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FOLLOWGATE -- 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.

   p0  = output start time (seconds)
   p1  = input start time (seconds)
   p2  = duration (seconds)
   p3  = carrier amplitude multiplier (relative multiplier)
   p4  = modulator amplitude multiplier (relative multiplier)
   p5  = power gauge window length (samples; try 100)
   p6  = smoothness -- how much to smooth the power gauge output (0-1; try .8)
   p7  = attack time -- how long it takes the gate to fully open once the
      modulator power rises above the threshold (seconds)
   p8  = release time -- how long it takes the gate to fully close once the
      modulator power falls below the threshold (seconds)
   p9  = pan (0-1 stereo; 0.5 is middle)
   p10 = power threshold (multiplier of total power, 0.0-1.0 (try 0.5))
   p11 = range (multiplier of threshold, 0.0-1.0 (usually small, try 0.005))

   p3 (carrier amp), p4 (modulator amp), p6 (smoothness), p7 (attack time),
   p8 (release time), p9 (pan), p10 (threshold) and p11 (range) 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