forge

Thin Flutter wrapper around opus_dart and opus_flutter. Encodes fixed-size PCM16LE frames to Opus packets and decodes them back.

Features

• ForgeEncoder / ForgeDecoder, a minimal encode/decode API over opus_dart's SimpleOpusEncoder/SimpleOpusDecoder
• Input-size validation on encode with a clear error message
• Packet-loss concealment and forward-error-correction support on decode
• PCM16LE ⇄ Int16List conversion helpers with explicit endianness

Getting started

Load libopus once, before constructing any encoder or decoder:

await ForgeInit.ensure();

Safe to call from more than one place — the actual native library load only happens once, and every call after that awaits the same future.

Supported formats:

• Sample rates: 8000, 12000, 16000, 24000, 48000 Hz
• Channels: 1 or 2

Usage

Encoding

final encoder = ForgeEncoder(
  sampleRate: 48000,
  channels: 2,
  samplesPerChannel: 960, // 20ms at 48kHz
);

final opusPacket = encoder.encode(pcmFrame); // pcmFrame: PCM16LE, interleaved

encoder.dispose(); // releases native memory when done

pcmFrame needs to be exactly samplesPerChannel * channels * 2 bytes (encoder.expectedInputBytes) — frames straight from framer's PcmFramer, configured with a matching frame size, work well here.

samplesPerChannel has to correspond to one of Opus's supported frame durations at the given sample rate (2.5/5/10/20/40/60 ms).

application (defaults to Application.audio) tunes the encoder for its content: audio for music/general audio, voip for speech, or restrictedLowdelay when latency matters more than quality.

Decoding

final decoder = ForgeDecoder(sampleRate: 48000, channels: 2);

final pcmFrame = decoder.decode(opusPacket); // PCM16LE bytes out

decoder.dispose();

sampleRate/channels need to match what the encoder on the sending side used — Opus packets don't carry that information themselves.

Pass null instead of a packet to signal a lost one; Opus's built-in packet loss concealment synthesizes a plausible continuation instead of a gap. Pass fec: true on the next packet to recover a previous lost one from embedded forward-error-correction data, if the encoder had FEC turned on.

Additional information

pcm16leBytesToSamples / samplesToPcm16leBytes (also exported from forge.dart) convert between raw interleaved PCM16LE bytes and the Int16List samples opus_dart's encode/decode calls expect, with an explicit little-endian read/write.