Opus codec¶
Encodings - opus
ENC-05 - status: draft - audio
Standard Opus and MLow payloads share one RTP audio stream; the receiver routes each frame to a decoder by the top two bits of its first payload byte.
MLow (mlow) and standard Opus (RFC 6716) are interleaved on the same SRTP-protected RTP audio stream and the same SSRC. MLow is the default for 1:1 calls; standard Opus frames MAY appear on the same stream. A receiver MUST decode both.
Codec selection is per-frame, by the first payload byte; there is no negotiation, no RTP header field, no payload-type split. A receiver MUST branch on the top two bits of the first byte, on every frame:
(firstByte & 0xC0) == 0xC0 → standard Opus packet, decode with RFC 6716 decoder
(firstByte & 0xC0) != 0xC0 → MLow "smpl" frame, route to MLow decoder ([mlow](../encodings/mlow.md))
For a standard Opus packet, read frame duration from config = firstByte >> 3
(RFC 6716 Table 2). Call-audio output sample rate is 16 kHz; decoded length =
16 * frameMs samples.
config < 12 → SILK, frameMs ∈ {10, 20, 40, 60} = [config & 3]
config 12..15 → Hybrid, frameMs ∈ {10, 20} = [(config-12) & 1]
config >= 16 → CELT, frameMs ∈ {2.5, 5, 10, 20} = [config & 3]
Requires: mlow, call-offer
Breakdown: mlow-frame, media-loop, rtp-framing
Implemented by
| Flavor | Status | Source | Notes |
|---|---|---|---|
whatsapp-rust |
working | — | — |
meowcaller |
partial | — | codec modules partial; first-byte routing present, Opus decode delegated to a stock libopus binding |
Annotation wacrg:ENC-05 — a flavor marks its implementation site in source with this comment; a script clones the source, finds it, and attaches the commit blame/permalink.
Contributors
| Contributor | Role |
|---|---|
 Rajeh Taher |
wrote initial spec |
protocol history / diff - blame
Open questions - Conditions under which a sender emits standard-Opus frames instead of MLow on a live 1:1 call (e.g. interop, fallback, or capability gating) are unspecified. - Whether group calls or any specific transport mode default to standard Opus rather than MLow is unspecified. - Whether standard-Opus payloads use a wider band (full 48 kHz Opus) than the 16 kHz call-audio output, or are always decoded down to 16 kHz, is unspecified.
References - RFC 6716 — Definition of the Opus Audio Codec
Changelog¶
- 2026-06-21 — Initial spec entry.