alexa-voice-service/lib/utils/downsampleBuffer.js

Alexa Voice Service wrapper for the browser.

'use strict';

/**
 * @credit http://stackoverflow.com/a/26245260
 */
function downsampleBuffer(buffer, inputSampleRate, outputSampleRate) {
  if (inputSampleRate === outputSampleRate) {
    return buffer;
  }

  if (inputSampleRate < outputSampleRate) {
    throw new Error('Output sample rate must be less than input sample rate.');
  }

  const sampleRateRatio = inputSampleRate / outputSampleRate;
  const newLength = Math.round(buffer.length / sampleRateRatio);
  let result = new Float32Array(newLength);
  let offsetResult = 0;
  let offsetBuffer = 0;

  while (offsetResult < result.length) {
    let nextOffsetBuffer = Math.round((offsetResult + 1) * sampleRateRatio);
    let accum = 0;
    let count = 0;

    for (var i = offsetBuffer; i < nextOffsetBuffer && i < buffer.length; i++) {
      accum += buffer[i];
      count++;
    }

    result[offsetResult] = accum / count;
    offsetResult++;
    offsetBuffer = nextOffsetBuffer;
  }

  return result;
}

module.exports = downsampleBuffer;