dsp: a tonewheel organ, a guitar amp, a mix-bus glue

dsp/fx/amp.dsp

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+# Amp -- two valve stages, a tone stack and a speaker cabinet.
+#
+# Most of an electric guitar's sound is what happens after the string,
+# and `guitar.dsp' stops at the string. This is the rest of the chain,
+# in the order an amplifier puts it.
+#
+# THE INPUT IS TIGHTENED FIRST. A high-pass at 100 Hz in front of the
+# gain, because low end driven into a valve turns to mud: an amp that
+# keeps the bass out of the preamp and puts it back in the tone stack
+# is how a high-gain chord stays a chord.
+#
+# TWO STAGES, AND THE FIRST IS BIASED. A tanh is symmetric and makes
+# odd harmonics only; a valve is not, because it is biased off center,
+# and the even harmonics that asymmetry adds are the warmth people mean
+# by "tube". So stage one is tanh of the signal plus a bias, less tanh
+# of the bias alone, which is zero for silence; a low-pass at
+# 6 kHz between the stages takes the fizz out before stage two clips
+# it again, symmetrically. `Gain' drives both.
+#
+# THE TONE STACK is one state-variable filter at 700 Hz, its three
+# outputs -- which sum to the input -- weighted by `Bass', `Mid' and
+# `Treble'. At 5 a knob is flat and each step either side is about
+# 2.4 dB, so all three at 5 is the signal unchanged. It sits after the
+# gain, where an amp has it.
+#
+# THE CABINET is what makes it a guitar amp rather than a fuzz. A
+# 12-inch speaker passes nothing much below 80 Hz -- where the box
+# resonates, hence the bump -- and nothing above 5 kHz, with a peak
+# where the cone breaks up just under that. Two low-passes give the
+# 24 dB slope; `Cab' at 0 bypasses it for a direct, harsh box.
+#
+# Mono in the middle, because an amp is: the two sides are summed in.
+#
+# AND AN ECHO PEDAL, because a channel carries one effect and a lead
+# guitar without its delay is not the sound anybody means. It hangs off
+# the cabinet, left at `Delay' and right half again as late, which is
+# where the stereo comes from; `Echo' at 0 is off.
+
+name "Amp";
+author "Misha Nasledov";
+description "A guitar amplifier: a tightened input, two valve stages, a three-band tone stack and a 12-inch cabinet.";
+category "Effects";
+
+    @gain = 5;
+    @gain.widget = 1;
+    @gain.min = 0;
+    @gain.max = 10;
+    @gain.label = "Gain";
+
+    @bass = 5;
+    @bass.widget = 1;
+    @bass.min = 0;
+    @bass.max = 10;
+    @bass.label = "Bass";
+    @bass.group = "Tone";
+
+    @mid = 5;
+    @mid.widget = 1;
+    @mid.min = 0;
+    @mid.max = 10;
+    @mid.label = "Mid";
+    @mid.group = "Tone";
+
+    @treble = 5;
+    @treble.widget = 1;
+    @treble.min = 0;
+    @treble.max = 10;
+    @treble.label = "Treble";
+    @treble.group = "Tone";
+
+    @bias = 0.2;
+    @bias.widget = 1;
+    @bias.min = 0;
+    @bias.max = 0.5;
+    @bias.label = "Bias";
+
+    @cab = 1;
+    @cab.widget = 1;
+    @cab.min = 0;
+    @cab.max = 1;
+    @cab.label = "Cab";
+
+    @echo = 0;
+    @echo.widget = 1;
+    @echo.min = 0;
+    @echo.max = 1;
+    @echo.label = "Echo";
+    @echo.group = "Echo";
+
+    @delay = 375 ms;
+    @delay.widget = 1;
+    @delay.min = 20 ms;
+    @delay.max = 1000 ms;
+    @delay.label = "Delay";
+    @delay.group = "Echo";
+
+    @feedback = 0.35;
+    @feedback.widget = 1;
+    @feedback.min = 0;
+    @feedback.max = 0.9;
+    @feedback.label = "Feedback";
+    @feedback.group = "Echo";
+
+    @level = 1;
+    @level.widget = 1;
+    @level.min = 0;
+    @level.max = 4;
+    @level.label = "Level";
+
+node ionode {
+    channels = 2;
+
+    in0 = 0;
+    in1 = 0;
+
+    out0 = out->out + echol->out * @echo;
+    out1 = out->out + echor->out * @echo;
+};
+
+node tight filt::svf {
+    in = (ionode->in0 + ionode->in1) * 0.5;
+    cutoff = 100;
+    res = 0;
+};
+
+node stage1 dist::saturate {
+    in = tight->out_high * (1 + @gain * 3) + @bias;
+    factor = 1;
+};
+
+node rest dist::saturate {
+    in = @bias;
+    factor = 1;
+};
+
+node couple filt::svf {
+    in = stage1->out - rest->out;
+    cutoff = 6000;
+    res = 0;
+};
+
+node stage2 dist::saturate {
+    in = couple->out_low;
+    factor = 1 + @gain * 0.5;
+};
+
+node stack filt::svf {
+    in = stage2->out;
+    cutoff = 700;
+    res = 0.2;
+};
+
+node tone math::add {
+    in0 = stack->out_low * exp2((@bass - 5) * 0.4) +
+          stack->out_band * exp2((@mid - 5) * 0.4);
+    in1 = stack->out_high * exp2((@treble - 5) * 0.4);
+};
+
+# The box: a resonant high-pass at 85 Hz, then two low-passes, the first
+# with the cone's breakup peak on it.
+node box filt::svf {
+    in = tone->out;
+    cutoff = 85;
+    res = 0.4;
+};
+
+node cone filt::svf {
+    in = box->out_high;
+    cutoff = 4200;
+    res = 0.5;
+};
+
+node cone2 filt::svf {
+    in = cone->out_low;
+    cutoff = 5000;
+    res = 0;
+};
+
+node cabmix mixer::fade {
+    in0 = tone->out;
+    in1 = cone2->out_low;
+    fade = @cab;
+};
+
+# A driven stage comes out near full scale however hard it was hit, so
+# the master is divided by the gain: `Gain' adds dirt, `Level' adds
+# volume, and a quiet clean channel stays as loud as it went in.
+node out mixer::mul {
+    in0 = cabmix->out;
+    in1 = @level / (1 + @gain * 2);
+};
+
+node echol delay::echo {
+    in = out->out;
+    size = 1600 ms;
+    delay = @delay;
+    feedback = @feedback;
+    dry = 0;
+};
+
+node echor delay::echo {
+    in = out->out;
+    size = 1600 ms;
+    delay = @delay * 1.5;
+    feedback = @feedback;
+    dry = 0;
+};
+
+io ionode;