--- /dev/null
+
+/* Stereo amplifier stage with bass, treble, gain and balance controls and a
+ dB meter. */
+
+declare name "amp -- stereo amplifier stage";
+declare author "Albert Graef";
+declare version "1.0";
+
+import("math.lib");
+import("music.lib");
+
+/* Fixed bass and treble frequencies. You might want to tune these for your
+ setup. */
+
+bass_freq = 300;
+treble_freq = 1200;
+
+/* Bass and treble gain controls in dB. The range of +/-20 corresponds to a
+ boost/cut factor of 10. */
+
+bass_gain = nentry("bass", 0, -20, 20, 0.1);
+treble_gain = nentry("treble", 0, -20, 20, 0.1);
+
+/* Gain and balance controls. */
+
+gain = db2linear(nentry("gain", 0, -96, 96, 0.1));
+bal = hslider("balance", 0, -1, 1, 0.001);
+
+/* Balance a stereo signal by attenuating the left channel if balance is on
+ the right and vice versa. I found that a linear control works best here. */
+
+balance = *(1-max(0,bal)), *(1-max(0,0-bal));
+
+/* Generic biquad filter. */
+
+filter(b0,b1,b2,a0,a1,a2) = f : (+ ~ g)
+with {
+ f(x) = (b0/a0)*x+(b1/a0)*x'+(b2/a0)*x'';
+ g(y) = 0-(a1/a0)*y-(a2/a0)*y';
+};
+
+/* Low and high shelf filters, straight from Robert Bristow-Johnson's "Audio
+ EQ Cookbook", see http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt. f0
+ is the shelf midpoint frequency, g the desired gain in dB. S is the shelf
+ slope parameter, we always set that to 1 here. */
+
+low_shelf(f0,g) = filter(b0,b1,b2,a0,a1,a2)
+with {
+ S = 1;
+ A = pow(10,g/40);
+ w0 = 2*PI*f0/SR;
+ alpha = sin(w0)/2 * sqrt( (A + 1/A)*(1/S - 1) + 2 );
+
+ b0 = A*( (A+1) - (A-1)*cos(w0) + 2*sqrt(A)*alpha );
+ b1 = 2*A*( (A-1) - (A+1)*cos(w0) );
+ b2 = A*( (A+1) - (A-1)*cos(w0) - 2*sqrt(A)*alpha );
+ a0 = (A+1) + (A-1)*cos(w0) + 2*sqrt(A)*alpha;
+ a1 = -2*( (A-1) + (A+1)*cos(w0) );
+ a2 = (A+1) + (A-1)*cos(w0) - 2*sqrt(A)*alpha;
+};
+
+high_shelf(f0,g) = filter(b0,b1,b2,a0,a1,a2)
+with {
+ S = 1;
+ A = pow(10,g/40);
+ w0 = 2*PI*f0/SR;
+ alpha = sin(w0)/2 * sqrt( (A + 1/A)*(1/S - 1) + 2 );
+
+ b0 = A*( (A+1) + (A-1)*cos(w0) + 2*sqrt(A)*alpha );
+ b1 = -2*A*( (A-1) + (A+1)*cos(w0) );
+ b2 = A*( (A+1) + (A-1)*cos(w0) - 2*sqrt(A)*alpha );
+ a0 = (A+1) - (A-1)*cos(w0) + 2*sqrt(A)*alpha;
+ a1 = 2*( (A-1) - (A+1)*cos(w0) );
+ a2 = (A+1) - (A-1)*cos(w0) - 2*sqrt(A)*alpha;
+};
+
+/* The tone control. We simply run a low and a high shelf in series here. */
+
+tone = low_shelf(bass_freq,bass_gain)
+ : high_shelf(treble_freq,treble_gain);
+
+/* Envelop follower. This is basically a 1 pole LP with configurable attack/
+ release time. The result is converted to dB. You have to set the desired
+ attack/release time in seconds using the t parameter below. */
+
+t = 0.1; // attack/release time in seconds
+g = exp(-1/(SR*t)); // corresponding gain factor
+
+env = abs : *(1-g) : + ~ *(g) : linear2db;
+
+/* Use this if you want the RMS instead. Note that this doesn't really
+ calculate an RMS value (you'd need an FIR for that), but in practice our
+ simple 1 pole IIR filter works just as well. */
+
+rms = sqr : *(1-g) : + ~ *(g) : sqrt : linear2db;
+sqr(x) = x*x;
+
+/* The dB meters for left and right channel. These are passive controls. */
+
+left_meter(x) = attach(x, env(x) : hbargraph("left", -96, 10));
+right_meter(x) = attach(x, env(x) : hbargraph("right", -96, 10));
+
+/* The main program. */
+
+process = hgroup("0-amp", hgroup("1-tone", tone, tone) :
+ hgroup("2-gain", (_*gain, _*gain)))
+ : vgroup("3-balance", balance)
+ : vgroup("4-meter", (left_meter, right_meter));