@audio/encode-mp3

Encode PCM audio samples to MP3 format.
WASM (libmp3lame via wasm-media-encoders) — works in both node and browser.

import mp3 from '@audio/encode-mp3';

const encoder = await mp3({ sampleRate: 44100, channels: 1, bitrate: 128 });
const chunk = encoder.encode(channelData); // → Uint8Array (MP3 frames)
const tail = encoder.flush();              // → Uint8Array (remaining)
// concatenate chunk + tail, then write encoder.head() over the start

Gapless

The output opens with an Info (CBR) or Xing (VBR) frame carrying LAME's tag: the frame and byte counts, a seek TOC, and the encoder delay (576 samples) and padding that decoders trim (ffmpeg, mpg123, Apple's AudioToolbox, LAME's own), so a decoded file has the source's exact length and timing. The totals are known at the end: head() after flush() returns the finished frame (after the ID3 tag when one leads) to write over the start; the whole-file encode.mp3(...) of @audio/encode does it for you. Unpatched (a pipe), the placeholder still carries the delay, and with frames (the exact length upfront) the frame count and padding too.

Options

Streaming

const encoder = await mp3({ sampleRate: 44100, channels: 1, bitrate: 192 });
const a = encoder.encode(chunk1); // → Uint8Array
const b = encoder.encode(chunk2); // → Uint8Array
const c = encoder.flush();        // → Uint8Array
// complete MP3 = concat(a, b, c), encoder.head() written over its start
encoder.free();

Streaming output

chapters: [{ time, title }] (seconds) become ID3v2 CTOC/CHAP frames (ID3v2 Chapter Frame Addendum), in writeMeta too. stream: true puts the tag (meta, chapters) ahead of the first frames; the last chapter's end is unknown until head() returns the finished tag after flush(), unless frames gives the exact length upfront.

License

MIT

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