There are several types of signals commonly found on a Eurorack modular synthesizer.  The specification for these signals is owned by Doepfer, the company that created the Eurorack standard used in their A-100 System.  Technical Details A-100 System.  Today many companies make Eurorack “compatible” modules, and some of those companies are pretty loose in how they match the original specification.  Strangely, it still seems to work out for the most part – but it means that every module should be designed to handle over-voltage conditions gracefully, both positive and negative.

In general, signals can be broken down into several categories:

• Analog audio
• Analog control voltage
• Digital gates, triggers, and clocks

Analog audio

Analog audio is by it’s definition not digital. If necessary it can be represented inside a digital module with a sampling rate of at least 44K, and bit depth of 16, (the rate used for CD audio) but when it leaves the module it must be converted back to the analog domain.

Analog audio signals consist of voltage that varies over time. Doepfer specifies that the peak to peak voltage of the signal falls into the -5 t0 +5 range, although I  have modules that run more in the -7 to +7 volts range, and that for the most part works just fine.  The frequency of an audio signal (which maps to the sounds pitch) is usually in the range of 20 hz (cycles per second), and 20,000 hz.  Different people hear differently.  If you spent much of your childhood hanging out around rock bands, it is very unlikely that your hearing goes near that high.  What?

In a modular synthesizer analog audio can be used as a control voltage (more on  that in a moment), and sometimes control voltages cross over into the audio range.

LFO Audio

Typically very low frequency audio signals are produced by Low Frequency Oscillators (LFOs) in the range of around .100 hz through 20 hz. Although these signals are similar to higher audio signals, they are usually thought of as control voltages.

VCO Audio

Audio signals are produced by Voltage Controlled Oscillators (VCOs) and they usually run from around 20 hz to 18,000 hz.  This upper limit is not specified, and varies quite a bit.  Usually there is no need for signals higher than that.

Noise

Most synthesizers contain noise generators as well as oscillators.  These also produce signals in the -5 to +5 volt range, but they have no fixed pitch.  In the audio domain they are used for chifs (a burst of unpitched noise at the start of a musical note), wind noises and distortion.  When they are set to run very slowly, they become a source of random  control voltages; also very useful.

Analog Control Voltage

Here things begin to get out of control.  There are several kinds of control voltages, and not all of these signals are interchangeable.  The basic categories are:

• DC signals
• AC signals
• 1 volt per octave pitch signals

AC signals

AC signals (typically -2.5 to +2.5 volts or higher) like LFOs and random noise generators typically are used for tremolo, vibrato and periodic filter effects. Doepfer specifies -2.5 to +2.5 although even their modules vary a bit -t the quadrature LFO goes well beyond this for example.

DC signals

DC signals (typically 0 to 5, 7, or 8 volts) are used to control the loudness of a signal out of a VCA.  Envelope generators fall into this catagory.  Doepfer specifies 0 to 8 volts for envelope loudness signals.

Some modules such as Voltage Controlled Amplifiers (VCAs) expect DC signals.  This means that they only react to half of a AC signal, the part above 0 volts.  Sometimes a VCA has two inputs, a envelope input that is DC, and a modulation input that is AC . but in many modules this is not supported.

1 volt per octave pitch signals

Finally, control voltage can be used to control the pitch of an VCO.  In this form the signal is defined by Doepfer to be 0 to 8 volts, which gives a range of 8 octaves.  That’s enough to cover the range of a large keyboard.  Of course, not all modules can generate signals beyond +5 volts, and not all modules will respond to higher voltages.  The important thing about CV when used for pitch is that one volt is defined to represent one octave.  This (at least) is consistent across all modules, and allows VCOs a range of 5 octaves or more.

Digital signals

Digital signals are always made up of only two voltages, typically 0 and around 5 volts.  Beyond that, they are used for several things which all have different criteria. These fall into subclasses as well:

Trigger signals

Triggers are generated when the signal goes to 5 volts.  Since nothing happens when the signal drops to 0, these signals are often very asymmetrical, just being short pulses over longer periods of time.

Gate signals

Gate signals control envelope generators.  A typical envelope generator has 4 stages: attack, decay, sustain, and release.   Attack is triggered when the signal goes positive, and release is triggered when the gate goes back to 0. It is important to remember that the release can go on for quite a long time, so that it may get cut off by the next attack.  In any case, it always continues after the gate signal ends.

Clock signals

Clock signals are usually 0 to 5 volts like the other digital signals, however they are periodic; unlike one-shot signals like triggers and gates, a clock signal continues to change from 0 to 5 volts at a regular rate.  The AC signal from a LFO, if it is set to be a square wave, can function as a clock signal, even if the negative part of the signal is discarded.  Clock signals can also be made up of a series of triggers.  All the action happens on the positive edge of the signal, where it goes from 0 to 5 so how long the signal stays at 5 volts is not really important.

As always, if you have comments to add, corrections to make, or suggestions for further study, please leave a comment below.