A slew limiter caps how fast a voltage can change, turning stepped CV into glide (portamento)

A slew limiter (also called a lag processor, glide, or portamento module) constrains the maximum rate at which a control voltage can change: when the input jumps to a new value, the output ramps toward it at a bounded speed instead of snapping. Applied to a pitch CV from a sequencer or keyboard, this produces portamento — notes slide into one another rather than stepping. Applied to a stepped random CV or a filter-cutoff CV, it smooths the staircase into gentle wandering curves and softens sudden sweeps. More slew means slower, more pronounced glide; zero slew passes steps through untouched. Rise and fall rates are often independently adjustable, enabling asymmetric glides — e.g. glide up but snap down for a legato feel, or a fast attack and slow decay for envelope-like shapes. Because this rise/fall behaviour is the same circuitry that shapes an envelope, function generators such as Make Noise Maths can act as a slew limiter, so a dedicated module is often unnecessary.

Examples

Keyboard/sequencer pitch CV → slew limiter → VCO (1V/oct): notes glide between pitches (portamento). Set rise=slow, fall=instant so each note glides up to the next but snaps down — a legato feel. S/H random CV → slew → VCF cutoff: stepped filter jumps become smooth wandering sweeps. On Maths, dial up the rise/fall on channel 1 or 4 to slew an incoming stepped CV.

Assessment

Explain what property of the input a slew limiter constrains and how that produces portamento. Predict what happens to a stepped CV as slew time is increased from zero. How would you set rise and fall to get legato (glide up, snap down) versus symmetric portamento?