Chapter 7: Modulators – sounds are moved

Modulators are modules that (in very different ways) generate control voltages. The control voltages are then used to influence the modules discussed so far: tone generators, filters and other audio components. For example, one control voltage could move the frequency of an oscillator slightly up and down (vibrato), another slowly opens a filter when a key is pressed, a third control voltage lets the VCA quickly increase to maximum when a key is pressed, and then slowly reduce the volume again.

  • LFOs: Low Frequency Oscillators generate control voltages that are repeated periodically, just like their “fast brothers”, the VCOs.
  • Envelope generators: Control voltages are generated “on command” here, i.e. an envelope generator requires a trigger or gate signal and then generates a sequence of control voltages whose phases can be set on the module.
  • Sequencers: These are quite complex machines that can generate a sequence of control voltages (and trigger/gate signals). They are often used for ostinato tone sequences, but can also control filters, amplitudes, etc.
  • Random Generators: Modules that can generate random (or “almost random”) control voltages – often the crucial “spice” for more elaborate sounds.

But we remember: there is no fundamental distinction between audio signal and control voltage in modular systems. Of course, the already discussed sound generators (VCOs, noise generators) can also be used as interesting modulators.


So an LFO is something like a slow-running oscillator. We have a periodic oscillation with a reasonably constant frequency and with waveforms (sine, triangle, square, pulse and sawtooth) as we know them from the oscillators working in the audio sector. As with VCOs, the voltage of LFO signals is generally symmetrical around 0 V (e.g. +/- 2.5 V). The corner frequency of a filter will always be altered alternately up and down with an LFO. In some special cases there is only a positive voltage range, e.g. 0 to +5 V. This is useful for square-wave signals to use them as gate signals / triggers.

The frequency of many LFOs can be controlled purely manually, but there are also a number of voltage-controlled LFOs (VCLFOs). Some specimens have a “reset” input that restarts oscillation when a trigger signal is applied. This corresponds to the hardsync input of a conventional VCO.

And then there are LFOs with features that you won’t find in most VCOs. For example, the freely adjustable waveform between triangle and sawtooth with the A-146, the ability to switch between cyclic operation or an “attack-decay” envelope with the A-143-1 or with the A-171-2, up to the delayed use of the LFO with the A-147-2 VC Delayed LFO.

Posts on individual LFOs

Envelopes & Co

The step from the freely oscillating LFO to the envelope is small, as can be seen very nicely with the A-143-1 Complex Envelope Generator. The difference? An envelope always needs a signal when it should start, usually also when it should end again, the LFO doesn’t care about that, it runs and runs and runs.

And then envelopes also have something like “phases” in their course: The time it takes to rise from 0 volts to the maximum voltage after the start signal is called the attack time. In the phase that follows – decay – the voltage drops again. That’s it for simple envelopes, the simplest ones only have a single phase – decay (A-142-1 and A-142-4). With the widespread ADSR envelope curves (Attack – Decay – Sustain – Release), the voltage at the end of the decay phase does not necessarily drop back to 0 volts, but to a level that can be set with the “sustain” parameter. The voltage remains at this level as long as a button is pressed or more technically: as long as a gate voltage is present at the control input of the module. This is followed by a final phase in this type of envelope called release – the time it takes to reach 0 volts.

Posts on individual envelope generators


Sequencers are perhaps “the” archetypal components in modular systems. With the “Berlin School” at the beginning of the 1970s, analog sequencers had a decisive influence on an entire music genre. Klaus Schulze or Tangerine Dream would hardly be imaginable without a sequencer, although both of them got along quite well without them at first.

From today’s perspective – with DAWs and their almost limitless storage space – an analog sequencer does something incredibly primitive. In a “true analog” sequencer, we typically have 8 steps, each with one or more potentiometers that we can use to set a voltage for that step. In addition, there is a row of switches that we can use to determine which of the steps has a trigger or gate signal. Everything the sequencer then does is go step by step and output a trigger/gate signal and a control voltage (or several of them in parallel, depending on the model).

The standard use for trigger and control voltage is to start envelopes and control pitch to create a small ostinato melody – the sequence. At the end of our 8 steps, the sequence starts over. The more the sequencer allows us to intervene in these processes, the more interesting the possible uses become. Some devices allow to reverse the direction of the sequence or even to play it randomly, first and last step can be shifted while playing, the speed of the sequence can be modulated per step so that we get tones of different lengths, etc.

