Downsampling a PCM audio buffer in javascript

Question

I am attempting to downsample the sample rate i am getting from audioContext. I believe it is coming in at 44100, and i want it to be 11025. I thought i could just average every 3 samples and it plays back at the correct rate, but the pitch of is too high, as if we were all on helium.

What is the correct way to downsample a float32Array from 44100 to a int16Array at 11025 samples.

var context = new Flash.audioContext();
var audioInput = context.createMediaStreamSource(stream);
var recorder = context.createScriptProcessor(null, 1, 1);
recorder.onaudioprocess = onAudio;
audioInput.connect(recorder);
recorder.connect(context.destination);

var onAudio = function (e) {
    var left = e.inputBuffer.getChannelData(0);
    bStream.write(Flash.convertFloat32ToInt16(left));
}

var convertFloat32ToInt16 = function(buffer) {
    var l = buffer.length;
    var point = Math.floor(l/3);
    var buf = new Int16Array(point);
    for (var x = l; x > 0;) {
        var average = (buffer[x] + buffer[x-1] +  buffer[x-2]) / 3;
        buf[point] = average*0x7FFF;
        point -= 1;
        x -= 3;
    }
    return buf.buffer;
}

score 20 · Accepted Answer · answered Feb 09 '15 at 18:51

20

For anyone else who needs the answer.

var downsampleBuffer = function (buffer, sampleRate, outSampleRate) {
    if (outSampleRate == sampleRate) {
        return buffer;
    }
    if (outSampleRate > sampleRate) {
        throw "downsampling rate show be smaller than original sample rate";
    }
    var sampleRateRatio = sampleRate / outSampleRate;
    var newLength = Math.round(buffer.length / sampleRateRatio);
    var result = new Int16Array(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] = Math.min(1, accum / count)*0x7FFF;
        offsetResult++;
        offsetBuffer = nextOffsetBuffer;
    }
    return result.buffer;
}

this will return a sample rate smaller than the one passed to it, while also converting it to a int16Array instead of a float32.

answered Feb 09 '15 at 18:51

Neablis

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Thank god for this post. Helped me so much you have no idea. – Koby Douek May 18 '17 at 05:53
Im glad it was able to help someone :) that was a mess to solve. – Neablis May 18 '17 at 19:22
You save my day :'D – diewland Nov 21 '17 at 10:34
Thanks for this post. Save my effort – dinu0101 Jul 06 '18 at 09:09
1

I found that sound that converted by this method had lots of noises. – jz1108 Apr 01 '19 at 14:49
3

This solution was written 4 years ago and the entire audio library has changed since. Don't assume it will work as it did back then. – Neablis Apr 01 '19 at 18:19
Not the best output but it works in a pinch. I see it in many samples/demos now. – hasan Mar 11 '22 at 00:47
A [slight variation](https://github.com/mw999/react-recording/blob/master/src/utilities/audio.js#L51) of this solution leads to a shorter and sped up audio file. I am completely in the dark when it comes to audio compression, but shouldn't the length stay the same? – anddt Mar 23 '22 at 19:30

Downsampling a PCM audio buffer in javascript

1 Answers1