to see how it changed the mapping.

Below we can see that the 1/1 for Bohlen-Pierce is now mapped to MIDI Note A.69 @ 440 Hz:

Click the Jump to Raw SCL or Jump to Raw KBM links to view the actual loaded SCL and KBM files mapping data.

Definition of Scala Linear Keyboard Mapping Files (KBM)

“Keyboard mappings determine the allocation of scale degrees to keys on a MIDI keyboard, or MIDI note numbers in general”.

Software implementations of the complete Scala format will include both the SCL file: the actual scale degrees of a given microtuning or intonation system, and the KBM file, which specifies how the pitches of the intonation system are directly key-mapped to the notes of MIDI keyboard controllers. Both of these are human-readable text format files.

While there are a wide variety of different uses for the Scala KBM files, perhaps the most essential of them is the so called, Linear Keyboard Mapping, which specifies:

  • Key For 1/1 - The MIDI Note on the controller where the scale will start: the degree 0 of the microtuning. For example, this could be MIDI Note C.60, A.69, or potentially any MIDI Note unique to the musical scenario at hand.

  • Reference Frequency - The frequency (Hz, CPS) that will be mapped to the Reference Key, which could be for example, set to the standard A.69 at 440 Hz, or C.60 at 261.625565 Hz.

  • Reference Key - The MIDI Note on which the Reference Frequency will be mapped, which, as above, might typically be C.60 or A.69. It is the combination of the Reference Key and the Reference Frequency assigned to it, that will determine the common base pitch and relative mapping of frequencies to MIDI Notes across the musical range for any given intonation system being mapped to a MIDI controller.

So, as we can see, a Linear Keyboard Mapping is ‘linear’ in the sense that pitches of an intonation system are mapped sequentially across the musical range of MIDI Notes relative to settings of the three parameters: Key For 1/1, Reference Frequency and Reference Key.

An intimate understanding of how this works, and why it is important to practically all musical instrument tuning, is fundamental to working with alternative intonation systems, as well as microtonal and xenharmonic music composition, where high-precision intonation is a frequent feature and requirement.

Here is an example Linear Keyboard Mapping, which maps the Key For 1/1 to MIDI Note C.60, with the Reference Frequency at 440 Hz, and the Reference Key on MIDI Note 69:

Example of Linear Mapping

! Size of map:
! First MIDI note number to retune:
! Last MIDI note number to retune:
! Middle note where the first entry in the mapping is mapped to:
! Reference note for which frequency is given:
! Frequency to tune the above note to (floating point e.g. 440.0):
! Scale degree to consider as formal octave:
! Mapping.

This would be typical for mapping intonation systems to Halberstadt keyboards, such as 12-note Pythagorean, various forms of meantone tunings, and a range of other so-called syntonic temperaments built upon chains (or cycles) of fifths, where the 1/1 starting note should fall on C.60 (middle C), and the concert reference pitch on A.69 @ 440 Hz. With this KBM, all of the well known classical diatonic modes will fall on the white keys of the controller, with sharps and flats on the black keys.

Microtuning is deeply integrated in Surge, as a lot of its modules will adapt their behavior to suit the loaded custom scale. For example, you can read this article on the Surge Wiki about applying microtuning pitch modulation using the sequencer.

Continuous Controller information (CC)

The eight macros towards the right of the routing bar have automatically assigned CC’s.

The list is as follows:

Control 1 = CC 41

Control 2 = CC 42

Control 3 = CC 43

Control 4 = CC 44

Control 5 = CC 45

Control 6 = CC 46

Control 7 = CC 47

Control 8 = CC 48


Feel free to visit the Surge Synth Slack (here if you have questions about Surge, want to help in developing it further or if you come across any bugs or other issues.


“Surge Code” The Surge code is licensed under GPL3

“VSTGUI” The VSTGUI is licensed under the below Steinberg license

“Lato” The Lato Font Software is licensed under the SIL Open Font License, Version 1.1.

Surge uses a variety of other open source packages, whose individual licenses are available in the appropriate git submodules and directories of the source code. These include nanosvg, catch2, tinyxml, and others. All Surge source code is available at https://github.com/surge-synthesizer/surge.git.


Version 3, 29 June 2007

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In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission to practice a patent or covenant not to sue for patent infringement). To “grant” such a patent license to a party means to make such an agreement or commitment not to enforce a patent against the party.

If you convey a covered work, knowingly relying on a patent license, and the Corresponding Source of the work is not available for anyone to copy, free of charge and under the terms of this License, through a publicly available network server or other readily accessible means, then you must either (1) cause the Corresponding Source to be so available, or (2) arrange to deprive yourself of the benefit of the patent license for this particular work, or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream recipients. “Knowingly relying” means you have actual knowledge that, but for the patent license, your conveying the covered work in a country, or your recipient’s use of the covered work in a country, would infringe one or more identifiable patents in that country that you have reason to believe are valid.

