@euirim/microsoft-cognitiveservices-speech-sdk
Version:
Microsoft Cognitive Services Speech SDK for JavaScript
90 lines (88 loc) • 3.98 kB
JavaScript
// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT license.
export class RiffPcmEncoder {
constructor(actualSampleRate, desiredSampleRate) {
this.privChannelCount = 1;
this.encode = (needHeader, actualAudioFrame) => {
const audioFrame = this.downSampleAudioFrame(actualAudioFrame, this.privActualSampleRate, this.privDesiredSampleRate);
if (!audioFrame) {
return null;
}
const audioLength = audioFrame.length * 2;
if (!needHeader) {
const buffer = new ArrayBuffer(audioLength);
const view = new DataView(buffer);
this.floatTo16BitPCM(view, 0, audioFrame);
return buffer;
}
const buffer = new ArrayBuffer(44 + audioLength);
const bitsPerSample = 16;
const bytesPerSample = bitsPerSample / 8;
// We dont know ahead of time about the length of audio to stream. So set to 0.
const fileLength = 0;
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView
const view = new DataView(buffer);
/* RIFF identifier */
this.setString(view, 0, "RIFF");
/* file length */
view.setUint32(4, fileLength, true);
/* RIFF type & Format */
this.setString(view, 8, "WAVEfmt ");
/* format chunk length */
view.setUint32(16, 16, true);
/* sample format (raw) */
view.setUint16(20, 1, true);
/* channel count */
view.setUint16(22, this.privChannelCount, true);
/* sample rate */
view.setUint32(24, this.privDesiredSampleRate, true);
/* byte rate (sample rate * block align) */
view.setUint32(28, this.privDesiredSampleRate * this.privChannelCount * bytesPerSample, true);
/* block align (channel count * bytes per sample) */
view.setUint16(32, this.privChannelCount * bytesPerSample, true);
/* bits per sample */
view.setUint16(34, bitsPerSample, true);
/* data chunk identifier */
this.setString(view, 36, "data");
/* data chunk length */
view.setUint32(40, fileLength, true);
this.floatTo16BitPCM(view, 44, audioFrame);
return buffer;
};
this.setString = (view, offset, str) => {
for (let i = 0; i < str.length; i++) {
view.setUint8(offset + i, str.charCodeAt(i));
}
};
this.floatTo16BitPCM = (view, offset, input) => {
for (let i = 0; i < input.length; i++, offset += 2) {
const s = Math.max(-1, Math.min(1, input[i]));
view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
}
};
this.downSampleAudioFrame = (srcFrame, srcRate, dstRate) => {
if (dstRate === srcRate || dstRate > srcRate) {
return srcFrame;
}
const ratio = srcRate / dstRate;
const dstLength = Math.round(srcFrame.length / ratio);
const dstFrame = new Float32Array(dstLength);
let srcOffset = 0;
let dstOffset = 0;
while (dstOffset < dstLength) {
const nextSrcOffset = Math.round((dstOffset + 1) * ratio);
let accum = 0;
let count = 0;
while (srcOffset < nextSrcOffset && srcOffset < srcFrame.length) {
accum += srcFrame[srcOffset++];
count++;
}
dstFrame[dstOffset++] = accum / count;
}
return dstFrame;
};
this.privActualSampleRate = actualSampleRate;
this.privDesiredSampleRate = desiredSampleRate;
}
}
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