Understanding Signals
Signal Flow Basics ⚓︎
In a hardware modular synth, the patch cables transmit analog electrical signals from one module to another. These signals are measured in volts. Multiphonics works the same way. You can see it for yourself by connecting any output jack to a level module: it will either show the signal’s current voltage, or give an idea of its range if you are measuring a high frequency signal (the refresh rate of a computer monitor is too slow to show a precise value passed a certain frequency).
The signals produced by modules can be bipolar, meaning that they oscillate between negative and positive voltages, or unipolar, meaning that they are strictly positive.
Examples of bipolar signals are VCOs, LFOs or noise generators. Examples of unipolar signals are envelopes, gates, or clocks. These are described in more detail below.
A nice feature of Multiphonics is that all modules were designed so that their inputs and outputs work with a signal range of 10V peak-to-peak. This can either be from -5V to +5V for so-called bipolar signals, or 0V to +10V for unipolar signals. Because all inputs and outputs operate with roughly the same voltage range, anything can be freely connected to anything else.
Types of Signals ⚓︎
Audio Signals ⚓︎
Audio signals oscillate at a frequency in the hearing range, between 20 Hz and 20 kHz. A patch will normally have at least one module that produces audio signals, and it will be connected to the Output module either directly or through various filters, mixers and other sound processors.
Examples of modules that produce or process audio signals are the Classic VCO, the State Variable Filter, the Objeq Filter, the VCA and the various mixers.
Connecting an audio-rate signal into a module’s modulation input will work, but might not always behave exactly like it would on an analog synthesizer. This is explained in the Audio Rate and CV Rate section further down this page.
Many modules that process audio signals have stereo input and output jacks. You can tell the left and right channels apart by looking for a small dot on the top-left or top right of the jack. The stereo jacks can be placed vertically or horizontally, as shown in the next image.
Stereo modules can be used in mono by connecting only to one channel. By convention in the audio world, we will generally connect to the left channel for mono.
CV Signals ⚓︎
Any signal meant to control a module rather than to produce a sound is called a control voltage (CV) signal. They come in many flavors.
Gate and Trigger
Gate and trigger signals are the simplest kinds of CV signals. They have only two states: low and high, at 0V and 10V respectively.
Outputs labeled Gate will stay high for the duration of an event. For example, in the Keyboard module, the Gate output will be high for as long as a note is pressed on a MIDI keyboard.
Outputs labeled Trig only generate a very short pulse at 10V when an event happens. Trigger pulses are also generated by output jacks with different names, like End in the ADSR and Start of Cycle in the LFO.
In Multiphonics, the trigger pulse length is approximately 0.5 millisecond.
Some inputs are designed to process gates and triggers. Obviously, inputs labeled Gate expect a gate signal. Other inputs only look for a rising edge in the signal. A rising edge happens when the signal goes from 0V to 10V. These inputs will work with either gate or trigger signals. Any input labeled Trig, Reset or Clock operates that way.
Inputs that expect gate or trigger signals will also work with any kind of CV signal. In Multiphonics, the threshold between a low and a high signal is 2V. Therefore, gate inputs will be active for as long as their input is above 2V, while inputs that are triggered on a rising edge will look for the signal to go from below 2V to above 2V.
Pitch
Inputs and outputs labeled Pitch work with the 1V/octave convention used in most modular synths. This means that a variation of +1V will increase the pitch by one octave, and -1V will decrease the pitch by as much.
A special attribute of Multiphonics is that its pitch signals are bipolar and cover the whole range of MIDI notes. For all modules that produce pitch signals, 0V represents middle C (MIDI 60, 261.63 Hz), -1V is the C one octave below, 1V is the C one octave above, and so on. This allows the pitch to have approximately a 10V range (MIDI 0 is -5V and MIDI 127 is 5.58V), and to be used like any other modulation signal anywhere in the patch.
Pitch signals are produced by the Keyboard module, the Gate+CV Sequencer and the Pitch Detector, but any kind of CV signal can be connected to a pitch input. The Quantizer module can even be used to convert any random CV to the scale of your choice.
Envelope
Envelopes are unipolar signals that activate when triggered and evolve over time. The envelope generator modules in Multiphonics are the ever popular ADSR, its smaller sibling the µADSR and the Pluck Envelope. Envelopes can also be produced by other means, like processing a gate signal through a Slew Limiter or extracting it from an audio signal using the Envelope Follower or the RMS module.
In many patches, an envelope signal will be used to shape the volume of the sound when a note is played. This is done by patching the envelope into the CV input of a VCA.
They are also good modulation sources for any knob that has modulation inputs.
Clock
Clock signals are special gates that alternate between high and low (10V and 0V) at a given rate. In the Master Clock module found in the top row, the clock rate is set by the current tempo, the Clock Div setting and the Swing knob.
It can be patched in the Clock input of a sequencer, or anywhere a gate or trig signal is expected. Conversely, a module with a Clock input will also work with any kind of trigger signal; it does not absolutely need to be connected to an actual clock.
Tempo
The Tempo signal found in the Master Clock and Clock modules is a unique feature of Multiphonics that makes the synchronization of any time-based parameter possible.
- A 0V signal represents 60 BPM.
- Each increment of 1V doubles the tempo
- Each decrement of 1V halves the tempo
For a detailed explanation of using this signal to synchronize time and rate parameters in LFOs, envelope generators, or delays, see the Synchronizing Rates and Times section in the Master Clock manual.
Velocity ⚓︎
Velocity is another place where Multiphonics improves on most other modular synthesizers. As with Pitch signals, we decided to make our velocity signals bipolar with a range of ±5V.
The main velocity source is the Vel output on the Keyboard module. It converts MIDI velocity into a CV signal, where -5V corresponds to velocity 1, 0V to velocity 64, and +5V to velocity 127.
A Gate+CV Sequencer’s CV output can also be used as a velocity signal source.
Some modules have inputs labeled Vel, designed to be connected to a velocity CV signal. A Vel input controls the amplitude of the signal generated by the module using an industry-standard curve, with MIDI velocity 64 (0V) representing unity gain. These inputs have an attenuverter to adjust the depth of the effect. When fully clockwise (100%), MIDI velocity 127 (+5V) boosts gain by approximately +12 dB, while MIDI velocity 1 (-5V) reduces it to near silence.
Audio Rate and CV Rate ⚓︎
To find a good balance between reasonable CPU consumption and the freedom to connect anything anywhere, we decided to run modules at two different rates.
The rate at which the audio signals are processed is called the audio rate. It is always equal to the sampling rate at which your DAW or audio interface is running.
Multiphonics is designed to operate properly at sampling rates of 44.1 kHz or higher. Its oscillator modules don’t produce audible aliasing under normal operation and with reasonable amounts of modulation. For the vast majority of patches, increasing the sampling rate will not improve the sound quality.
The rate at which CV signals are processed is called the CV rate. It is fixed at 2000 Hz, meaning that CV signals are processed 2000 times per second. While this is not as fast as the audio rate, it is still well within the hearing range and is quick enough for crazy modulations.
Modulation inputs attached to parameter knobs generally run at CV rate, as do most other inputs designed for CV signals (Pitch, Vel, Env, Gate, Trig, Reset, etc).
Any audio signal connected to an input that runs at CV rate will be mercilessly downsampled to the CV rate. The aliasing artifacts caused by that downsampling may introduce unexpected glitches in your patch, but it may also produce some amazing noises.