Clocks

In the previous chapter the sequencer moved to its next step and the sample and hold took a new value at regular intervals. Each of those steps was set off by a pulse from a clock. On a modular synthesiser the clock is usually a separate module, and patching its output to several others is what keeps them all playing in time with each other.

Tempo is normally given in beats per minute. Dividing by 60 gives beats per second, which is a frequency in hertz, so 120 BPM is 2 Hz. A clock seldom runs at one pulse per beat, though. Many modular sequencers move one step for each pulse they receive, and patterns are usually written in sixteenth notes, so the clock driving them runs at four pulses per beat: 8 pulses a second at 120 BPM. MIDI clock, which carries tempo from one instrument to another over a MIDI cable, runs at 24 pulses per beat. Both figures are written as pulses per quarter note, or PPQN.

Swing

Swing moves every second pulse later and leaves the first pulse of each pair where it was, so the pair as a whole keeps its length and the beat does not drift. The setting is the position of the late pulse within the pair. At 50% it sits exactly halfway and the rhythm is straight. At 66.7% it sits two thirds of the way through, which turns each pair of sixteenths into the first and last notes of a triplet. Past about 75% the late pulse is so close to the next one that the two start to sound like a single event.

Because swing works on pairs, it only affects the subdivisions. The pulse on each beat never moves, and neither does the pulse halfway between two beats, because it starts the second pair of the beat. In the tool above the higher click on each beat stays put, and of the three lower clicks after it, the first and the third arrive late.

One clock, several rates

A patch often needs several rates at once: a hi-hat on every sixteenth, a kick on every beat, a chord change every bar. Separate free-running clocks drift apart, because no two of them run at exactly the same rate, so a patch usually runs one clock and derives the others from it. A clock divider counts incoming pulses and puts out one pulse for every two, three, four or however many it is set to. Its outputs are counted from the source, so they cannot drift away from it.

Set one output to divide by three and another to divide by four and you have a polyrhythm. The two land together on the first beat, separate, and land together again after twelve beats, because twelve is the smallest number that both three and four divide into. That is why the rows above are twelve beats long.

A clock multiplier goes the other way and adds pulses between the ones it receives. To do that it measures the gap between the last two incoming pulses and spaces the new ones as though the next gap will be the same. When the tempo changes, the new pulses are spaced for the old tempo until the multiplier has measured the new gap. The ×2 row above is calculated from the tempo setting rather than measured, so it follows a tempo change immediately.

Reset

Two sequencers on the same clock stay in step only if they started together and neither has missed a pulse since. They fall out of step when a module is switched on late, when a cable is plugged in halfway through a bar, or when a sequence length is changed while it runs. A reset input puts them back in step. A pulse arriving at reset sends a sequencer or a divider back to its first step, and sending one reset signal to every module brings them all back to step one at the same moment.

Clocks do not set note length

A clock pulse marks a moment and carries no information about how long the note it starts should last. Many parts of a synthesiser need only that moment: a drum voice fires once per pulse, and a sample and hold takes one value per pulse. A melody also needs each note to end at the right time, and that is set by a different signal, the gate.