feat(codec): add detection support for AC-3, E-AC-3 audio codecs (#275)

* ac3/eac3 detection

* fix tests

* review pass + follow codebase comments style

* Add ac3/eac3 to docs

* WIP mpeg-2

* Set registration_descriptor

* add support for ac3 system b in mpeg-ts

* Update docs with ac3 in .ts support

* fix formatting

* Remove default to fix linter

* Add eac3 to mpeg-ts

* fix formatting

* Fix failing test

* Refactor AC3/EAC3 code a little

---------

Co-authored-by: Vanilagy <[email protected]>

src/isobmff/isobmff-boxes.ts

@@ -20,6 +20,7 @@ import {
 	UNDETERMINED_LANGUAGE,
 	assertNever,
 	keyValueIterator,
+	Bitstream,
 } from '../misc';
 import {
 	AudioCodec,
@@ -43,7 +44,7 @@ import {
 	IsobmffVideoTrackData,
 	Sample,
 } from './isobmff-muxer';
-import { parseOpusIdentificationHeader } from '../codec-data';
+import { parseAc3SyncFrame, parseEac3SyncFrame, parseOpusIdentificationHeader } from '../codec-data';
 import { MetadataTags, RichImageData } from '../metadata';
 
 export class IsobmffBoxWriter {
@@ -907,6 +908,79 @@ const pcmC = (trackData: IsobmffAudioTrackData) => {
 	]);
 };
 
+/** AC3SpecificBox */
+const dac3 = (trackData: IsobmffAudioTrackData) => {
+	const frameInfo = parseAc3SyncFrame(trackData.info.firstPacket.data);
+	if (!frameInfo) {
+		throw new Error(
+			'Couldn\'t extract AC-3 frame info from the audio packet. '
+			+ 'Ensure the packets contain valid AC-3 sync frames (as specified in ETSI TS 102 366).',
+		);
+	}
+
+	const bytes = new Uint8Array(3);
+	const bitstream = new Bitstream(bytes);
+
+	bitstream.writeBits(2, frameInfo.fscod);
+	bitstream.writeBits(5, frameInfo.bsid);
+	bitstream.writeBits(3, frameInfo.bsmod);
+	bitstream.writeBits(3, frameInfo.acmod);
+	bitstream.writeBits(1, frameInfo.lfeon);
+	bitstream.writeBits(5, frameInfo.bitRateCode);
+	bitstream.writeBits(5, 0); // reserved
+
+	return box('dac3', [...bytes]);
+};
+
+/** EC3SpecificBox */
+const dec3 = (trackData: IsobmffAudioTrackData) => {
+	const frameInfo = parseEac3SyncFrame(trackData.info.firstPacket.data);
+	if (!frameInfo) {
+		throw new Error(
+			'Couldn\'t extract E-AC-3 frame info from the audio packet. '
+			+ 'Ensure the packets contain valid E-AC-3 sync frames (as specified in ETSI TS 102 366).',
+		);
+	}
+
+	// Calculate size
+	let totalBits = 16; // header: data_rate (13) + num_ind_sub (3)
+	for (const sub of frameInfo.substreams) {
+		totalBits += 23; // fixed fields per substream
+		if (sub.numDepSub > 0) {
+			totalBits += 9; // chan_loc
+		} else {
+			totalBits += 1; // reserved
+		}
+	}
+	const size = Math.ceil(totalBits / 8);
+
+	const bytes = new Uint8Array(size);
+	const bitstream = new Bitstream(bytes);
+
+	bitstream.writeBits(13, frameInfo.dataRate);
+	bitstream.writeBits(3, frameInfo.substreams.length - 1); // num_ind_sub
+
+	for (const sub of frameInfo.substreams) {
+		bitstream.writeBits(2, sub.fscod);
+		bitstream.writeBits(5, sub.bsid);
+		bitstream.writeBits(1, 0); // reserved
+		bitstream.writeBits(1, 0); // asvc = 0
+		bitstream.writeBits(3, sub.bsmod);
+		bitstream.writeBits(3, sub.acmod);
+		bitstream.writeBits(1, sub.lfeon);
+		bitstream.writeBits(3, 0); // reserved
+		bitstream.writeBits(4, sub.numDepSub);
+
+		if (sub.numDepSub > 0) {
+			bitstream.writeBits(9, sub.chanLoc);
+		} else {
+			bitstream.writeBits(1, 0); // reserved
+		}
+	}
+
+	return box('dec3', [...bytes]);
+};
+
 export const subtitleSampleDescription = (
 	compressionType: string,
 	trackData: IsobmffSubtitleTrackData,
@@ -1606,6 +1680,8 @@ const audioCodecToBoxName = (codec: AudioCodec, isQuickTime: boolean): string =>
 		case 'alaw': return 'alaw';
 		case 'pcm-u8': return 'raw ';
 		case 'pcm-s8': return 'sowt';
+		case 'ac3': return 'ac-3';
+		case 'eac3': return 'ec-3';
 	}
 
