Build it in seven steps

The previous chapter presented the voice complete, with its connections fixed. Building it in order shows what each part contributes, and it is how a voice is assembled on a modular synthesiser, where no two modules are connected until you patch them. Step through the build below with the voice playing. The diagram shows the parts that have been added in solid outline and the ones still to come in dashed outline.

What each step adds

1. Oscillator. One oscillator is connected straight to the output, with no filter or amplifier after it yet. Switch waveforms while it plays. Each waveform has a different set of harmonics, so the tone changes while the pitch stays where it is.
2. Add a filter. A lowpass filter now sits between the oscillator and the output. It reduces the harmonics above its cutoff. The note and the waveform are unchanged, and the oscillator sounds darker because fewer of its harmonics reach the output.
3. Add a pitch source. A sequencer now sets the oscillator's pitch, stepping through eight notes, and sends a gate on every step. The pitch changes on every step, but the sound plays continuously, because the gate is not connected to any part of the voice yet.
4. Add an envelope. An envelope generator now receives the sequencer's gate and produces a shape on every step. Its output is not connected to anything. The sound is the same as at the previous step. The diagram shows the envelope running, but its output is not yet connected to any setting.
5. Add an amplifier. An amplifier now sits after the filter, and the envelope controls its level. Each step is now a separate note that starts and fades, with silence after each one before the next begins.
6. Add an LFO. An LFO is sent to the filter's cutoff. The depth control sets how far it moves the cutoff. At a low depth the tone rises and falls slowly over several notes. At a high depth the cutoff sweeps across most of its range.
7. Send the envelope to the filter. The envelope is now also sent to the filter's cutoff, alongside the LFO. The LFO repeats at its own rate, and the envelope moves the cutoff once for each note. Each note now starts brighter and darkens as its envelope decays, while the slower sweep from the LFO continues underneath.

Why step four makes no difference

At step four the sound does not change, because an envelope generator produces a control signal, and a control signal changes the sound only when it is sent to a setting. On a modular synthesiser this is a common reason a newly added module has no audible effect: the module is running, but its output is not patched to any input. On a synthesiser with fixed connections the same thing happens when an envelope's amount is set to zero. The envelope only becomes audible at step five, when it controls the amplifier.

One envelope or two

At step seven the same envelope controls both the amplifier and the filter, so the note's level and its brightness rise and fall together. Most keyboard synthesisers, including the Minimoog, give the filter its own envelope so that the two can differ, which is covered in envelopes as modulation. On a modular synthesiser that is a second envelope module, triggered by the same gate. Later parts of this guide sequence the voice, process its control signals and route its modulation. The part beginning with modulation at audio rate connects the same parts in other ways, such as sending one oscillator to the pitch of another.