Slew

Many of the control signals in this guide change in steps: a sequencer jumps from one pitch to the next, a sample and hold jumps to a new random value on each tick, and a gate jumps between low and high. A slew limiter sits between such a source and its destination and limits the slew rate, the speed at which its output can change. A step at the input comes out as a ramp, and in the common linear design a large step makes a longer ramp than a small one. On a modular synthesiser it is a module of its own, sometimes called a lag processor. Keyboard synthesisers and software instruments build the same function into the pitch signal path and label it glide or portamento.

The tool below sends one of three stepped sources through a slew limiter to an oscillator's pitch. The plot shows the source in grey and the slewed output in blue over the last four seconds.

Rise and fall

Many slew limiters have separate controls for rising and falling. With both equal, upward and downward changes are smoothed at the same rate. With a long rise and a short fall, the output climbs slowly and drops quickly, so a melody slides up into high notes and falls almost at once to low ones. In this tool the rise and fall times are the time the output takes to move one octave in linear mode, and the time constant of the curve in exponential mode.

Constant rate, constant time and exponential

A linear slew limiter moves at a constant rate, so an octave takes twice as long as a tritone, which is half the distance. Some synthesisers offer a constant-time glide instead, where every glide takes the same time whatever the interval. An exponential slew limiter, also called a lag, moves fast at the start and slows as it nears the target, the way a capacitor charges through a resistor. It needs only a resistor and a capacitor, and many analogue portamento circuits work this way, which gives their glides a quick start and a slow settle into the note.

Other uses

Slew works on any control signal. On a stepped random signal it gives a smooth random line, a common source of slow drifting modulation. On a gate it gives an attack and release envelope: the output rises over the rise time when the gate goes high and falls over the fall time when it goes low. On a square wave it gives a trapezoid, and with long enough times a triangle. Function generators such as Make Noise's Maths are built around a slew circuit for this reason, and with their output fed back to trigger themselves they become the looping envelopes of patches that drive themselves.