audioc2

Pure JavaScript AMR-NB codec, hand-ported line-by-line from opencore-amr 0.1.6. No WebAssembly, no native dependencies, no build step.

Status: decoder and encoder complete and bit-exact. Output is verified byte-identical to the reference C implementation across all 8 modes (MR475…MR122), DTX/SID comfort-noise frames, and (decoder) bad-frame (BFI) error concealment. Encode → decode round-trips reproduce the reference codec sample-for-sample.

• Zero dependencies, ESM, Node ≥ 18 (also works in browsers/bundlers)
• ~500× realtime decoding, ~130× realtime encoding on a modern machine (V8)
• Drop-in replacement for the emscripten-compiled amrnb.js interface

Usage

High-level:

import { AmrNbDecoder, AmrNbEncoder, Mode } from 'audioc2';
import fs from 'node:fs';

const amr = fs.readFileSync('voice.amr');      // IETF storage format
const pcm = new AmrNbDecoder().decodeAll(amr); // Int16Array, 8 kHz mono

const amr2 = new AmrNbEncoder().encodeAll(pcm, Mode.MR122); // PCM -> AMR

Low-level (mirrors opencore-amr's wrapper.cpp / the emscripten module):

import {
  Decoder_Interface_init, Decoder_Interface_Decode, FRAME_SIZE,
} from 'audioc2';

const state = Decoder_Interface_init();
const pcm = new Int16Array(160);
// frame: Uint8Array of one frame, ToC byte first; bfi: 0 or 1
Decoder_Interface_Decode(state, frame, pcm, 0);

CLI:

node cli/amrnb-dec.js in.amr out.wav      # decode
node cli/amrnb-enc.js in.wav out.amr 7    # encode (mode 0-7)

How it was ported (and how it stays correct)

Every function is transcribed line by line from the opencore-amr C sources, preserving the 16/32-bit saturating fixed-point semantics exactly (src/common/basicop.js documents the conventions). Lookup tables are machine-extracted from the C sources by tools/extract-tables.mjs.

Verification (see test/):

• basicop.test.js — every fixed-point primitive compared against a natively compiled vector generator (2.8M cases incl. saturation edges).
• decode.test.js — frame-by-frame bit-exact comparison against golden output produced by the reference codec for all modes, DTX and BFI.
• encode.test.js — byte-exact comparison of the encoded bitstream against the reference encoder for all 8 modes (with and without DTX), plus a round-trip check.
• tools/native/ — builds the reference C decoder/encoder for golden cross-checks and divergence bisection (requires gcc and the opencore-amr 0.1.6 sources; not needed to run the library).

Regenerate fixtures/tables: npm run gen-reference, npm run gen-tables (both need the opencore-amr 0.1.6 source tree / reference module, see the paths in tools/).

License

Apache-2.0, same as opencore-amr, of which this is a derivative work (original code (C) 1998-2010 PacketVideo, portions derived from 3GPP TS 26.073 reference C code, (C) 2004 3GPP Organizational Partners).