Add reusable Opus identification header parsing function

src/codec.ts

@@ -967,6 +967,30 @@ export const parseAacAudioSpecificConfig = (bytes: Uint8Array | null) => {
 	};
 };
 
+export const parseOpusIdentificationHeader = (bytes: Uint8Array) => {
+	const view = toDataView(bytes);
+
+	const outputChannelCount = view.getUint8(9);
+	const preSkip = view.getUint16(10, true);
+	const inputSampleRate = view.getUint32(12, true);
+	const outputGain = view.getInt16(16, true);
+	const channelMappingFamily = view.getUint8(18);
+
+	let channelMappingTable: Uint8Array | null = null;
+	if (channelMappingFamily) {
+		channelMappingTable = bytes.subarray(19, 19 + 2 + outputChannelCount);
+	}
+
+	return {
+		outputChannelCount,
+		preSkip,
+		inputSampleRate,
+		outputGain,
+		channelMappingFamily,
+		channelMappingTable,
+	};
+};
+
 // From https://datatracker.ietf.org/doc/html/rfc6716, in 48 kHz samples
 const OPUS_FRAME_DURATION_TABLE = [
 	480, 960, 1920, 2880,

src/isobmff/isobmff-boxes.ts

@@ -16,6 +16,7 @@ import {
 import {
 	AudioCodec,
 	generateAv1CodecConfigurationFromCodecString,
+	parseOpusIdentificationHeader,
 	parsePcmCodec,
 	PCM_AUDIO_CODECS,
 	PcmAudioCodec,
@@ -824,19 +825,17 @@ export const dOps = (trackData: IsobmffAudioTrackData) => {
 	if (description) {
 		assert(description.byteLength >= 18);
 
-		const view = ArrayBuffer.isView(description)
-			? new DataView(description.buffer, description.byteOffset, description.byteLength)
-			: new DataView(description);
-
-		outputChannelCount = view.getUint8(9);
-		preSkip = view.getUint16(10, true);
-		inputSampleRate = view.getUint32(12, true);
-		outputGain = view.getInt16(16, true);
-		channelMappingFamily = view.getUint8(18);
-
-		if (channelMappingFamily) {
 		const bytes = toUint8Array(description);
-			channelMappingTable = bytes.subarray(19, 19 + 2 + outputChannelCount);
+		const header = parseOpusIdentificationHeader(bytes);
+
+		outputChannelCount = header.outputChannelCount;
+		preSkip = header.preSkip;
+		inputSampleRate = header.inputSampleRate;
+		outputGain = header.outputGain;
+		channelMappingFamily = header.channelMappingFamily;
+
+		if (header.channelMappingTable) {
+			channelMappingTable = header.channelMappingTable;
 		}
 	}
 

src/matroska/matroska-muxer.ts

@@ -34,6 +34,7 @@ import {
 	PCM_AUDIO_CODECS,
 	PcmAudioCodec,
 	generateAv1CodecConfigurationFromCodecString,
+	parseOpusIdentificationHeader,
 	parsePcmCodec,
 	validateAudioChunkMetadata,
 	validateSubtitleMetadata,
@@ -228,12 +229,11 @@ export class MatroskaMuxer extends Muxer {
 
 				const description = trackData.info.decoderConfig.description;
 				if (description) {
-					const view = ArrayBuffer.isView(description)
+					const bytes = toUint8Array(description);
-						? new DataView(description.buffer, description.byteOffset, description.byteLength)
+					const header = parseOpusIdentificationHeader(bytes);
-						: new DataView(description);
 
-					// Read the value from the Opus Identification Header
+					// Use the preSkip value from the header
-					seekPreRollNs = Math.round(1e9 * (view.getUint16(10, true) / 48000));
+					seekPreRollNs = Math.round(1e9 * (header.preSkip / 48000));
 				}
 			}
 

src/ogg/ogg-demuxer.ts

@@ -1,4 +1,4 @@
-import { AudioCodec, parseOpusTocByte } from '../codec';
+import { AudioCodec, parseOpusIdentificationHeader, parseOpusTocByte } from '../codec';
 import { Demuxer } from '../demuxer';
 import { Input } from '../input';
 import { InputAudioTrack, InputAudioTrackBacking } from '../input-track';
@@ -236,12 +236,12 @@ export class OggDemuxer extends Demuxer {
 		bitstream.description = firstPacket.data;
 		bitstream.lastMetadataPacket = secondPacket;
 
-		const view = toDataView(firstPacket.data);
+		const header = parseOpusIdentificationHeader(firstPacket.data);
-		bitstream.numberOfChannels = view.getUint8(9);
+		bitstream.numberOfChannels = header.outputChannelCount;
-		bitstream.sampleRate = view.getUint32(12, true);
+		bitstream.sampleRate = header.inputSampleRate;
 
 		bitstream.opusInfo = {
-			preSkip: view.getUint16(10, true),
+			preSkip: header.preSkip,
 		};
 	}