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IT filters

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minjaesong
2026-05-01 23:19:49 +09:00
parent a4adc428d0
commit fe59df18f7
7 changed files with 186 additions and 73 deletions
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133
tsvm_core/src/net/torvald/tsvm/peripheral/AudioAdapter.kt
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@@ -124,10 +124,11 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
internal val DBGPRN = false
const val SAMPLING_RATE = 32000
const val TRACKER_CHUNK = 512
const val TRACKER_C3 = 0x4000 // legacy alias (one octave below the new reference)
const val TRACKER_C4 = 0x5000 // reference C for instrument samplingRate (terranmon.txt:2000)
// Amiga period at TRACKER_C4 for a standard 8363 Hz instrument (NTSC clock 3579545 Hz).
// PT "C-2" period 428 ↔ TSVM TRACKER_C4 ↔ 8363 Hz; mod2taud uses the same convention.
const val MIDDLE_C = 0x5000 // reference C for instrument samplingRate (terranmon.txt:2000)
// Amiga period at MIDDLE_C for a standard 8363 Hz instrument (NTSC clock 3579545 Hz).
// PT "C-2" period 428 ↔ TSVM MIDDLE_C ↔ 8363 Hz; mod2taud uses the same convention.
// Trackers may use different labelling conventions (e.g. C5) for Middle C.
// For non-tracker context, Middle C shall be labelled as C4.
const val AMIGA_BASE_PERIOD = 428.0
}
@@ -1168,7 +1169,7 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
}
private fun computePlaybackRate(inst: TaudInst, noteVal: Int): Double =
inst.samplingRate.toDouble() / SAMPLING_RATE * 2.0.pow((noteVal - TRACKER_C4) / 4096.0)
inst.samplingRate.toDouble() / SAMPLING_RATE * 2.0.pow((noteVal - MIDDLE_C) / 4096.0)
// Applies one tick of Amiga-mode pitch slide. When the song is in Amiga tone mode, E/F coarse
// slide arguments are stored as raw tracker period units (the original ProTracker/ST3 byte),
@@ -1176,9 +1177,9 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
// linear mode: negative = pitch down (E effect), positive = pitch up (F effect), so a positive
// slideArg subtracts from the period (pitch rises).
private fun amigaSlide(noteVal: Int, slideArg: Int): Int {
val period = AMIGA_BASE_PERIOD * 2.0.pow(-(noteVal - TRACKER_C4).toDouble() / 4096.0)
val period = AMIGA_BASE_PERIOD * 2.0.pow(-(noteVal - MIDDLE_C).toDouble() / 4096.0)
val newPeriod = (period - slideArg).coerceAtLeast(1.0)
return (TRACKER_C4 + 4096.0 * log2(AMIGA_BASE_PERIOD / newPeriod)).roundToInt()
return (MIDDLE_C + 4096.0 * log2(AMIGA_BASE_PERIOD / newPeriod)).roundToInt()
}
private fun advanceEnvelope(voice: Voice, inst: TaudInst, tickSec: Double) {
@@ -1318,11 +1319,28 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
}
/**
* Recompute the biquad LPF coefficients for `voice` when its cutoff or
* resonance has changed since the last refresh. Cutoff 0..255 maps
* exponentially from ~110 Hz to ~14 kHz; resonance 0..255 maps linearly
* to Q ∈ [0.5, 6.0]. The filter is disabled at full-open (cutoff ≥ 0xFE
* with no resonance), avoiding the per-sample cost when transparent.
* Recompute the IT-compatible 2-pole resonant low-pass coefficients for
* `voice` when its cutoff or resonance has changed since the last refresh.
*
* Taud's filter range mirrors Impulse Tracker's at double resolution:
* Taud 0..254 maps to IT 0..127, while Taud 255 means "filter off" (the
* IT high-bit-clear sentinel). The filter is bypassed when cutoff = 255.
*
* The coefficient math and topology mirror OpenMPT/Schism Tracker (see
* reference_materials/tracker_filter/openmpt_Snd_flt.cpp and
* schism_filters.c). Notably this is NOT a biquad: the recurrence has no
* feedforward x[n-1] / x[n-2] terms.
