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/* Compressor unit. */
declare name "compressor -- compressor/limiter unit";
declare author "Albert Graef";
declare version "1.0";
import("music.lib");
/* Controls. */
// partition the controls into these three groups
comp_group(x) = hgroup("1-compression", x);
env_group(x) = vgroup("2-envelop", x);
gain_group(x) = vgroup("3-gain", x);
// compressor controls: ratio, threshold and knee size
ratio = comp_group(nentry("ratio", 2, 1, 20, 0.1));
threshold = comp_group(nentry("threshold", -20, -96, 10, 0.1));
knee = comp_group(nentry("knee", 3, 0, 20, 0.1));
// attack and release controls; clamped to a minimum of 1 sample
attack = env_group(hslider("attack", 0.002, 0, 1, 0.001)) : max(1/SR);
release = env_group(hslider("release", 0.5, 0, 10, 0.01)) : max(1/SR);
// gain controls: make-up gain, compression gain meter
makeup_gain = gain_group(hslider("makeup gain", 0, -96, 96, 0.1));
gain(x) = attach(x, x : gain_group(hbargraph("gain", -96, 0)));
/* Envelop detector. This is basically the same as in amp.dsp. */
t = 0.1;
g = exp(-1/(SR*t));
env = abs : *(1-g) : + ~ *(g);
rms = sqr : *(1-g) : + ~ *(g) : sqrt;
sqr(x) = x*x;
/* Compute the envelop of a stereo signal. Replace env with rms if you want to
use the RMS value instead. */
env2(x,y) = max(env(x),env(y));
/* Compute the compression factor for the current input level. The gain is
always 0 dB if we're below the reduced threshold, threshold-knee. Beyond
the real threshold value the level is scaled by 1/ratio. Between these two
extremes we return a convex combination of those factors. This is also
known as "soft-knee" compression: the compression kicks in gradually at
threshold-knee and reaches its full value at threshold. For special
effects, you can also achieve old-school "hard-knee" compression by setting
the knee value to zero. Also note that, before computing the gain, the
input level is first smoothed out using a 1 pole IIR to prevent clicks when
the input level changes abruptly. The attack and release times of this
filter are configured with the corresponding envelop controls of the
compressor. */
compress(env) = level*(1-r)/r
with {
// the (filtered) input level above the threshold
level = env : h ~ _ : linear2db : (_-threshold+knee) : max(0)
with {
h(x,y) = f*x+(1-f)*y with { f = (x<y)*ga+(x>=y)*gr; };
ga = exp(-1/(SR*attack));
gr = exp(-1/(SR*release));
};
// the knee factor, clamped to 0..1; we add a small perturbation in
// the denominator to prevent infinities and nan when knee<<1
p = level/(knee+eps) : max(0) : min(1) with { eps = 0.001; };
// the actual compression ratio
r = 1-p+p*ratio;
};
process(x,y) = (g*x,g*y)
with {
g = env2(x,y) : compress : gain : +(makeup_gain) : db2linear;
};
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