SynthPi Usage


User Interface

Signal Flow

Oscillators Section

Additive Organ
Sync, Ringmod, Glide, Mono

Filters Section

Frequency and Resonance
Keyboard Tracking
Filter Envelope
Filter Routing

Amplifier Section

Volume Envelope
Volume / Limiter

Modulators Section

Modulation Envelope

User Interface

SynthPi’s application control window.

After starting SynthPi you’ll notice two things: The application’s control window (image to the right) comes up and your computer’s standard web browser starts and displays the web page with the synthesizer user interface.

The control window shows all important status and logging information for the synthesizer. You can always re-open the web interface with the “Open Browser Interface” button and you can stop the synthesizer engine with the “Exit” button or by closing the control window.

All important stuff and all the fun is happening in the web browser window. Here you can see and edit all the synthesizer’s parameters in real time and do all the administrative stuff like patch loading, patch saving or MIDI and synth configuration.

The browser window’s main area is the synthesizer control interface. This is where you sculpt your sound.

The main area with all sound parameters.

All these knobs and switches can be accessed with your mouse. One important hint: double clicking a value resets the value to zero. That’s especially handy if you want to reset a modulation or effect value.

This interface is available from all web browsers in your local network. If you know the IP address of the machine SynthPi is running on, you can call it by entering http://<synthpi's.ip.address>:31415 in the browser’s address bar.

SynthPi’s web browser interface is organized in four blocks:

  • Sound generation, oscillators (yellow)
  • Filters (orange)
  • Amplifier and effects (blue)
  • Modulators (violet)

These blocks represent the signal flow that your sound travels until it reaches the speakers.

Signal Flow

Signal Flow Chart

The yellow block is the sound generation unit, where you can choose the synthesis method, the produced waveforms or harmonics, the pitch and tuning and some sound modulation like ring modulation or synchronization. There’s also a noise generator to produce a chip-tune (C64 SID) like noise sound.

The generated sound then goes through the orange colored filters (if at least one of them is turned on). Filters are used to attenuate or emphasize certain frequency ranges of the sound. The filters are the crucial tools to model your sound.

The last stage your sound has to pass is the blue colored amplifier and effects section. There’s an ADSR envelope that sculps the sound’s volume over time. The envelope for example defines if the sound starts immediatly after you pressed a note on your keyboard or if the volume should rise slowly. It also defines the time a sound decays after you released it’s key. And last but not least a bunch of effects can be applied like distortion, chorus, delay and reverberation. The last dial is the over-all volume.

The violet modulation block is used to modulate certain synth parameters over time. It has a low frequency oscillator (LFO) and a modulation envelope to make the sounds more vivid and lively.

So let’s dive into the four sections in detail.

Oscillators Section

The oscillators section offers three methods of synthesis to produce sound:

  • Waves” – Basic analog or sampled waveforms from different categories
  • Additive Organ” – Sine wave harmonics like used in classic tonewheel organs
  • Plucked” – A synthesized plucked string sound using a Karplus-Strong algorithm

Each of these three methods has slightly different controls to shape the generated sound.

Complete Oscillators Section in “Waves” Mode

The oscillators section consists of two sound generators (OSC1 and OSC2) which can be set to produce different waveforms.

OSC2 can be detuned slightly (“Fine“) or in semitone steps (“Semis“, -24 to 24 = up to two octaves lower or higher than the played note).

Use the “1 Mix 2” dial to balance the volume of OSC1 and 2. The “Gain” knob defines the outgoing volume passed on in the signal flow. This, for example, can be used to drive the filters harder or to prevent clipping.

The knob “Noise Level” adds a characteristic white noise to the sound.


soundselect-vaIn “Waves” mode the two dials select the waveform produced by OSC1 and OSC2.

These waveforms are continuously variable. The “OSC1/2 Wave” knob doesn’t instantly switch between different waves but morphes them gradually.

There are five different waveform categories available, selectable with the button below each knob:

  • Basic analog: Sine – triangle – sawtooth – rectangle – pulse waves
  • Overtones: Triangle waves with up to the 8th harmonic overtone
  • Strings: Sampled waveforms from string instruments (e.g. cello and violin)
  • Keyboards: Sampled waveforms from keyboard instruments (e.g. piano, electric piano and organs)
  • Voices: Sampled waveforms from male human voice vowels

The shape of the selected waveform is displayed in the knobs’ background.

OSC1 and OSC2 can be set to different waveforms and categories and OSC2 can be tuned relative to OSC1.

Additive Organ

soundselect-organThis synthesis method produces sounds like vintage electric organs. The “Harmonics” knob dials through different configurations of sine wave overtones comparable to the drawbar settings on Hammond organs.

The bars displayed in the background of the dials represent the overtone levels of the selected sound.

OSC1 and OSC2 can be set to different overtone settings. And of course OSC2 can be tuned relative to OSC1.


soundselect-pluckedThis mode is attempting to model the sound of a plucked string. It is an implementation of a Karplus-Strong algorithm.

Simply put it works like this: A really short tone burst is fed into a delay line. The delay line’s length is defined by the intended pitch. The delay line’s feedback dampening ratio correlates to the dampening or decay of the sound coming from the modeled string.