But sequencers aren’t limited to playing a series of notes. With an extremely fast-running sequencer, a so-called “graphic oscillator” can be implemented, in which the length of the sequence corresponds to the passage of a single oscillation. Or we use sequencers as complex envelope generators, which then do not work cyclically, but in a “one shot” mode: After running through the 8 steps, the sequencer stops again and has to be restarted by a trigger signal.

A special form – the pure trigger sequencer without the output of other control voltages – is discussed below with the trigger modules.

Posts on individual sequencers

Random and semi-random

Playing with chance (or maybe even “letting chance play”) also has a long tradition in modular systems. This time, however, it was not Moog who was in the lead in development, but Don Buchla (whose systems, by the way, usually do without a conventional keyboard – which, from the point of view of the “West Coast” modularists, is only a limitation anyway). The Doepfer modules A-149-1 and A-149-2 are borrowed from Buchla’s very tricky random modules “Source of Uncertainty”.

Mathematicians will confirm: Generating a “real” random value (in our case usually a random voltage level) is not that easy. “Pseudo-random numbers” are sometimes mentioned: These look “random” by all means, but are in principle calculable, which a “real” random number is not. In a modular system, this is most similar to a “sample & hold” circuit: an input signal is sampled at certain intervals and the current value is output until the next sample time is reached. If the input signal is a completely normal oscillation from a VCO or LFO, then the result would theoretically be calculable and even loses its random character at certain sampling intervals (e.g. at a very low frequency of the input signal and very high frequency of the sampling intervals). It is precisely at this point that the “pseudo” becomes musically interesting again: by skilfully selecting the input signal and sampling rate, you can, for example, move between a “melody” and “random” with smooth transitions.

Posts on individual random generators

Trigger and gates

So far we have considered a number of very different modulation sources. Modulation using control voltages is probably one of the basic principles that make up a synthesizer. Without modulations, the thing would be an electronic organ, to put it somewhat exaggeratedly. For this reason, there are many very different modulation sources that all have one thing in common: They generate control voltages that we can use to influence other modules.

There is one very specific type of control voltage that we haven’t looked at in detail yet: triggers. Or gate signals. We’ll come back to the difference. Triggers and gates are different from all other control voltages: while the latter can take on very different values, e.g. to control the cutoff-frequency of a filter, triggers only have to exceed a threshold value. They are signals to start a process. An envelope for example. Starting (and also stopping) a sequencer, but also switching to the next step of a sequencer. Since these are all processes for which we need the most precise timing possible, triggers are mostly individual square or pulse oscillations: the rising edge should be there as quickly as possible. Theoretically, we could of course also use the rising edge of a sine or triangle signal, but when exactly is the threshold value exceeded that starts the envelope generator? Most of the time we don’t know exactly at what voltage an envelope starts (5.3 volts? 7.89 volts?) nor what voltage a modulation source is delivering. The extremely steep edge of a rectangle is much safer. When the trigger signal starts, the threshold of the triggered module is already exceeded.

For triggers, it doesn’t matter how long such a trigger signal emits voltage before it falls back to 0 volts. Only the rising edge counts here. The gate signal is different. In principle, this is the same control signal, but we have a receiver that also reacts to the falling edge. ADSR-type envelopes are an example of this. As long as the gate signal emits voltage, the envelope remains at the sustain level (after going through the attack and decay phases). The release phase is only started on a falling edge – the gate signal falls back to 0 volts. Gates and triggers are often used synonymously, and in principle they are the same signals that are only used slightly differently by the receiver: Only the rising edge at the trigger, both rising and falling edges at the gate.

If I describe gates and triggers as voltages here, then that is – from a synthesizer historical point of view – not quite correct. Older analog synthesizers, such as the early Moog systems, did not use voltages as triggers, but rather the shorting of a line. The voltage triggers are more practical, however, because (a) we do not have to be careful of accidentally short-circuiting another module with a trigger. As a result, early Moog modular systems had a very different type of connector for the triggers. And because we can (b) also use something like LFOs for triggering.

Some of the random modules also generate “random triggers”: Here, of course, the randomness does not consist in a certain voltage level, but in the point in time at which a trigger is generated. These can be completely free events or “clocked randomness”, in which, for example, a random decision is made for each external trigger signal as to whether a trigger is generated at the module output or not.

Trigger sequencers also belong in the category of trigger generators. First of all, of course, almost every analog sequencer can output trigger signals in addition to the control voltages for pitch etc. The classic A-155 falls into this category. However, there are special modules that are limited to the output of trigger signals but make more tracks available, e.g. to control percussive sounds, such as the eight-track A-157 Trigger Sequencer.

Posts on modules that can generate triggers

Posts on trigger sequencers

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