Simple envelope follower, controlling an amplitude gate.

in RTcmix/insts/jg/FOLLOWER

quick syntax:


CAPITALIZED parameters are pfield-enabled for table or dynamic control (see the maketable or makeconnection scorefile commands). Parameters after the [bracket] are optional and default to 0 unless otherwise noted.

Param Field Parameter Units Dynamic Optional Notes
p0 output start time seconds no no  
p1 input start time seconds no no  
p2 duration seconds no no  
p3 carrier amplitude multiplier relative multiplier yes no  
p4 modulator amplitude multiplier relative multiplier yes no  
p5 power gauge window length samples no no try 100
p6 smoothness 0-1; try .8 yes no how much to smooth the power gauge output
p7 attack time seconds yes no how long it takes the gate to fully open once the modulator power rises above the threshold
p8 release time seconds yes no how long it takes the gate to fully close once the modulator power falls below the threshold
p9 pan 0-1 stereo; 0.5 is middle yes no  
p10 power threshold multiplier of total power, 0.0-1.0 yes no try 0.5
p11 range multiplier of threshold, 0.0-1.0 yes no usually small, try 0.005

Parameters labled as Dynamic can receive dynamic updates from a table or real-time control source.

Author: John Gibson, 1/5/03; rev for v4, JGG, 7/24/04

FOLLOWGATE extracts the amplitude of an audio input signal and uses it to control an audio signal gate applied to a separate signal. See the FOLLOWER instrument for the more general amplitude-envelope follower, and the FOLLOWBUTTER instrument for another example of how an amplitude-envelope follower may be employed.

Usage Notes

FOLLOWGATE uses the amplitude envelope of the modulator to control the action of a gate applied to the carrier. The gate opens when the power of the modulator rises above a threshold and closes when the power falls below the threshold. The bottom of the gate can be adjusted (via the “RANGETABLE” parameter, p11) so that it sits flush against the “floor” or rides above the floor, letting some signal through even when the gate is closed. The carrier is supplied as the “left” channel, the modulator as the “right” channel.

The “left” input channel comes from the bus with the lower number; the “right” input channel from the bus with the higher number. See the bus_config scorefile command for information about how RTcmix busses are used.

Currently in RTcmix it’s not possible for an instrument to take input from both an “in” bus and an “aux in” bus at the same time. So, for example, if you want the modulator to come from a microphone, which must enter via an “in” bus, and the carrier to come from a WAVETABLE instrument via an “aux” bus, then you must route the mic into the MIX instrument as a way to convert it from “in” to “aux in”. If you want the carrier to come from a file, then it must first go through MIX (or some other instrument) to send it into an aux bus. Since the instrument is usually taking input from an aux bus, the input start time for this instrument must be zero. The only exception would be if you’re taking the carrier and modulator signals from the left and right channels of the same sound file.

The envelope follower consists of a power gauge that measures the average power of the modulator signal. The “windowsize” parameter (p5) is the number of samples to average. Large values (> 1000) track only gross amplitude changes; small values (< 10) track very minute changes. If the power level changes abruptly, as it does especially with long windows, you’ll hear zipper noise. Reduce this by increasing the “SMOOTHNES” parameter (p6) This applies a low-pass filter to the power gauge signal, smoothing any abrupt changes.

You’ll probably always need to boost the modulator amplitude multiplier (p4) beyond what you’d expect, because we’re using the RMS power of the modulator to affect the carrier, and this is always lower than the peak amplitude of the modulator signal.

The output of FOLLOWGATE can be either mono or stereo.

Sample Scores

very basic:

   rtsetparams(44100, 2)
   source_listen = 0  // set to 1 to hear carrier and modulator separately
   dur = 20
   // play carrier to bus 0
   bus_config("WAVETABLE", "aux 0 out")
   wavet = maketable("wave", 1000, 1, .5, .2)
   amp = 15000
   WAVETABLE(0, dur, amp, freq = 440, 0, wavet)
   WAVETABLE(0, dur, amp, freq * 1.02, 0, wavet)
   // play modulator to bus 1
   bus_config("WAVETABLE", "aux 1 out")
   env = maketable("line", 1000, 0,0, 1,1, 19,1, 20,0)
   incr = base_incr = 0.15
   notedur = base_incr * 0.3
   freq = 1000
   for (st = 0; st < dur; st += incr) {
      amp = irand(5000, 30000)
      WAVETABLE(st, notedur, amp * env, freq, 0, wavet)
      incr = base_incr * irand(0.5, 2)
   // apply modulator's amp envelope to carrier
   bus_config("FOLLOWGATE", "aux 0-1 in", "out 0-1")
   setline(0,0, 1,1, 10,1, 20,0)
   caramp = 2.0
   modamp = 5.0
   winlen = 100      // number of samples for power gauge to average
   smooth = 0.0      // how much to smooth the power gauge curve
   attack = 0.002
   release = 0.02
   pan = 0.5
   if (source_listen) {
      bus_config("MIX", "aux 0-1 in", "out 0-1")
      MIX(0, 0, dur, 1, 0, 1)
   else {
      thresh = makeconnection("mouse", "x", min=0, max=2, dflt=0.5, lag=50, "threshold")
      range = makeconnection("mouse", "y", min=0, max=0.1, dflt=0.005, lag, "range")
      FOLLOWGATE(0, inskip = 0, dur, caramp, modamp, winlen, smooth, attack, release, pan, thresh, range)

See Also