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Move synth pitch modulation to control rate — fix playback hiccups (v0.4.4)

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On-device profiling (worst case: 4 channels x 3 oscillators + cutoff & pitch
modulation, dense pattern at 200 BPM) showed the render callback averaging 2.8ms
against a 2.0ms budget with continuous xruns — the hiccups. The cost was the
per-sample Math.pow used for pitch modulation across 3 oscillators x many voices.

Recompute modulated oscillator pitch at control rate (every 16 samples, like the
filter cutoff) instead of per sample; LFO/envelopes move slowly enough that this
is inaudible. Render dropped to ~1.27ms avg with zero new xruns. No crashes under
load (the ConcurrentHashMap + stable SVF filter fixes from 0.4.3 hold).

Patch bump to 0.4.4 (versionCode 11).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Reactorcoremeltdown
2026-07-18 01:48:39 +02:00
parent 78ebd88548
commit 00f52c3434
2 changed files with 30 additions and 23 deletions
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4
app/build.gradle.kts
View File

@@ -22,8 +22,8 @@ android {
// (android.media.midi) and AAudio low-latency audio we rely on.
minSdk = 26
targetSdk = 34
versionCode = 10
versionName = "0.4.3"
versionCode = 11
versionName = "0.4.4"
// We provide our own instrumentation runner if/when tests are added.
testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

49
app/src/main/java/space/rcmd/android/sizzle/audio/SynthVoice.kt
View File

@@ -34,6 +34,7 @@ class SynthVoice(private val sampleRate: Int) {
private val phase = DoubleArray(SynthPatch.OSC_COUNT)
private val staticSemis = FloatArray(SynthPatch.OSC_COUNT) // oct+semi+fine, no modulation
private val staticInc = DoubleArray(SynthPatch.OSC_COUNT) // phase increment when un-modulated
private val modInc = DoubleArray(SynthPatch.OSC_COUNT) // modulated increment, refreshed at control rate
private var pitchModActive = false // any mod slot routes to pitch?
// Topology-preserving (Cytomic) 2-pole state-variable low-pass. Unconditionally
@@ -97,6 +98,7 @@ class SynthVoice(private val sampleRate: Int) {
val semis = patch.oscOct[i] * 12f + patch.oscSemi[i] + patch.oscFine[i] / 100f
staticSemis[i] = semis
staticInc[i] = (baseFreq * fastPow2(semis / 12.0) / sampleRate).coerceIn(0.0, 0.49)
modInc[i] = staticInc[i] // valid until the first control-rate refresh
}
pitchModActive = (0 until SynthPatch.MOD_SLOTS).any { patch.modSrc[it] != 0 && patch.modDst[it] in 0..2 }
}
@@ -136,27 +138,9 @@ class SynthVoice(private val sampleRate: Int) {
}
}
// ---- oscillators + noise ----
var mix = 0f
for (i in 0 until SynthPatch.OSC_COUNT) {
val level = patch.oscLevel[i]
if (level <= 0.0001f) continue
val inc = if (pitchModActive) {
var semis = staticSemis[i] + mPitch * 12f
if (i == 1) semis += mOsc2 * 12f
if (i == 2) semis += mOsc3 * 12f
(baseFreq * fastPow2(semis / 12.0) / sampleRate).coerceIn(0.0, 0.49)
} else {
staticInc[i]
}
var ph = phase[i] + inc
if (ph >= 1.0) ph -= 1.0
phase[i] = ph
mix += waveform(patch.oscWave[i], ph, inc, mPwm) * level
}
if (patch.noiseLevel > 0.0001f) mix += nextRand() * patch.noiseLevel
// ---- filter (coefficients refreshed at control rate) ----
// ---- control-rate update (every CTRL_INTERVAL samples): the expensive
// Math.pow for cutoff + pitch runs ~1/16 as often. LFO/envelopes move slowly
// enough that stepping the coefficients this often is inaudible. ----
if (ctrl <= 0) {
ctrl = CTRL_INTERVAL
val exp = (patch.cutoff + patch.filterEnvAmt * filtE + patch.keytrack * keyNorm + mCut)
@@ -165,8 +149,31 @@ class SynthVoice(private val sampleRate: Int) {
val g = tan(Math.PI * hz / sampleRate).toFloat()
val k = (2f - 1.96f * patch.resonance).coerceIn(0.05f, 2f) // lower k = more resonance
ca1 = 1f / (1f + g * (g + k)); ca2 = g * ca1; ca3 = g * ca2
if (pitchModActive) {
for (i in 0 until SynthPatch.OSC_COUNT) {
var semis = staticSemis[i] + mPitch * 12f
if (i == 1) semis += mOsc2 * 12f
if (i == 2) semis += mOsc3 * 12f
modInc[i] = (baseFreq * fastPow2(semis / 12.0) / sampleRate).coerceIn(0.0, 0.49)
}
}
}
ctrl--
// ---- oscillators + noise ----
var mix = 0f
for (i in 0 until SynthPatch.OSC_COUNT) {
val level = patch.oscLevel[i]
if (level <= 0.0001f) continue
val inc = if (pitchModActive) modInc[i] else staticInc[i]
var ph = phase[i] + inc
if (ph >= 1.0) ph -= 1.0
phase[i] = ph
mix += waveform(patch.oscWave[i], ph, inc, mPwm) * level
}
if (patch.noiseLevel > 0.0001f) mix += nextRand() * patch.noiseLevel
// ---- filter (coefficients computed at control rate, above) ----
val filtered = svf(mix)
// ---- amplifier ----