*
* frequency = 110 Hz × 2^(itCutoff / 24 + 0.25) (IT 0..127)
* dmpfac = 10 ^ (-itResonance × 0.009375) (= 24/128/20 dB)
* r = mixingFreq / (2π × frequency)
* d = dmpfac × r + dmpfac − 1
* e = r²
* denom = 1 + d + e
* A0 = 1 / denom
* B0 = (d + 2e) / denom
* B1 = −e / denom
* y[n] = A0 × x[n] + B0 × y[n−1] + B1 × y[n−2]
*/
private fun refreshVoiceFilter(voice: Voice) {
val cut = voice.currentCutoff.coerceIn(0, 255)
@@ -1331,49 +1349,42 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
voice.filterCutoffCached = cut
voice.filterResonanceCached = res
if (cut >= 0xFE && res == 0) {
if (cut >= 255) {
voice.filterActive = false
return
}
// Exponential cutoff: 110 Hz × 2^(cut × log2(14000/110) / 255).
// log2(14000/110) ≈ 6.992, so exponent ≈ cut × 0.0274.
val cutoffHz = 110.0 * 2.0.pow(cut * 6.992 / 255.0)
val nyquist = SAMPLING_RATE * 0.5 - 1.0
val f0 = cutoffHz.coerceIn(20.0, nyquist)
val Q = 0.5 + (res / 255.0) * 5.5
val itCutoff = cut * 0.5 // 0..127
val itResonance = if (res >= 255) 0.0 else res * 0.5 // 0..127
val w0 = 2.0 * PI * f0 / SAMPLING_RATE
val cosW = cos(w0)
val sinW = sin(w0)
val alpha = sinW / (2.0 * Q)
val nyquist = SAMPLING_RATE * 0.5 - 1.0
val frequency = (110.0 * 2.0.pow(itCutoff / 24.0 + 0.25)).coerceAtMost(nyquist)
val dmpfac = 10.0.pow(-itResonance * (24.0 / 128.0) / 20.0)
val b0 = (1.0 - cosW) * 0.5
val b1 = 1.0 - cosW
val b2 = b0
val a0 = 1.0 + alpha
val a1 = -2.0 * cosW
val a2 = 1.0 - alpha
val r = SAMPLING_RATE / (2.0 * PI * frequency)
val d = dmpfac * r + dmpfac - 1.0
val e = r * r
val denom = 1.0 + d + e
voice.filterB0 = b0 / a0
voice.filterB1 = b1 / a0
voice.filterB2 = b2 / a0
voice.filterA1 = a1 / a0
voice.filterA2 = a2 / a0
voice.filterA0 = 1.0 / denom
voice.filterB0 = (d + e + e) / denom
voice.filterB1 = -e / denom
voice.filterActive = true
}
/** Apply the cached biquad LPF to one mono sample. Caller must have called
* refreshVoiceFilter at the start of the tick. */
/** Apply the cached IT-style 2-pole LPF to one mono sample. Caller must
* have called refreshVoiceFilter at the start of the tick. The history
* taps are clipped to ±2.0 to tame resonance ringing on extreme settings,
* matching OpenMPT's ClipFilter helper. */
private fun applyVoiceFilter(voice: Voice, x0: Double): Double {
if (!voice.filterActive) return x0
val y0 = voice.filterB0 * x0 +
voice.filterB1 * voice.filterX1 +
voice.filterB2 * voice.filterX2 -
voice.filterA1 * voice.filterY1 -
voice.filterA2 * voice.filterY2
voice.filterX2 = voice.filterX1; voice.filterX1 = x0
voice.filterY2 = voice.filterY1; voice.filterY1 = y0
val y1Clipped = voice.filterY1.coerceIn(-2.0, 2.0)
val y2Clipped = voice.filterY2.coerceIn(-2.0, 2.0)
val y0 = voice.filterA0 * x0 +
voice.filterB0 * y1Clipped +
voice.filterB1 * y2Clipped
voice.filterY2 = voice.filterY1
voice.filterY1 = y0
return y0
}
@@ -1499,9 +1510,9 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
voice.rowPan = (voice.channelPan ushr 2).coerceIn(0, 63)
}
// Filter cutoff/resonance defaults — adjusted per-tick by the pf envelope when in filter mode.
