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Performance: Sparse Mel Filterbank Application #10

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Performance: Sparse Mel Filterbank Application #10

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5 changes: 5 additions & 0 deletions .jules/bolt.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
# .jules/bolt.md

## 2024-05-21 - Sparse Mel Filterbank
Learning: Mel filterbank matrix is ~98% sparse; iterating over full matrix in hot loop is a major bottleneck (3.75x slow).
Action: Always check for sparsity in numerical processing loops; precompute indices if possible.
19 changes: 17 additions & 2 deletions src/mel.js
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Expand Up @@ -265,6 +265,19 @@ export class MelSpectrogram {
this._fftRe = new Float64Array(this.nFft);
this._fftIm = new Float64Array(this.nFft);
this._powerBuf = new Float32Array(this.nFreqBins);

// Precompute sparse filterbank indices
this._fbStart = new Int32Array(this.nMels);
this._fbEnd = new Int32Array(this.nMels);
for (let m = 0; m < this.nMels; m++) {
const fbOffset = m * this.nFreqBins;
let start = 0;
while (start < this.nFreqBins && this.melFilterbank[fbOffset + start] === 0) start++;
this._fbStart[m] = start;
let end = this.nFreqBins;
while (end > start && this.melFilterbank[fbOffset + end - 1] === 0) end--;
this._fbEnd[m] = end;
}
}

/**
Expand Down Expand Up @@ -315,7 +328,7 @@ export class MelSpectrogram {
// 4. STFT + Power + Mel + Log
const rawMel = new Float32Array(this.nMels * nFrames);
const { _fftRe: fftRe, _fftIm: fftIm, _powerBuf: powerBuf } = this;
const { hannWindow: window, melFilterbank: fb, nMels, twiddles: tw, nFft, nFreqBins, hopLength, logZeroGuard } = this;
const { hannWindow: window, melFilterbank: fb, nMels, twiddles: tw, nFft, nFreqBins, hopLength, logZeroGuard, _fbStart, _fbEnd } = this;

for (let t = 0; t < nFrames; t++) {
const offset = t * hopLength;
Expand All @@ -325,7 +338,9 @@ export class MelSpectrogram {
for (let m = 0; m < nMels; m++) {
let melVal = 0;
const fbOff = m * nFreqBins;
for (let k = 0; k < nFreqBins; k++) melVal += powerBuf[k] * fb[fbOff + k];
const start = _fbStart[m];
const end = _fbEnd[m];
for (let k = start; k < end; k++) melVal += powerBuf[k] * fb[fbOff + k];
rawMel[m * nFrames + t] = Math.log(melVal + logZeroGuard);
}
}
Expand Down