@audio/decode-aac

Decode AAC/M4A and ALAC audio to PCM float samples. FAAD2 WASM handles AAC; a pure-JS port of Apple's reference decoder handles ALAC. The M4A sample entry selects the codec.

Install

npm i @audio/decode-aac

Usage

import decode from '@audio/decode-aac'

// M4A or raw ADTS; auto-detected
let { channelData, sampleRate } = await decode(uint8array)
// channelData: Float32Array[] (one per channel)
// sampleRate: number

Streaming

import { decoder } from '@audio/decode-aac'

let dec = await decoder()
let { channelData, sampleRate } = dec.decode(chunk)
dec.free()

decoder() is asynchronous. Its decode() and flush() methods are synchronous.

API

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

Whole-file decode. Auto-detects M4A (MP4 container) vs raw ADTS.

decoder(opts?): Promise<AACDecoder>

Creates a decoder instance for manual control.

• dec.decode(data): decode a Uint8Array or ArrayBuffer chunk.
• dec.flush(): discard buffered partial data and return an empty result.
• dec.free(): release WASM memory.

Without options the byte stream is auto-detected (M4A or ADTS). Container demuxers that hold the codec config out-of-band pass it up-front and feed raw access units — one frame, or an array of frames, per decode() call:

let dec = await decoder({ asc })   // AudioSpecificConfig (esds, Matroska CodecPrivate, WAVEFORMATEX extra bytes)
let dec = await decoder({ alac })  // ALAC magic cookie

This is how @audio/decode-mp4, @audio/decode-webm and @audio/decode-avi decode AAC and ALAC tracks from video files.

AudioData

{ channelData: Float32Array[], sampleRate: number }

Formats

• M4A / MP4 / MOV with AAC audio (LC and HE-AAC v1/v2 with SBR or PS) — QuickTime sound descriptions v0, v1 and v2
• M4A / MP4 with bit-exact pure-JS ALAC decoding at 16, 20, 24, or 32 bits
• Raw ADTS streams (.aac)

Metadata

import { parseMeta } from '@audio/decode-aac/meta'

let { meta, sampleRate } = parseMeta(m4aBytes)
// meta: { title, artist, album, year, genre, track, ..., pictures }

License

ॐ · AAC decoding GPL-2.0 (FAAD2), ALAC decoding Apache-2.0 (port of Apple's ALAC reference)