Patches that drive themselves

Every patch so far has had a direction: a clock or a key press starts each event, and the signals flow from sources to destinations. Feedback sends a signal from later in that chain back to an earlier point. This chapter covers three examples: an envelope that triggers itself when it ends and so keeps repeating, a sequencer whose pitch also sets its own clock rate so that its rhythm follows its melody, and an oscillator that modulates its own frequency and so changes its own waveform. On a modular synthesiser any of these is one or two extra cables, and on other synthesisers some are available as a setting, such as an envelope's loop mode.

A loop from an envelope

An attack and decay envelope runs once when it receives a trigger. Many envelope generators also send a trigger out of an end-of-cycle output when they finish. Connecting that output back to the trigger input makes the envelope start again each time it ends, so it repeats indefinitely, and each cycle lasts as long as its attack and decay times added together. A looping envelope is an LFO whose shape you can set by its attack and decay: a slow rise and fast fall is a ramp, an equal rise and fall is a triangle, and a fast rise with a long fall is a series of plucks. Function generators such as Make Noise's Maths have a cycle switch that makes this connection internally.

A sequencer in a loop

A sequencer normally takes its timing from a clock and its notes from its steps, and the two are independent. In the patch above the pitch control signal is also sent to the clock's rate, so each step's pitch sets how long it lasts before the next one, and each melody produces its own rhythm. On a modular synthesiser this is one cable from the sequencer's pitch output to the rate input of the clock that is driving it, usually through an attenuator to set the amount.

The same idea extends to larger patches. A sequencer can choose its own next step from its own output, a random source can be sampled by a clock whose rate is set by the random source, and several loops can feed each other. Patches built this way, often called generative, keep producing new material, and the player sets up the system rather than playing each note.

Feedback at audio rate

Sending an oscillator's output back to its own frequency input is feedback FM. The oscillator speeds up while its output is high and slows down while it is low, which bends the sine wave towards a ramp and adds harmonics. Yamaha's FM synthesisers, the DX7 among them, include feedback on one of their operators for this reason. With more feedback the loop stops settling into a repeating shape and the output becomes noise.

In the browser, a feedback loop has to pass through a delay of at least 128 samples, about 3 milliseconds, so the feedback in this tool arrives a little late. An analogue self-patch, or the feedback inside an FM synthesiser, has far less delay, so the waveforms and the feedback levels at which noise appears differ from this tool. The clearest difference is the rise in pitch between about 25% and 40% feedback: the late feedback raises the oscillator's average frequency, and without the delay, as in an analogue oscillator whose output is sent to its own frequency input, the same settings lower it instead.