The “Harmonics” knob allows you to set the harmonic content of the pluck exciter – this is more or less how intense the virtual string gets plucked.

Damp” sets the amount of dampening and therefore how long the string’s sound will sustain.

The settings “Harmonics” and “Damp” apply to both: OSC1 and OSC2. But, as always, OSC2 can be tuned relative to OSC1.

Sync, Ringmod, Glide and Mono

The bottom row of the oscillators section has some further modulation and configuration options.


  • With “Sync” turned on, the waveform of OSC2 restarts every time OSC1’s waveform starts a new cycle. This setting is good for typical synthesizer sync sounds (hard sync). It is especially effective when OSC2 is tuned higher than OSC1 and even more if OSC2’s pitch is modulated with an LFO or envelope from the modulation section.
  • Ringmod” or amplitude modulation is a function, where the waves of OSC1 and OSC2 are multiplied. This results in a inharmonic, bell-like sound. The value of the “Ringmod” knob is the factor OSC2’s frequency is scaled to before it is multiplicated with OSC1. When set to zero (x0.00) no ring modulation will occur.
  • Glide” (often also called portamento) sets the time the pitch takes to slide from one note to the next. This works best when the oscillator section is set to “Mono” and is very unpredictable when played polyphonically.
  • With “Mono” turned on only one note can be played at a time (monophonic synthesizer).

Filters Section

filtersFilters are the most important section for shaping the overall timbre of your sound.

SynthPi comes with two multi mode filters which can be independently turned on and off.

There are six different filter types available:

  • “LP24”: 24db (4 poles) Low-pass Filter
  • “LP”: Low-pass Filter
    With low-pass filters only frequencies below the filter frequency pass through. This leads to a softer, mellower sound. The cut-off curve of “LP24” is steeper than  “LP”.
  • “BP”: Band-pass Filter
    A band-pass filter passes frequencies around the filter frequency and rejects frequencies outside that range resulting in a sort-of nasal sound.
  • “HP”: High-pass Filter
    With high-pass filters only frequencies above the filter frequency pass through. The sound gets harsher.
  • “Notch”: Notch Filter
    A notch filter attenuates the filter frequency. This is a very subtle change to the sound but can be used to make a sound more definite.
  • AllP”: All-pass Filter
    All frequencies pass through this filter but it adds a subtle delay to the wave leading to some kind of “phasing” effect. Can also be very dramatic with lots of filter resonance.

Except for the first filter type all filters are 12db (2 poles) filters.

Frequency and Resonance

The knobs for frequency and resonance define the effect of the filter.

“Freq” sets the cut-off frequency for the filter.

The “Res” control is used to emphasize the amplitude of the frequencies around the cut-off frequency. The sound gets some kind of quacky, buzzy or zappy. If the resonance is turned up to high values, the filter starts to self-oscillate. This can lead to a ringing sound or to a fixed sine wave tone at the cut-off frequency.

Keyboard Tracking

The “Keyboard” knob sets the degree to which the cut-off frequency follows the pitch of the note you play.

Set to 0, the cut-off frequency doesn’t change. With the middle keyboard tracking value 100 the filter frequency follows the note pitch 1:1. And even higher values accentuate this effect.

And if you like: With setting the filter to a cut-off frequency of 0, the resonance very high (self oscillation) and the keyboard tracking to 100 you can play the filter like a tuned oscillator.

Filter Envelope

filters-envelopeWith the envelope you can change the filters frequency over time after a key has been pressed. These are the four stages of the envelope:

  1. “Attack” adjusts the time the envelope takes from zero to 100% when a key is pressed.
  2. “Decay” is the time it takes after “Attack” is finished to settle at the “Sustain” value.
  3. The “Sustain” value is held as long as you hold the key.
  4. When you release the note’s key, the “Release” time determines how long it takes until the envelope’s value returns to zero.

This said, the dial “Env Depth” determines how much the envelope progression affects the filter’s frequency. It can be set to a positive or negative value.

The “Velocity” switch turns on keyboard velocity. The harder you press a key on your MIDI keyboard the more accentuated the envelope’s effect on the filter is.

With “Loop Env” you can switch the envelope into loop mode. In loop mode the envelope will not rest at the “Sustain” level as long as the key is pressed. It will restart the “Attack”/”Decay” cycle over and over again and will only go into “Release” mode when the key is released.

Filter Routing

filter-routeSince SynthPi has two independent filters, you’ll need to define in which way these filters are hooked together.

With the “Filter Routing” panel you can define the signal flow from the oscillators through both filters. There are three options: Serial, Parallel and Per Oscillator.

  • “Serial”: The sound goes from the oscillators through filter 1 and filter 1’s output goes through filter 2.
  • “Parallel”: The sound goes from the oscillators to filter 1 and filter 2 in parallel and both filter’s results are mixed together.
  • “Per OSC”: The sound from OSC1 goes through filter 1 and from OSC2 through filter 2. The filter’s outputs are mixed together.
Serial | Parallel | Per OSC

If set to “Parallel” or “Per OSC” and one of the two filters is turned off, the unfiltered input signal will be fed forward.