If, pursuant to or in connection with a single transaction or arrangement, you convey, or propagate by procuring conveyance of, a covered work, and grant a patent license to some of the parties receiving the covered work authorizing them to use, propagate, modify or convey a specific copy of the covered work, then the patent license you grant is automatically extended to all recipients of the covered work and works based on it.

A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the rights that are specifically granted under this License. You may not convey a covered work if you are a party to an arrangement with a third party that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license (a) in connection with copies of the covered work conveyed by you (or copies made from those copies), or (b) primarily for and in connection with specific products or compilations that contain the covered work, unless you entered into that arrangement, or that patent license was granted, prior to 28 March 2007.

Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.

12. No Surrender of Others’ Freedom.

If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.

13. Use with the GNU Affero General Public License.

Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.

14. Revised Versions of this License.

The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.

If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Program.

Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.

15. Disclaimer of Warranty.


16. Limitation of Liability.


17. Interpretation of Sections 15 and 16.

If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.


How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year>  <name of author>

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.

Also add information on how to contact you by electronic and paper mail. If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:

<program>  Copyright (C) <year>  <name of author>

This program comes with ABSOLUTELY NO WARRANTY; for details type show w. This is free software, and you are welcome to redistribute it under certain conditions; type show cfor details.

The hypothetical commands show w and show c should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.

You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see https://www.gnu.org/licenses/.

The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read https://www.gnu.org/licenses/why-not-lgpl.html.


(c) 2018, Steinberg Media Technologies, All Rights Reserved

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of the Steinberg Media Technologies nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.



Copyright (c) 2010-2015, Łukasz Dziedzic (dziedzic@typoland.com), with Reserved Font Name Lato.

This Font Software is licensed under the SIL Open Font License, Version 1.1. This license is copied below, and is also available with a FAQ at: http://scripts.sil.org/OFL

SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007

PREAMBLE The goals of the Open Font License (OFL) are to stimulate worldwide development of collaborative font projects, to support the font creation efforts of academic and linguistic communities, and to provide a free and open framework in which fonts may be shared and improved in partnership with others.

The OFL allows the licensed fonts to be used, studied, modified and redistributed freely as long as they are not sold by themselves. The fonts, including any derivative works, can be bundled, embedded, redistributed and/or sold with any software provided that any reserved names are not used by derivative works. The fonts and derivatives, however, cannot be released under any other type of license. The requirement for fonts to remain under this license does not apply to any document created using the fonts or their derivatives.

DEFINITIONS “Font Software” refers to the set of files released by the Copyright Holder(s) under this license and clearly marked as such. This may include source files, build scripts and documentation.

“Reserved Font Name” refers to any names specified as such after the copyright statement(s).

“Original Version” refers to the collection of Font Software components as distributed by the Copyright Holder(s).

“Modified Version” refers to any derivative made by adding to, deleting, or substituting – in part or in whole – any of the components of the Original Version, by changing formats or by porting the Font Software to a new environment.

“Author” refers to any designer, engineer, programmer, technical writer or other person who contributed to the Font Software.

PERMISSION & CONDITIONS Permission is hereby granted, free of charge, to any person obtaining a copy of the Font Software, to use, study, copy, merge, embed, modify, redistribute, and sell modified and unmodified copies of the Font Software, subject to the following conditions:

1) Neither the Font Software nor any of its individual components, in Original or Modified Versions, may be sold by itself.

2) Original or Modified Versions of the Font Software may be bundled, redistributed and/or sold with any software, provided that each copy contains the above copyright notice and this license. These can be included either as stand-alone text files, human-readable headers or in the appropriate machine-readable metadata fields within text or binary files as long as those fields can be easily viewed by the user.

3) No Modified Version of the Font Software may use the Reserved Font Name(s) unless explicit written permission is granted by the corresponding Copyright Holder. This restriction only applies to the primary font name as presented to the users.

4) The name(s) of the Copyright Holder(s) or the Author(s) of the Font Software shall not be used to promote, endorse or advertise any Modified Version, except to acknowledge the contribution(s) of the Copyright Holder(s) and the Author(s) or with their explicit written permission.

5) The Font Software, modified or unmodified, in part or in whole, must be distributed entirely under this license, and must not be distributed under any other license. The requirement for fonts to remain under this license does not apply to any document created using the Font Software.


This license becomes null and void if any of the above conditions are not met.



This project is maintained by the community at the GitHub Surge Synthesizer open source project