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Back when mixing was each effect's own business, they all did a plain linear blend except for the vibrato, which did an equal-power one with a sin/cos pair. When the mix moved into the effect chain that distinction got flattened, and the vibrato quietly lost it. It shouldn't have. The two laws differ in what they assume about whether the wet and dry are correlated, and each is wrong by 3dB in the other's case. When the wet is a filtered version of the dry - which is most of what's here - the amplitudes add, half of each gives you back the original, and an equal-power mix puts a 3dB hump right in the middle of the sweep where everyone leaves the knob. When they're uncorrelated the powers add instead, and it's the linear mix that leaves a 3dB hole there. So it isn't one law for everything, it is a property of the effect, and the effect header is where it should say so. Add a 'MIX:' annotation alongside PRIORITY and DEFAULT_MIX, defaulting to LINEAR, and mark the four whose wet really is decorrelated from the dry: vibrato, echo, reverb and the pitch shifter. Phaser and flanger stay linear on purpose, even though it's tempting. An allpass or comb has the same magnitude spectrum as the input and only differs in phase, and the cancellation is the whole point - equal power would fill the notches back in by 3dB and make them sound weak. Nothing uses the field yet; do_effect_step() still does what it did. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
33 lines
1.1 KiB
C
33 lines
1.1 KiB
C
// NAME: Vibrato [VIB]
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// PRIORITY: 70
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// MIX: POWER // a modulated delay - the wet is the dry displaced in time
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// POT: "Rate" FREQUENCY(0.1 8.0) = 2.0 Hz
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// POT: "Depth" LINEAR(0.0 5.0) = 0.875 ms
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// Vibrato: LFO-modulated delay line for a classic Doppler shift.
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// Blending dry signal with the wet signal produces a rich chorus-like effect.
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// At 100% wet only the pitch-shifted signal is audible; fun for rotary speaker emulation.
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#define VIBRATO_CENTER_SAMPLES (6.0f * SAMPLES_PER_MSEC)
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static struct {
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struct lfo_state lfo;
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float depth; // delay amplitude in samples
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unsigned int idx;
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float samples[1024]; // ~21ms at 48kHz; max read = center(6ms) + depth(5ms) = 528 samples
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} vibrato;
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static void vibrato_init(unsigned char pot[10])
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{
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set_lfo_freq(&vibrato.lfo, vibrato_pot0(pot[0]));
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vibrato.depth = vibrato_pot1(pot[1]) * SAMPLES_PER_MSEC;
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}
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static float vibrato_step(float in)
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{
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float d = VIBRATO_CENTER_SAMPLES + vibrato.depth * lfo_step(&vibrato.lfo, lfo_sinewave);
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sample_array_write(in, &vibrato.idx, vibrato.samples);
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float wet = sample_array_read(d, &vibrato.idx, vibrato.samples);
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return wet;
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}
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