The “1 Mix 2” knob is only affective if the filter routing is set to “Parallel” or “Per OSC”.

Amplifier Section

amplifierThe final stage of the signal flow is the amplifier and effects section.

Volume Envelope

The volume envelope gives the sound it’s basic recognizable shape.

The volume envelope curve determines how the amplitude of the waveform changes over time: Starting when a note’s key is pressed, evolving as long as it is held and finishing after the key has been released.

volume-envThese are the four stages of the envelope:

  1. “Attack” adjusts the time it takes for the sound to reach full volume.
  2. “Decay” – After “Attack” is finished – and the key is still pressed – “Decay” defines the time it takes to decrease the volume until it reaches the “Sustain” value.
  3. After “Decay” is finished the “Sustain” volume value is held as long as you hold the key.
  4. When you release the note’s key, the “Release” time determines how long it takes for the sound to decay to silence.

The “Velocity” switch turns on keyboard velocity. The harder you press a key on your MIDI keyboard the more accentuated the envelope’s effect on the sound’s volume is. The velocity level increases the overall volume (louder) and shortens the “Attack” time (faster).

With “Loop Env” you can switch the envelope into loop mode. In loop mode the envelope will not rest at the “Sustain” level as long as the key is pressed. It will restart the “Attack”/”Decay” cycle over and over again and will only go into “Release” mode when the key is released.


effectsHere you can put the final spices to your sound. The available ingredients are:

  • Drive
    Adds an amount of overdrive or distortion to the sound.
  • Chorus
    This gives the sound a wider, shimmering timbre
  • Spread
    Spreads the two oscillators OSC1 and OSC2 to the left and right in the stereo panorama. Caution: This effect doesn’t work with filter routing serial.
  • Delay
    A delay or echo effect. “Delay” sets the amount of perceivable echo from “none” to “only echo”. “Time” is the echo’s time to repeat and “Feedback” defines how often the delay repeats.
  • Reverb
    A simple “one knob” reverberation. The more this is turned up, the bigger the cathedral and amount of reverb will get.

Volume / Limiter

Limiter is active

The final knob is the volume knob. But, there’s something special about this knob.

When the produced sounds somewhere in the signal flow get too loud to be processed without distortion a limiter kicks in. The limiter tries to keep everything in balance and reduces the output volume to an appropriate level. You can see this happening on the volume knob itself: a red arch is drawn representing the level of limiting.

If limiting occurs, you should somehow tame parts of the sound generation in your signal flow (e.g. reduce the “Gain” value in the oscillators section or the “Res” setting of a filter). Except, of course, you like what you are hearing!

Modulators Section


In addition to the sound generation signal flow described above, there’s a section offering two modulation sources and various modulation targets. These modulations can be applied to several parameters in the other sections and change the activated values to give your sound more movement and variety.

The two modulation sources are a so-called Low Frequency Oscillator (LFO) and a modulation envelope triggered with every keystroke on your MIDI keyboard.


The low frequency oscillator creates waves oscillating in low frequencies (0.03 to 30 Hertz) with six different waveforms.


The “LFO Rate” knob sets the LFO’s frequency from very slow 0.03 Hz (one cycle every 30 seconds) up to 30 Hz.

The six switches “LFO Waveform” define the modulation waveform of the LFO.

lfo-wavesThe images to the right show the LFO waveforms “Sine“, “Triangle“, ramp “Up“, ramp “Down” and “Square” (images: wiki-media commons, CC-BY-SA-3.0,2.5,2.0,1.0). The last waveform “S&Hold” (sample and hold) creates a sequence of random values.

If the switch “Key Reset” is on, the LFO’s waveform restarts every time a key is pressed.

With the “Amount” knob you can set the amount of LFO modulation, that is applied to the selected targets to the right. The  modulation wheel (mod-wheel) on your MIDI keyboard adds to this amount level. So you can apply modulation by using the MIDI mod-wheel controller.

The dials right to the arrow set how much a parameter is affected by the LFO modulation. These dials operate in positive and negative direction. Hence, the neutral position (no effect) is in the middle position.

The available targets for LFO modulation are: The overall “Pitch” of both oscillators, the pitch of oscillator OSC2 only (“Pitch2“), the waveform of oscillator 1 and 2 (“Wave1“, “Wave2“), the filter frequency of both filters (“Filter1“, “Filter2“) and the overall volume of SynthPi’s output (“Vol“).

Modulation Envelope

The second modulation source is a modulation envelope which is triggered with every played note.


The envelope’s parameters “Attack“, “Decay“, “Sustain“, “Release” and “Loop” work the same way as the filter’s envelopes or the volume envelope in the amplifier section.

The modulation envelope’s target dials set the amount of envelope modulation in a positive or negative way. The neutral position (no effect) is in the middle with value zero.

The available targets are the pitch of both oscillators (“Pitch“), the pitch of oscillator 2 only (“Pitch2“), the waveform of both oscillators (“Wave“), the noise level produced in the oscillator section (“Noise“) and the oscillator’s ring modulation amount (“Ringmod“).