--- /dev/null
+
+/* Expander unit. */
+
+/* This is pretty much the same as compressor.dsp, but here the given ratio is
+ applied to *attenuate* levels *below* the threshold. */
+
+declare name "expander -- expander/noise gate unit";
+declare author "Albert Graef";
+declare version "1.0";
+
+import("music.lib");
+
+/* Controls. */
+
+exp_group(x) = hgroup("1-expander", x);
+env_group(x) = vgroup("2-envelop", x);
+gain_group(x) = vgroup("3-gain", x);
+
+ratio = exp_group(nentry("ratio", 2, 1, 20, 0.1));
+threshold = exp_group(nentry("threshold", -40, -96, 10, 0.1));
+knee = exp_group(nentry("knee", 3, 0, 20, 0.1));
+
+attack = env_group(hslider("attack", 0.001, 0, 1, 0.001)) : max(1/SR);
+release = env_group(hslider("release", 0.1, 0, 10, 0.01)) : max(1/SR);
+
+gain(x) = attach(x, x : gain_group(hbargraph("gain", -96, 0)));
+
+t = 0.1;
+g = exp(-1/(SR*t));
+env = abs : *(1-g) : + ~ *(g);
+rms = sqr : *(1-g) : + ~ *(g) : sqrt;
+sqr(x) = x*x;
+
+env2(x,y) = max(env(x),env(y));
+
+expand(env) = level*(1-r)
+with {
+ 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));
+ };
+ p = level/(knee+eps) : max(0) : min(1) with { eps = 0.001; };
+ r = 1-p+p*ratio;
+};
+
+process(x,y) = (g*x,g*y)
+with {
+ g = env2(x,y) : expand : gain : db2linear;
+};