 	// Logic diverges here
@@ -1645,6 +1721,8 @@ const audioCodecToConfigurationBox = (codec: AudioCodec, isQuickTime: boolean) =
 		case 'opus': return dOps;
 		case 'vorbis': return esds;
 		case 'flac': return dfLa;
+		case 'ac3': return dac3;
+		case 'eac3': return dec3;
 	}
 
 	// Logic diverges here

src/isobmff/isobmff-demuxer.ts

@@ -27,6 +27,11 @@ import {
 	FlacBlockType,
 	HevcDecoderConfigurationRecord,
 	Vp9CodecInfo,
+	parseEac3Config,
+	getEac3SampleRate,
+	getEac3ChannelCount,
+	AC3_SAMPLE_RATES,
+	AC3_ACMOD_CHANNEL_COUNTS,
 } from '../codec-data';
 import { Demuxer } from '../demuxer';
 import { Input } from '../input';
@@ -952,6 +957,10 @@ export class IsobmffDemuxer extends Demuxer {
 							track.info.codec = 'ulaw';
 						} else if (lowercaseBoxName === 'alaw') {
 							track.info.codec = 'alaw';
+						} else if (lowercaseBoxName === 'ac-3') {
+							track.info.codec = 'ac3';
+						} else if (lowercaseBoxName === 'ec-3') {
+							track.info.codec = 'eac3';
 						} else {
 							console.warn(`Unsupported audio codec (sample entry type '${sampleBoxInfo.name}').`);
 						}
@@ -1463,6 +1472,51 @@ export class IsobmffDemuxer extends Demuxer {
 				track.info.codecDescription = description;
 			}; break;
 
+			case 'dac3': { // AC3SpecificBox
+				const track = this.currentTrack;
+				if (!track) {
+					break;
+				}
+				assert(track.info?.type === 'audio');
+
+				const bytes = readBytes(slice, 3);
+				const bitstream = new Bitstream(bytes);
+
+				const fscod = bitstream.readBits(2);
+				bitstream.skipBits(5 + 3); // Skip bsid and bsmod
+				const acmod = bitstream.readBits(3);
+				const lfeon = bitstream.readBits(1);
+
+				if (fscod < 3) {
+					track.info.sampleRate = AC3_SAMPLE_RATES[fscod]!;
+				}
+
+				track.info.numberOfChannels = AC3_ACMOD_CHANNEL_COUNTS[acmod]! + lfeon;
+			}; break;
+
+			case 'dec3': { // EC3SpecificBox
+				const track = this.currentTrack;
+				if (!track) {
+					break;
+				}
+				assert(track.info?.type === 'audio');
+
+				const bytes = readBytes(slice, boxInfo.contentSize);
+				const config = parseEac3Config(bytes);
+
+				if (!config) {
+					console.warn('Invalid dec3 box contents, ignoring.');
+					break;
+				}
+
+				const sampleRate = getEac3SampleRate(config);
+				if (sampleRate !== null) {
+					track.info.sampleRate = sampleRate;
+				}
+
+				track.info.numberOfChannels = getEac3ChannelCount(config);
+			}; break;
+
 			case 'stts': {
 				const track = this.currentTrack;
 				if (!track) {

src/isobmff/isobmff-muxer.ts

@@ -111,6 +111,7 @@ export type IsobmffTrackData = {
 		 * ADTS-wrapped data.
 		 */
 		requiresAdtsStripping: boolean;
+		firstPacket: EncodedPacket;
 	};
 } | {
 	track: OutputSubtitleTrack;
@@ -429,8 +430,9 @@ export class IsobmffMuxer extends Muxer {
 					!this.isFragmented
 					&& (PCM_AUDIO_CODECS as readonly string[]).includes(track.source._codec),
 				requiresAdtsStripping,
+				firstPacket: packet,
 			},
-			timescale: meta.decoderConfig.sampleRate,
+			timescale: decoderConfig.sampleRate,
 			samples: [],
 			sampleQueue: [],
 			timestampProcessingQueue: [],