Janus SLU

Poly slew limiter with independent rise and fall · Vulpus Labs

A slew limiter puts a speed limit on a signal.

The output chases the input. If the input moves slowly, the output keeps up exactly and nothing happens at all — a slew limiter does nothing to a signal that is already under its limit. If the input moves faster than the limit, or jumps, the output cannot follow: it sets off towards the new value at the fastest speed it is allowed and arrives late.

This behaviour has different musical meanings depend on what you patch into it:

• For pitch CV, a jump between notes becomes a slide — portamento;
• For a gate, the instant on/off becomes a ramp up and a ramp down — an attack and a release;
• For a stepped signal like a random or a square LFO, the steps become slopes;
• For an audio-rate signal, the limiter cannot keep up at all, and what comes out is distorted.

Janus SLU sets two separate speed limits. RISE is the limit on the way up and FALL is the limit on the way down. Configured separately, they enable a signal to leap up and sink slowly back, or ease in and cut off.

IN and OUT

IN — poly signal input. Pitch, a gate, an envelope, an LFO, audio; it is all volts, and the module has no opinion about which it is smoothing.

OUT — poly, one value per lane.

Every lane has its own state, so a chord goes through as a chord: six voices at six different pitches are six independent glides, not one smeared average. Polyphony follows the patch, and a lane that comes back into use starts at its input rather than sliding up to it from zero — otherwise the first note of every patch would swoop in from nowhere.

TIME

Fader — 0 to 30 seconds, resting at 1.

The number is seconds to cross ten volts, which is the full range of the rack. It is a speed, not a duration: the time you actually hear depends on how far the signal has to travel. Half the distance takes half the time.

This matters more than it sounds, because the distances in a patch are mostly small. Pitch is one volt per octave, so a semitone is a twelfth of a volt — a hundred and twentieth of the ten volts the fader is quoted against:

So glides between notes live at the top of the fader and smoothing lives at the bottom.

SHAPE

LIN (down, the default) / EXP (up). One switch for the rise, one for the fall.

This is the difference between the two ways of getting somewhere late.

LIN travels at one steady speed and stops when it arrives. Like a lift: it sets off, it moves at its rated speed the whole way, and it stops dead at the floor. The arrival is exact and on time — if the fader says the move takes 200 ms, it takes 200 ms — and there is a slight corner at the end where the movement stops.

EXP starts fast and slows as it gets close. Like a door with a closer on it, or a car coasting to a halt: most of the distance is covered early, and the last part is a long, soft approach. There is no corner — it settles rather than stops. Strictly it never quite arrives, so the module lands it once the remaining distance is too small to hear.

On a pitch glide, LIN reaches the note and the slide stops; EXP eases into it, which is what most people mean by glide. On an envelope, LIN gives straight ramps and EXP gives the curved attack and decay of the classic analogue shape.

Both settings take the same time. The switch changes the journey, not its length, and you can throw it mid-glide without the signal jumping or the timing changing.

The two switches are separate on purpose. A straight ramp up into a curved fall away — or the reverse — are shapes you cannot get from one switch.

CV

AMT (−/+, centre detent) over MONO and POLY. The two jacks are added together and the knob scales the result, one set per direction. Centre is off.

A volt doubles the time; minus a volt halves it. At full knob, ±5 V covers a factor of 32 either way. Working in doublings rather than in seconds means the same movement on the CV is the same musical change wherever the fader is set, and the time can never be pushed to zero or below.

MONO shifts the whole module; POLY takes a separate voltage per voice, so a fan from Janus KEY gives every note its own glide time — a velocity-dependent portamento, for instance. Both jacks at once does both.

LOCK

The small button on the FALL row ties the two faders together, so moving one moves the other.

It ties the faders, not the times. Each direction keeps its own CV inputs and its own knob, so a locked pair still comes apart under modulation.