Also known as: Opus, Opus codec
Opus is an open, royalty-free audio codec standardised by the IETF as RFC 6716, designed to handle everything from low-bitrate speech to high-fidelity music in a single format.1 It is the default codec of WebRTC and is ubiquitous in VoIP, conferencing, and streaming. Opus is a hybrid: it fuses SILK — a linear-predictive speech coder in the CELP tradition — with CELT, a low-delay transform coder, and switches between or blends them depending on the content and bitrate.
How it works
Opus reuses two prior codecs. SILK, contributed by Skype, is a linear-predictive coder good at speech at low-to-moderate bitrates; CELT, from Xiph.Org, is an MDCT transform coder engineered for very low algorithmic delay and good music quality. Opus runs SILK below roughly 8–12 kHz audio bandwidth, CELT for wideband/fullband and low-latency needs, and a hybrid mode in between where SILK codes the low band and CELT the high band. A single stream can change mode, bitrate, bandwidth, and channel count seamlessly at any 20 ms boundary.
That flexibility is the point. Opus offers bitrates from about 6 kbps to 510 kbps, sampling rates up to 48 kHz, mono and stereo, and configurable frame sizes from 2.5 to 60 ms — so an application dials in latency versus quality without changing codecs. Built-in variable bitrate, forward error correction, and packet-loss concealment make it robust over lossy networks, which is why WebRTC and most modern VoIP stacks adopted it. Being royalty-free removed the licensing barrier that codecs like AMR and ACELP-based ones carry.
Relevance to SDR
Opus is a network audio codec, not an over-the-air radio waveform, so it is not decoded from RF. It is relevant to this guide as the leading open, royalty-free alternative in the same design space as the licensed cellular codecs, and it frequently sits at the receiving end of the chain: SDR decoders and trunking loggers often re-encode recovered voice to Opus for storage or streaming because it is compact and unencumbered. GopherTrunk decodes the on-air vocoder to PCM; delivering or archiving that audio as Opus is a downstream packaging choice rather than part of the RF decode itself.
Sources
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Opus (audio format) — Wikipedia, on the SILK+CELT hybrid structure, bitrate and latency range, and royalty-free IETF status. ↩