Decrease bitrate on WAV file created with recorderjs

Question

I'm trying to use recorderjs on an app engine site where users upload short audio recordings (say, 1 to a dozen seconds long). I've noticed that the WAV files I'm uploading are much larger than I expected. For example, I just created a recording that lasts roughly 9 seconds, and the uploaded blob is 1736769 bytes, which is > 1.5 megabytes.

Question:

How do I modify the recorderjs code (or my own code -- maybe I'm using recorderjs incorrectly) so that my audio blobs have a lower bitrate? I'd like a 10 second recording to be safely under 1 MB.

My guess is that I would need to modify the encodeWAV function in here, or maybe exportWAV, but I'm not sure how. Would it make sense to just drop every other element of the interleaved buffer in exportWAV? Is there a more intelligent way to do it? How does the bitrate of the exported WAV depend on properties of my computer (e.g. the sampling rate of my soundcard)?

I can add some details on my own code if it might be helpful.

Edit: if you'd like to see a live example, install google chrome beta and try this page. On my computer, a recording 5-10 seconds long is over 1 MB.

Many thanks,

Adrian

Answer 1

In my case Chrome records audio at 96kHz and Firefox at 44.1kHz, that makes huge WAV files. I implemented a downsampling function inside recorderWorker.js where you can select the sample ratio you want, like 16000.

function downsampleBuffer(buffer, rate) {
    if (rate == sampleRate) {
        return buffer;
    }
    if (rate > sampleRate) {
        throw "downsampling rate show be smaller than original sample rate";
    }
    var sampleRateRatio = sampleRate / rate;
    var newLength = Math.round(buffer.length / sampleRateRatio);
    var result = new Float32Array(newLength);
    var offsetResult = 0;
    var offsetBuffer = 0;
    while (offsetResult < result.length) {
        var nextOffsetBuffer = Math.round((offsetResult + 1) * sampleRateRatio);
        var accum = 0, 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;
}

and i call it when exporting the wav file:

function exportWAV(rate, type) {
    var bufferL = mergeBuffers(recBuffersL, recLength);
    var bufferR = mergeBuffers(recBuffersR, recLength);
    var interleaved = interleave(bufferL, bufferR);
    var downsampledBuffer = downsampleBuffer(interleaved, rate);
    var dataview = encodeWAV(rate, downsampledBuffer, false);
    var audioBlob = new Blob([ dataview ], {
        type : type
    });

    this.postMessage(audioBlob);
}