webamp/js/media/StereoBalanceNode.js

// The Web Audio API does not offer an easy way to make a stereo balance
// control. This is an attempt to fill that void, using an API similar to
// [StereoPannerNode](https://developer.mozilla.org/en-US/docs/Web/API/StereoPannerNode)
export default function StereoBalanceNode(context, options = { balance: 0 }) {
  let balance = 0;

  // ChannelSplitterNode cannot be told to use a `channelInterperatation` of
  // "speakers". This means that if we get a mono file, we will end up only
  // playing in the left speaker. So instead we use this dummy gain node to
  // convert whatever source we get (stereo, mono, or n channels) into a stereo
  // signal.
  // Idea credit: https://github.com/WebAudio/web-audio-api/issues/975#issue-177242377
  const upMixer = context.createGain();
  upMixer.channelCount = 2;
  upMixer.channelCountMode = "explicit";
  upMixer.channelInterpretation = "speakers";

  const splitter = context.createChannelSplitter(2);

  // Create the gains for left and right
  const leftGain = context.createGain();
  const rightGain = context.createGain();

  const merger = context.createChannelMerger(2);

  upMixer.connect(splitter);

  splitter.connect(leftGain, 0);
  splitter.connect(rightGain, 1);

  leftGain.connect(merger, 0, 0);
  rightGain.connect(merger, 0, 1);

  // -1 (left) to 1 (right)
  function set(rawValue) {
    const value = Number(rawValue);
    leftGain.gain.value = value > 0 ? 1 - value : 1;
    rightGain.gain.value = value > 0 ? 1 : 1 + value;
    balance = value;
  }

  function get() {
    return balance;
  }

  // Create our own version of an [AudioParam](https://developer.mozilla.org/en-US/docs/Web/API/AudioParam).
  // We don't currently support any of the "over time" methods, but maybe some day
  // we'll want to.
  const audioParam = {};
  Object.defineProperties(audioParam, {
    value: { get, set, enumerable: true, configurable: true },
  });

  // The way the `.connect` API works, we can't actually construct our own
  // AudioNode. Instead we have to take an existing node and monkey patch it.
  Object.defineProperties(upMixer, {
    balance: {
      value: audioParam,
      enumerable: true,
      writable: false,
      configurable: true,
    },
    connect: {
      value: AudioNode.prototype.connect.bind(merger),
      enumerable: false,
      writable: false,
      configurable: true,
    },
    disconnect: {
      value: AudioNode.prototype.disconnect.bind(merger),
      enumerable: false,
      writable: false,
      configurable: true,
    },
  });

  if (balance !== options.balance) {
    set(options.balance);
  }

  return upMixer;
}