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downsample to 8khz #201

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downsample to 8khz #201

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58 changes: 47 additions & 11 deletions src/recorder.js
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Original file line number Diff line number Diff line change
Expand Up @@ -18,8 +18,8 @@ export class Recorder {
Object.assign(this.config, cfg);
this.context = source.context;
this.node = (this.context.createScriptProcessor ||
this.context.createJavaScriptNode).call(this.context,
this.config.bufferLen, this.config.numChannels, this.config.numChannels);
this.context.createJavaScriptNode).call(this.context,
this.config.bufferLen, this.config.numChannels, this.config.numChannels);

this.node.onaudioprocess = (e) => {
if (!this.recording) return;
Expand Down Expand Up @@ -88,18 +88,21 @@ export class Recorder {
} else {
interleaved = buffers[0];
}
let dataview = encodeWAV(interleaved);
let audioBlob = new Blob([dataview], {type: type});

this.postMessage({command: 'exportWAV', data: audioBlob});
var downsampledBuffer = downsampleBuffer(interleaved, targetRate);
var dataview = encodeWAV(downsampledBuffer);
// let dataview = encodeWAV(interleaved);
let audioBlob = new Blob([dataview], { type: type });

this.postMessage({ command: 'exportWAV', data: audioBlob });
}

function getBuffer() {
let buffers = [];
for (let channel = 0; channel < numChannels; channel++) {
buffers.push(mergeBuffers(recBuffers[channel], recLength));
}
this.postMessage({command: 'getBuffer', data: buffers});
this.postMessage({ command: 'getBuffer', data: buffers });
}

function clear() {
Expand Down Expand Up @@ -152,6 +155,35 @@ export class Recorder {
}
}

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);
// Use average value of skipped samples
var accum = 0, count = 0;
for (var i = offsetBuffer; i < nextOffsetBuffer && i < buffer.length; i++) {
accum += buffer[i];
count++;
}
result[offsetResult] = accum / count;
// Or you can simply get rid of the skipped samples:
// result[offsetResult] = buffer[nextOffsetBuffer];
offsetResult++;
offsetBuffer = nextOffsetBuffer;
}
return result;
}

function encodeWAV(samples) {
let buffer = new ArrayBuffer(44 + samples.length * 2);
let view = new DataView(buffer);
Expand All @@ -170,10 +202,14 @@ export class Recorder {
view.setUint16(20, 1, true);
/* channel count */
view.setUint16(22, numChannels, true);
// /* sample rate */
// view.setUint32(24, sampleRate, true);
// /* byte rate (sample rate * block align) */
// view.setUint32(28, sampleRate * 4, true);
/* sample rate */
view.setUint32(24, sampleRate, true);
view.setUint32(24, 8000, true);
/* byte rate (sample rate * block align) */
view.setUint32(28, sampleRate * 4, true);
view.setUint32(28, 8000 * 4, true);
/* block align (channel count * bytes per sample) */
view.setUint16(32, numChannels * 2, true);
/* bits per sample */
Expand Down Expand Up @@ -215,7 +251,7 @@ export class Recorder {
}

clear() {
this.worker.postMessage({command: 'clear'});
this.worker.postMessage({ command: 'clear' });
}

getBuffer(cb) {
Expand All @@ -224,7 +260,7 @@ export class Recorder {

this.callbacks.getBuffer.push(cb);

this.worker.postMessage({command: 'getBuffer'});
this.worker.postMessage({ command: 'getBuffer' });
}

exportWAV(cb, mimeType) {
Expand All @@ -241,7 +277,7 @@ export class Recorder {
}

static
forceDownload(blob, filename) {
forceDownload(blob, filename) {
let url = (window.URL || window.webkitURL).createObjectURL(blob);
let link = window.document.createElement('a');
link.href = url;
Expand Down
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