Web Audio API - Live Stream 'clicks' between chunks.

问题

I am trying to stream audio through a websocket on a node.js (express) server to a web browser. The audio is coming from an iOS device as 16-bit, mono wav files sampled at 4k (4000 samples per second).

Here's my code:

Server Code:

webSocketServer.on('connection', function connection(client) {
  client.on('message', function(message) { 
    webSocketServer.clients.forEach(function each(connection) {
      connection.send(message, { binary: true }
    );
  });
});

Client Code:

webSocket = new WebSocket('ws://' + window.location.hostname + ':8080/');
webSocket.binaryType = 'arraybuffer'
webSocket.onmessage = function(message) {
  var arrayBuffer = message.data // wav from server, as arraybuffer
  var source = audioContext.createBufferSource();
  audioContext.decodeAudioData(arrayBuffer, function(buffer){
    source.buffer = buffer
    source.connect(audioContext.destination)
    source.start(time);
    time += source.buffer.duration
  }, function(){
    console.log('error')
  })
};

decodeAudioData()appears to be working, however the audio buffer it returns is half the length I was expecting. (eg 4000 samples will only give me 0.5 seconds of audio. I originally thought this was because the wav is 16 bit and not 32, but switching to 32 caused decodeAudioData() to trigger it's error callback.

I figured this workaround could be added to the success callback:

source.playbackRate.value = 0.5 // play at half speed
time += source.buffer.duration * 2 // double duration

This gets the timing to work perfectly, but I am left with one problem: There is an audible 'click' or 'pop' between audio chunks. After spacing out the chunks by one second (time += (source.buffer.duration * 2) + 1), I was able to find that the click happens at the very beginning of each chunk.

So my main two head-scratchers are:

1) Why is the decoded audio playing at twice the speed I am expecting? Is my sampling rate of 4k too low for the Web Audio API? Why can't I decode 32-bit wav's?

2) I have some experience with digital audio workstations (ableton, logic) and I know that clicking sounds can arise if a wave 'jumps' from a sample back down to zero or vice versa (ie: starting/ending a sine wave in the midst of a phase). Is that what's going on here? Is there a way to get around this? Crossfading each individual sample seems silly. Why doesn't each chunk pickup where the last one left off?

回答1:

1) The audio I was receiving was actually at 2k by mistake, but the wav header still said 4k, thus the double speed error.

2) See the last paragraph of Chris Wilsons answer here:

Finally - this is not going to work well if the sound stream does not match the default audio device's sample rate; there are always going to be clicks, because decodeAudioData will resample to the device rate, which will not have a perfect duration. It will work, but there will likely be artifacts like clicks at the boundaries of chunks. You need a feature that's not yet spec'ed or implemented - selectable AudioContext sample rates - in order to fix this.

Brion Vibbers AudioFeeder.js works great without any clicks but requires raw 32bit pcm data. Also be wary of upsampling artifacts!