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This is the actual effect format update mentioned in the previous
commit: express the boiler plate details in the comment at the top of
the effect so that it can be auto-generated.
Part of that is that now the default values can be much more sanely
expressed: instead of setting the defaults as the particular integer
values the pot takes, express them in the user-visible units.
So the default noise gate level value can now be described as
// POT: "Level" LINEAR(-100.0 -40.0) = -70.0 dB
instead of the inscrutable
EFFECT_POT("Level", desc_dB, gate_pot0_level, 0), // -70dB
where the default value of 0 ended up indeed being -70dB, but we had to
have a comment to that effect because it was so non-obvious (it came
from the fact that gate_pot0_level() did a linear interpolation between
-100 and -40, and -70 is the middle value).
So now the pot values make a lot more sense and defaults are easier to
see and set.
This also means that instead of having the pot values be -60 to +60 (in
order to make the default "noon" be zero and needing no initializer),
since we auto-generate the defaults we can make the actual internal
values be the more straightforward 0..120 range - not only matching what
we did for MIDI, but also simplifying a lot of the helper math cases.
As a result, the pot arrays are now done as 'unsigned char'.
While doing all this, I also noticed that the ->describe() function is
just entirely historical: it used to depend on the pot value for when
the enumerations were done as a separate hack. But these days
enumeration pots are an integral part of the pot description, and the
->describe() functions were all just returning the unit string.
So get rid of the function pointer indirection, and just make it have a
simple and straightforward 'const char *unit' string.
This all makes the 'gen_effect' python script a bit bigger, but that is
more than made up for by the boiler plate removal in the effects
themselves, so not only does it make things clearer, this all removes
more lines than it adds.
And equally importantly, it means that now the Web UI actually can
convert the raw pot values into sensible units, because it too can take
the pot descriptions into account. Some other minor UI updates while at
it.
All the python and JS scripting itself was obviously all done by AI.
Some day I might care enough to learn to actually write python, but for
now I'm perfectly happy just reading the end result.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
119 lines
4.3 KiB
C
119 lines
4.3 KiB
C
// NAME: Preamp [PRE]
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// PRIORITY: 20
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// POT: "Level" LINEAR(-20.0 20.0) = 0.0 dB
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// POT: "Sat" LINEAR(0.5 4.0) = 1.2 x
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// POT: "Voice" ENUM(Tube JFET) = Tube
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// Two-stage cascaded triode (Tube) or famous single-stage JFET preamp model.
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// SATURATION drives both waveshapers; VOICE selects topology; LEVEL trims output.
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// One-pole LPF: y[n] = x[n] + a*(y[n-1] - x[n]), 6 dB/oct.
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// Pole: a = pow2(-2*pi*fc / (fs*ln2)); 9.06472 = 2*pi/ln2.
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struct preamp_onepole {
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float a, z;
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};
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static inline void preamp_onepole_set(struct preamp_onepole *f, float fc)
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{
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f->a = pow2(-9.06472028f * fc / SAMPLES_PER_SEC);
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}
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static inline float preamp_onepole_step(struct preamp_onepole *f, float x)
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{
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f->z = x + f->a * (f->z - x);
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return f->z;
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}
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// Tube model: two-stage 12AX7 triode, class-A biased, interstage DC block.
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// Fixed operating-point asymmetry for 60's vintage character (~0.15 grid bias).
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#define PREAMP_TUBE_ASYMM 0.15f
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#define PREAMP_TUBE_DC_R 0.995f // ~38 Hz HPF pole at 48 kHz
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// tanhf(0.15f) and tanhf(0.045f) precomputed; saves two tanhf calls per sample.
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#define PREAMP_TUBE_TANHB 0.14888f
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#define PREAMP_TUBE_TANHA2 0.04496f
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// Small-signal gain at default drive (1.2): stage1 * stage2 ≈ 0.885; norm targets ~1.05.
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#define PREAMP_TUBE_NORM 1.186f
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// JFET model: from schematic of a famous common-emitter 2N5457-like stage.
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// Emitter-bypass shelf, asymmetric Class-A waveshaper, Miller-cap rolloff.
