Manipulate audio buffer for add filters and effects in Javascript

Question

I'm working on an audio editor based on web, in Javascript. It's not a live audio player (like a virtual dj console) where filters and effects are applied in real time. It's a web app that allows user to cut, copy, paste track segments. The track is loaded from a file. I'm using Web Audio Api, React with Wavesurfer.js library (link) and its plugins. Thank's by Regions plugin (link) and some guides (link) I can allow user to cut, copy, paste and change volume track segments.

Until now I manipulated the array buffer (Float32Array Object) and the AudioBuffer object of the track.

For example it's easy to adjust volume to a track segment/region:

function adjustGainToRegion(region, gain) {
  const start = region.start;
  const end = region.end;

  const originalAudioBuffer = waveSurfer.getDecodedData();

  var newAudioBuffer = audioContext.createBuffer(
    originalAudioBuffer.numberOfChannels,
    originalAudioBuffer.length,
    originalAudioBuffer.sampleRate
  );

  for (var channel = 0; channel < originalAudioBuffer.numberOfChannels; channel++) {
    var originalChanData = originalAudioBuffer.getChannelData(channel);
    var newChanData = newAudioBuffer.getChannelData(channel);

    var midData = originalChanData.subarray(start * originalAudioBuffer.sampleRate, end * originalAudioBuffer.sampleRate);

    newChanData.set(originalChanData);

    for(let i=0;i<midData.length;i++){
      newChanData[i] = newChanData[i] * gain;
    }
  }

  return newAudioBuffer;
}

But, how to manipulate the AudioBuffer object or array buffer for apply filters (highpass, lowpass, bandpass, notch) and effects (reverb, delay, echo, ect...)? Is it possible? Or is a workaround needed?

EDIT

The above function isn't correct, because it always updates the newAudioBuffer from track start (0s) and not from region start.

function adjustGainToRegion(region, gain) {
  const start = region.start;
  const end = region.end;
  const regionDuration = Math.round(end - start);

  const originalAudioBuffer = waveSurfer.getDecodedData();

  var newAudioBuffer = audioContext.createBuffer(
      originalAudioBuffer.numberOfChannels,
      originalAudioBuffer.length,
      originalAudioBuffer.sampleRate
    );

  for (var channel = 0; channel < originalAudioBuffer.numberOfChannels; channel++) {
    var originalChanData = originalAudioBuffer.getChannelData(channel);
    var newChanData = newAudioBuffer.getChannelData(channel);

    var beforeData = originalChanData.subarray(0, start * originalAudioBuffer.sampleRate);
    var midData = originalChanData.subarray(start * originalAudioBuffer.sampleRate, end * originalAudioBuffer.sampleRate);
    var afterData = originalChanData.subarray(Math.floor(end * originalAudioBuffer.sampleRate), (originalAudioBuffer.length * originalAudioBuffer.sampleRate));

    for(let i=0;i<midData.length;i++){
      midData[i] = midData[i] * gain;
    }

    newChanData.set(beforeData);
    newChanData.set(midData, (start * newAudioBuffer.sampleRate));
    newChanData.set(afterData, (start + regionDuration) * newAudioBuffer.sampleRate);
  }

  return newAudioBuffer;
}

Answer 1

This can be done with an OfflineAudioContext. You could for example rewrite your code to use an AudioBufferSourceNode to slice the original audio data and a GainNode to manipulate the gain.

function adjustGainToRegion(region, gain) {
    const originalAudioBuffer = waveSurfer.getDecodedData();
    const offlineAudioContext = new OfflineAudioContext({
        length: Math.round((region.end - region.start) * originalAudioBuffer.sampleRate),
        numberOfChannels: originalAudioBuffer.numberOfChannels,
        sampleRate: originalAudioBuffer.sampleRate
    });
    const audioBufferSourceNode = new AudioBufferSourceNode(
        offlineAudioContext,
        { buffer: originalAudioBuffer }
    );
    const gainNode = new GainNode(offlineAudioContext, { gain });

    audioBufferSourceNode
        .connect(gainNode)
        .connect(offlineAudioContext.destination);
    audioBufferSourceNode.start(0, region.start);

    return offlineAudioContext.startRendering();
}

You can for example replace or combine the GainNode with a BiquadFilterNode or any other node provided by the Web Audio API.