@audio/decode-eac3

@audio/decode-eac3

Decode E-AC-3 (Dolby Digital Plus) — and plain AC-3 — to PCM float samples.
FFmpeg's eac3 decoder — a slim LGPL-2.1-or-later libavcodec/libavutil build (no GPL components) — compiled to a single-file WASM ES module. No side files, loads from any CDN, Node, workers and AudioWorklets.

import decode, { decoder } from '@audio/decode-eac3'

let { channelData, sampleRate } = await decode(eac3Bytes)

let dec = await decoder()
let head = dec.decode(chunk1)   // synchronous; partial frames carry over
let tail = dec.flush()
dec.free()

E-AC-3 (Dolby Digital Plus) is specified in ATSC A/52:2018 Annex E. FFmpeg's eac3 decoder shares its core with ac3dec.c and dispatches on the bitstream's own bitstream_id field (bsid ≤ 10 → classic AC-3 syntax, bsid = 16 → E-AC-3), so this package decodes both raw AC-3 and E-AC-3 sync-frame streams (.ac3, .ec3, .eac3) — container demuxers (@audio/decode-mp4, @audio/decode-webm, @audio/decode-avi) route their E-AC-3 tracks here. Sync frames (syncword 0x0B77) are cut in JS using FFmpeg's own public av_ac3_parse_header for the frame length (frmsiz for E-AC-3, the frmsizecod/fscod table for classic AC-3) — same shape as @audio/decode-ac3.

Up to 5.1 (7.1 with dependent-substream channel maps FFmpeg supports), channels come out in WAV order already: FFmpeg's ac3dec.c builds a native (bitmask) AVChannelLayout for every decoded frame, and native layouts enumerate channels in ascending AV_CH_* bit order — which is WAV order (FL, FR, FC, LFE, BL, BR, …; FFmpeg labels AC-3/E-AC-3's rear pair SIDE_LEFT/SIDE_RIGHT rather than BACK_LEFT/BACK_RIGHT, but the bit position — and so the output slot — is the same). No downmix is requested, so the stream's own channel count is preserved.

Dynamic range compression (drc_scale) is forced to 0, disabling it — FFmpeg's AC-3 decoder applies it by default (drc_scale = 1.0). @audio/decode-ac3 (liba52) never applies dialnorm or DRC either; forcing it off here keeps output level comparable between the two packages for the same AC-3 input — verified in test.js (RMS within ±0.5 dB on ../decode-ac3/fixtures/stereo.ac3, decoded by both). Neither package applies dialnorm — FFmpeg's ac3dec.c doesn't read or apply it at all, only DRC.

API

decode(src: Uint8Array | ArrayBuffer): Promise<AudioData>

Decode a complete stream.

decoder(): Promise<Eac3Decoder>

Synchronous streaming decoder: decode(chunk) returns the samples of every complete frame in the data so far, flush() drops a trailing partial frame, free() releases WASM memory. errors counts frames that failed to decode.

AudioData

{ channelData: Float32Array[], sampleRate: number }

Use when: you need to play or transcode E-AC-3 (Dolby Digital Plus) — or AC-3 — in the browser or Node.

License

ॐ · LGPL-2.1-or-later, inherited from the bundled FFmpeg eac3/ac3 decoder (built without --enable-gpl — see build.sh and LICENSE.ffmpeg). FFmpeg source: the shared lib/ffmpeg submodule (release/7.1), also used by @audio/decode-ape.