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#define PREAMP_JFET_BIAS 0.12f // class-A operating offset
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#define PREAMP_JFET_MAKEUP 1.6f // ~+4 dB output boost
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#define PREAMP_JFET_SHELF_D 0.50f // shelf depth: ~6 dB LF cut below corner
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#define PREAMP_JFET_SHELF 120.0f // Hz, emitter-bypass corner
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#define PREAMP_JFET_MILLER 9000.0f // Hz, C_cb * R_c HF rolloff
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#define PREAMP_JFET_DC_R 0.997f // ~71 Hz HPF pole at 48 kHz
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// tanhf(0.12f) precomputed.
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#define PREAMP_JFET_TANHB 0.11943f
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// Small-signal gain at default drive (1.2): waveshaper * MAKEUP ≈ 1.89; norm targets ~1.05.
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#define PREAMP_JFET_NORM 0.555f
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static struct {
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float level, drive;
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int voice;
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struct { float dc_x, dc_y; } tube;
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struct {
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struct preamp_onepole shelf, miller;
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float dc_x, dc_y;
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} jfet;
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} preamp = {
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.level = 1.0f,
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.drive = 1.2f,
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};
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static inline void preamp_init(unsigned char pot[10])
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{
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preamp.level = db_to_level(preamp_pot0(pot[0]));
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preamp.drive = preamp_pot1(pot[1]);
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preamp.voice = pot[2];
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// Shelf and miller corners are fixed; computed here because pow2() needs runtime tables.
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preamp_onepole_set(&preamp.jfet.shelf, PREAMP_JFET_SHELF);
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preamp_onepole_set(&preamp.jfet.miller, PREAMP_JFET_MILLER);
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}
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static inline float preamp_tube_step(float x, float drive)
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{
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// Stage 1: asymmetric bias pushes the operating point off-centre on
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// the tanh curve, generating even-order harmonics (2nd-harmonic mechanism).
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float s1 = tanhf(x * (drive * 0.7f) + PREAMP_TUBE_ASYMM) - PREAMP_TUBE_TANHB;
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// Interstage DC block (~38 Hz HPF) strips stage-1 DC before stage 2
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// so the bias offsets don't accumulate across the cascade.
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float dc = s1 - preamp.tube.dc_x + PREAMP_TUBE_DC_R * preamp.tube.dc_y;
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preamp.tube.dc_x = s1;
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preamp.tube.dc_y = dc;
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// Stage 2: re-saturates the cleaned signal at reduced asymmetry.
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return tanhf(dc * (drive * 0.9f) + PREAMP_TUBE_ASYMM * 0.3f) - PREAMP_TUBE_TANHA2;
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}
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static inline float preamp_jfet_step(float x, float drive)
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{
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// Emitter-bypass low-shelf: subtracts a fraction of the LPF output to
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// attenuate below 120 Hz by ~6 dB, matching the bypass capacitor rolloff.
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float lf = preamp_onepole_step(&preamp.jfet.shelf, x);
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float in = x - PREAMP_JFET_SHELF_D * lf;
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// Asymmetric Class-A waveshaper; BIAS offsets the operating point so
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// positive and negative swings saturate at different rates.
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// Subtracting tanhf(BIAS) removes the static DC component.
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float shaped = tanhf(in * drive + PREAMP_JFET_BIAS) - PREAMP_JFET_TANHB;
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// DC block for the drive-dependent residual offset.
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float dc = shaped - preamp.jfet.dc_x + PREAMP_JFET_DC_R * preamp.jfet.dc_y;
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preamp.jfet.dc_x = shaped;
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preamp.jfet.dc_y = dc;
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// Miller-cap HF rolloff then characteristic output boost.
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return preamp_onepole_step(&preamp.jfet.miller, dc) * PREAMP_JFET_MAKEUP;
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}
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static inline float preamp_step(float in)
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{
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float out = (preamp.voice == 0)
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? preamp_tube_step(in, preamp.drive) * PREAMP_TUBE_NORM
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: preamp_jfet_step(in, preamp.drive) * PREAMP_JFET_NORM;
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return out * preamp.level;
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}
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