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refactor(filter): cache coefficients and extract cutoff calculation

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Add updateCoefficients() that short-circuits when cutoff, resonance,
and type are unchanged. Cache ladderG and formantCoefficients so
processSample() avoids redundant exp()/tan() calls per sample.

Extract static getCutoffHz() from Filter::process() so FilterBank
can compute the keytracked cutoff once per parallel branch instead
of per sample. Simplify formant() and screamer() signatures to use
cached state.
This commit is contained in:
biggy
2026-09-09 14:32:49 +02:00
parent 3b2ecc50f2
commit dc6fd88ed1
3 changed files with 90 additions and 35 deletions
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Source/Filter.cpp
+76 -29
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@@ -5,7 +5,7 @@ namespace serum
namespace
{
constexpr double kPi = 3.14159265358979323846;
constexpr double kPi = juce::MathConstants<double>::pi;
inline float clampF (float v, float lo, float hi) noexcept
{
@@ -30,6 +30,9 @@ void Filter::prepare (double sampleRate, int maxBlockSize)
void Filter::reset()
{
lastCutoffHz = lastRes = -1.0f;
lastType = -1;
ladderG = 0.0;
ic1eq = ic2eq = 0.0;
lastG = lastK = 0.0;
a1 = a2 = a3 = 0.0;
@@ -40,6 +43,57 @@ void Filter::reset()
combDamp = 0.0;
}
void Filter::updateCoefficients (float cutoffHz, float res, int type) noexcept
{
if (cutoffHz == lastCutoffHz && res == lastRes && type == lastType)
return;
lastCutoffHz = cutoffHz;
lastRes = res;
lastType = type;
// Ladder stages are cascaded one-poles, which need an exponential coefficient
// (always in (0,1]) for unconditional stability. The TPT SVF (formant/screamer)
// uses cached tan()-based coefficients instead.
switch ((FilterModel) type)
{
case FilterModel::LadderLP:
case FilterModel::LadderHP:
case FilterModel::LadderBP:
case FilterModel::Diode:
{
const double fc = clampD (cutoffHz, 20.0, sr * 0.45);
ladderG = 1.0 - std::exp (-2.0 * kPi * fc / sr);
break;
}
case FilterModel::Formant:
{
// morph (0..1) sweeps the three bandpass centres to produce vowel-like spectra.
const double m = clampD (maps::hzToCutoff (cutoffHz), 0.0, 1.0);
const double base[3] = { 400.0, 1200.0, 2600.0 };
const double k = clampD (2.0 * (1.0 - (double) res), 0.05, 2.0);
for (int i = 0; i < 3; ++i)
{
const double fc = base[i] * (0.7 + 1.6 * m) * (i == 2 ? 0.9 : 1.0);
const double g = std::tan (kPi * clampD (fc, 30.0, sr * 0.45) / sr);
auto& c = formantCoefficients[(size_t) i];
c[0] = 1.0 / (1.0 + g * (g + k));
c[1] = g * c[0];
c[2] = g * c[1];
}
break;
}
case FilterModel::Screamer:
{
const double g = std::tan (kPi * clampD (cutoffHz, 30.0, sr * 0.45) / sr);
const double k = clampD (2.0 * (1.0 - (double) res), 0.05, 2.0);
updateSvf (g, k);
break;
}
default:
break;
}
}
void Filter::updateSvf (double g, double k) noexcept
{
if (g == lastG && k == lastK)
@@ -143,22 +197,16 @@ double Filter::comb (double in, double freqHz, double res, double drive) noexcep
return (double) y;
}
double Filter::formant (double in, double morph, double res) noexcept
double Filter::formant (double in) noexcept
{
// morph (0..1) sweeps the three bandpass centres to produce vowel-like spectra.