voice.currentCutoff = if (inst.defaultCutoff > 0) inst.defaultCutoff else 0xFF
// 255 = filter off (IT high-bit-clear); 0..254 = active range matching IT 0..127 at double resolution.
voice.currentCutoff = inst.defaultCutoff
voice.currentResonance = inst.defaultResonance
voice.filterX1 = 0.0; voice.filterX2 = 0.0
voice.filterY1 = 0.0; voice.filterY2 = 0.0
voice.filterCutoffCached = -1 // force coefficient refresh on first tick
voice.filterResonanceCached = -1
@@ -1942,7 +1953,6 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
voice.fadeoutVolume = 1.0
voice.autoVibPhase = 0
voice.autoVibTicksSinceTrigger = 0
voice.filterX1 = 0.0; voice.filterX2 = 0.0
voice.filterY1 = 0.0; voice.filterY2 = 0.0
voice.rowVolume = applyRetrigVolMod(voice.rowVolume, voice.retrigVolMod)
voice.channelVolume = voice.rowVolume
@@ -1962,9 +1972,11 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
voice.playbackRate = computePlaybackRate(inst, finalPitch)
// Filter envelope (filter mode): scale current cutoff by env value (0..1, 0.5 = unity).
// If the instrument has no initial cutoff (255 = off), the envelope drives the filter
// from the maximum active value (254) so the filter can become audible during the note.
if (voice.hasPfEnv && voice.envPfIsFilter) {
val baseCut = if (inst.defaultCutoff > 0) inst.defaultCutoff else 0xFF
voice.currentCutoff = (baseCut * (voice.envPfValue * 2.0)).toInt().coerceIn(0, 0xFF)
val baseCut = if (inst.defaultCutoff < 255) inst.defaultCutoff else 254
voice.currentCutoff = (baseCut * (voice.envPfValue * 2.0)).toInt().coerceIn(0, 254)
}
// Refresh biquad filter coefficients once per tick (only recomputes when changed).
@@ -2264,19 +2276,18 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
var autoVibPhase = 0 // 8-bit phase counter
var autoVibTicksSinceTrigger = 0 // for sweep ramp-up
// Filter / cutoff state — drives the per-voice 2-pole resonant LPF.
var currentCutoff = 0xFF // 0..255 (0xFF = open / unfiltered)
var currentResonance = 0 // 0..255
// Biquad state (updated per output sample) and cached coefficients
// (recomputed per tick when cutoff/resonance change).
// Filter / cutoff state — drives the per-voice IT-compatible 2-pole resonant LPF.
// Convention: 255 = filter off (matches IT's high-bit-clear sentinel);
// 0..254 = active range mirroring IT 0..127 at double resolution.
var currentCutoff = 0xFF
var currentResonance = 0xFF
// IT 2-pole IIR-only state (updated per output sample) and cached coefficients
// (recomputed per tick when cutoff/resonance change). Recurrence:
// y[n] = A0 × x[n] + B0 × y[n-1] + B1 × y[n-2]
var filterActive = false
var filterB0 = 1.0
var filterA0 = 1.0
var filterB0 = 0.0
var filterB1 = 0.0
var filterB2 = 0.0
var filterA1 = 0.0
var filterA2 = 0.0
var filterX1 = 0.0
var filterX2 = 0.0
var filterY1 = 0.0
var filterY2 = 0.0
// Snapshot of cutoff/resonance the cached coefficients correspond to.
@@ -2591,7 +2602,7 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
* Layout:
* 0..3 u32 sample pointer
* 4..5 u16 sample length
* 6..7 u16 sampling rate at TRACKER_C4 (0x5000)
* 6..7 u16 sampling rate at Middle C (0x5000) // NOTE: Taud treats middle C as C4, but some trackers show you C4 even if they are internally C5. Best practice: copy the value as-is.
* 8..9 u16 play start
* 10..11 u16 loop start
* 12..13 u16 loop end
@@ -2621,7 +2632,7 @@ class AudioAdapter(val vm: VM) : PeriBase(VM.PERITYPE_SOUND) {
var samplePtr: Int, // 32-bit sample bin offset
var sampleLength: Int,
var samplingRate: Int, // rate at TRACKER_C4
var samplingRate: Int, // rate at MIDDLE_C
var samplePlayStart: Int,
var sampleLoopStart: Int,
var sampleLoopEnd: Int,