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

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>

app/build.gradle.kts

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

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

@@ -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 ----
+        // ---- control-rate update (every CTRL_INTERVAL samples): the expensive
-        var mix = 0f
+        // Math.pow for cutoff + pitch runs ~1/16 as often. LFO/envelopes move slowly
-        for (i in 0 until SynthPatch.OSC_COUNT) {
+        // enough that stepping the coefficients this often is inaudible. ----
-            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) ----
         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 ----