const double m = clampD (morph, 0.0, 1.0);
const double base[3] = { 400.0, 1200.0, 2600.0 };
const double k = clampD (2.0 * (1.0 - res), 0.05, 2.0);
double out = 0.0;
const double gains[3] = { 1.0, 0.8, 0.5 };
for (int i = 0; i < 3; ++i)
{
const double fc = base[i] * (0.7 + 1.6 * m) * (i == 2 ? 0.9 : 1.0);
const double g = std::tan (kPi * clampD (fc, 30.0, sr * 0.45) / sr);
const double a1 = 1.0 / (1.0 + g * (g + k));
const double a2 = g * a1;
const double a3 = g * a2;
const auto& c = formantCoefficients[(size_t) i];
const double a1 = c[0];
const double a2 = c[1];
const double a3 = c[2];
const double v3 = in - formantState[(size_t) i][1];
const double v1 = a1 * formantState[(size_t) i][0] + a2 * v3;
const double v2 = formantState[(size_t) i][1] + a2 * formantState[(size_t) i][0] + a3 * v3;
@@ -169,11 +217,9 @@ double Filter::formant (double in, double morph, double res) noexcept
return clampD (out * 0.5, -8.0, 8.0);
}
double Filter::screamer (double in, double cutoffHz, double res, double drive) noexcept
double Filter::screamer (double in, double drive) noexcept
{
const double g = std::tan (kPi * clampD (cutoffHz, 30.0, sr * 0.45) / sr);
const double k = clampD (2.0 * (1.0 - res), 0.05, 2.0);
const double band = svfBand (in, g, k);
const double band = svfBand (in, lastG, lastK);
const double driven = std::tanh (band * (1.0 + drive * 12.0));
return driven * (1.0 - drive * 0.4);
}
@@ -183,11 +229,8 @@ float Filter::processSample (float in, float cutoffHz, float res, float drive, i
res = clampF (res, 0.0f, 0.98f);
drive = clampF (drive, 0.0f, 1.0f);
// Ladder stages are cascaded one-poles, which need an exponential coefficient
// (always in (0,1]) for unconditional stability. The TPT SVF (formant/screamer)
// computes its own tan()-based g internally.
const double fc = clampD (cutoffHz, 20.0, sr * 0.45);
const double g = 1.0 - std::exp (-2.0 * kPi * fc / sr);
updateCoefficients (cutoffHz, res, type);
const double g = ladderG;
double out = (double) in;
switch ((FilterModel) type)
@@ -221,10 +264,10 @@ float Filter::processSample (float in, float cutoffHz, float res, float drive, i
out = comb (in, cutoffHz, res, drive);
break;
case FilterModel::Formant:
out = formant (in, maps::hzToCutoff (cutoffHz), res);
out = formant (in);
break;
case FilterModel::Screamer:
out = screamer (in, cutoffHz, res, drive);
out = screamer (in, drive);
break;
default:
break;
@@ -233,18 +276,22 @@ float Filter::processSample (float in, float cutoffHz, float res, float drive, i
return (float) clampD (out, -8.0, 8.0);
}
float Filter::getCutoffHz (float cutoffNorm, float keytrack, float noteHz) noexcept
{
// Keytrack shifts the cutoff with note pitch.
const float noteNumber = (noteHz > 0.0f) ? (69.0f + 12.0f * std::log2f (noteHz / 440.0f)) : 60.0f;
const float baseHz = maps::cutoffToHz (cutoffNorm);
const float keyFactor = std::pow (2.0f, keytrack * (noteNumber - 60.0f) / 12.0f);
return clampF (baseHz * keyFactor, 20.0f, 18000.0f);
}
void Filter::process (float* samples, int numSamples, float cutoffNorm, float res,
float drive, float keytrack, float noteHz, int type, int slope) noexcept
{
if (samples == nullptr || numSamples <= 0)
return;
// Keytrack shifts the cutoff with note pitch.
const float noteNumber = (noteHz > 0.0f) ? (69.0f + 12.0f * std::log2f (noteHz / 440.0f)) : 60.0f;
const float baseHz = maps::cutoffToHz (cutoffNorm);
const float keyFactor = std::pow (2.0f, keytrack * (noteNumber - 60.0f) / 12.0f);
const float cutoffHz = clampF (baseHz * keyFactor, 20.0f, 18000.0f);
const float cutoffHz = getCutoffHz (cutoffNorm, keytrack, noteHz);
for (int i = 0; i < numSamples; ++i)
samples[i] = processSample (samples[i], cutoffHz, res, drive, type, slope);
}