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// Fast Fourier Transform (adapted from spectral_editor/dct.js)
// Radix-2 Cooley-Tukey algorithm
// Bit-reversal permutation (in-place)
function bitReversePermute(real, imag, N) {
let temp_bits = N;
let num_bits = 0;
while (temp_bits > 1) {
temp_bits >>= 1;
num_bits++;
}
for (let i = 0; i < N; ++i) {
let j = 0;
let temp = i;
for (let b = 0; b < num_bits; ++b) {
j = (j << 1) | (temp & 1);
temp >>= 1;
}
if (j > i) {
const tmp_real = real[i];
const tmp_imag = imag[i];
real[i] = real[j];
imag[i] = imag[j];
real[j] = tmp_real;
imag[j] = tmp_imag;
}
}
}
// In-place radix-2 FFT
// direction: +1 for forward, -1 for inverse
function fftRadix2(real, imag, N, direction) {
const PI = Math.PI;
for (let stage_size = 2; stage_size <= N; stage_size *= 2) {
const half_stage = stage_size / 2;
const angle = direction * 2.0 * PI / stage_size;
let wr = 1.0;
let wi = 0.0;
const wr_delta = Math.cos(angle);
const wi_delta = Math.sin(angle);
for (let k = 0; k < half_stage; ++k) {
for (let group_start = k; group_start < N; group_start += stage_size) {
const i = group_start;
const j = group_start + half_stage;
const temp_real = real[j] * wr - imag[j] * wi;
const temp_imag = real[j] * wi + imag[j] * wr;
real[j] = real[i] - temp_real;
imag[j] = imag[i] - temp_imag;
real[i] = real[i] + temp_real;
imag[i] = imag[i] + temp_imag;
}
const wr_old = wr;
wr = wr_old * wr_delta - wi * wi_delta;
wi = wr_old * wi_delta + wi * wr_delta;
}
}
}
// Forward FFT: Time domain → Frequency domain
function fftForward(real, imag, N) {
bitReversePermute(real, imag, N);
fftRadix2(real, imag, N, +1);
}
// Real FFT wrapper for MQ extraction
// Input: Float32Array (time-domain signal)
// Output: Float32Array (interleaved [re0, im0, re1, im1, ...])
function realFFT(signal) {
const N = signal.length;
// Must be power of 2
if ((N & (N - 1)) !== 0) {
throw new Error('FFT size must be power of 2');
}
const real = new Float32Array(N);
const imag = new Float32Array(N);
// Copy input to real part
for (let i = 0; i < N; ++i) {
real[i] = signal[i];
}
// Compute FFT
fftForward(real, imag, N);
// Interleave output
const spectrum = new Float32Array(N * 2);
for (let i = 0; i < N; ++i) {
spectrum[i * 2] = real[i];
spectrum[i * 2 + 1] = imag[i];
}
return spectrum;
}
// STFT Cache - Pre-computes and caches windowed FFT frames
class STFTCache {
constructor(signal, sampleRate, fftSize, hopSize) {
this.signal = signal;
this.sampleRate = sampleRate;
this.fftSize = fftSize;
this.hopSize = hopSize;
this.frames = []; // Array of {time, offset, squaredAmplitude}
this.compute();
}
compute() {
this.frames = [];
this.maxDB = -Infinity;
const numFrames = Math.floor((this.signal.length - this.fftSize) / this.hopSize);
for (let frameIdx = 0; frameIdx < numFrames; ++frameIdx) {
const offset = frameIdx * this.hopSize;
const time = offset / this.sampleRate;
// Extract frame
const frame = this.signal.slice(offset, offset + this.fftSize);
// Apply Hann window
const windowed = new Float32Array(this.fftSize);
for (let i = 0; i < this.fftSize; ++i) {
const w = 0.5 - 0.5 * Math.cos(2 * Math.PI * i / this.fftSize);
windowed[i] = frame[i] * w;
}
// Compute FFT, store only squared amplitudes (re*re + im*im, no sqrt)
const fftOut = realFFT(windowed);
const squaredAmplitude = new Float32Array(this.fftSize / 2);
for (let i = 0; i < this.fftSize / 2; ++i) {
const re = fftOut[i * 2];
const im = fftOut[i * 2 + 1];
squaredAmplitude[i] = re * re + im * im;
const db = 10 * Math.log10(Math.max(squaredAmplitude[i], 1e-20));
if (db > this.maxDB) this.maxDB = db;
}
this.frames.push({time, offset, squaredAmplitude});
}
if (!isFinite(this.maxDB)) this.maxDB = 0;
}
setHopSize(hopSize) {
if (hopSize === this.hopSize) return;
this.hopSize = hopSize;
this.compute();
}
setFFTSize(fftSize) {
if (fftSize === this.fftSize) return;
this.fftSize = fftSize;
this.compute();
}
getNumFrames() {
return this.frames.length;
}
getFrameAtIndex(frameIdx) {
if (frameIdx < 0 || frameIdx >= this.frames.length) return null;
return this.frames[frameIdx];
}
getFrameAtTime(t) {
if (this.frames.length === 0) return null;
// Find closest frame
const frameIdx = Math.floor(t * this.sampleRate / this.hopSize);
return this.getFrameAtIndex(frameIdx);
}
getSquaredAmplitude(t) {
const frame = this.getFrameAtTime(t);
return frame ? frame.squaredAmplitude : null;
}
// Get magnitude in dB at specific time and frequency
getMagnitudeDB(t, freq) {
const sq = this.getSquaredAmplitude(t);
if (!sq) return -80;
const bin = Math.round(freq * this.fftSize / this.sampleRate);
if (bin < 0 || bin >= this.fftSize / 2) return -80;
return 10 * Math.log10(Math.max(sq[bin], 1e-20));
}
}
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