src/utils/oversample.js

/**
 * Oversample (part of) an array of samples by a fixed factor, using a
 * lengthy windowed sinc filter. Very crude.
*/
export default class Oversampler {

  oversample(inbuf, instart, inlen) {

    const before = performance.now();
    
    const outlen = inlen * this.factor;
    const flen = this.filter.length; // known to be odd
    const tail = Math.round((flen-1)/2); // so filter is tail + 1.0 + tail
    const outbuf = new Float32Array(outlen);

    let i0 = instart - Math.floor(tail / this.factor);
    if (i0 < 0) i0 = 0;
    let i1 = instart + inlen + Math.floor(tail / this.factor);
    if (i1 > inbuf.length) i1 = inbuf.length;

    for (let i = i0; i < i1; ++i) {
      const x = inbuf[i];
      const outoff = (i - instart) * this.factor - tail;
      for (let j = 0; j < flen; ++j) {
        const outix = outoff + j;
        if (outix < 0 || outix >= outlen) continue;
        outbuf[outix] += x * this.filter[j];
      }
    }

    console.log("oversample: " + (i1-i0) + " samples in, " + outlen + " out in " +
                (performance.now() - before) + " ms");
    
    return outbuf;
  }
  
  constructor(factor) {

    if (factor !== 8) {
      throw "Only oversampling factor currently supported is 8";
    }

    this.factor = factor;
    this.filter = 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  }
}