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Opus codec

Definition

Opus is the de-facto open-source general-purpose audio codec, released in 2012 and standardised in RFC 6716. It combines the SILK codec (voice) and CELT (music) into a single interactive codec covering 6 kbps (NarrowBand) to 510 kbps (FullBand), with frame sizes from 2.5 ms to 60 ms. It became "the codec of choice for the wide variety of applications on the internet" after release; Meta "used Opus for all its RTC needs" prior to MLow (Source: sources/2024-06-13-meta-mlow-metas-low-bitrate-audio-codec).

Why it's the comparison benchmark

From Meta's 2024 MLow announcement: "The last widely known, good open-source codec was Opus, released in 2012" — 12 years between widely-deployed general-purpose codecs. That longevity is why Opus is the reference point every new codec (including Meta's MLow) is measured against.

Known operating limits (per MLow announcement)

  • 6 kbps floor in NarrowBand mode (0–4 kHz). "Does not adequately capture all the sound frequencies produced by human voices — and so doesn't sound as clear or natural." NB vs WB distinction matters here: at 6 kbps, Opus is in NB, not WB.
  • 19 kbps minimum for inband FEC at 10% packet loss. Below this, Opus "is not able to encode any inband FEC". A 30% receiver-side packet-loss test at 14 kbps with no inband FEC is the operational blind spot MLow closes.

POLQA comparison (Meta 2024)

Codec Bitrate Mode POLQA MOS
Opus 6 kbps NarrowBand 1.89
MLow 6 kbps WideBand 3.9

MLow claims ~2× the Opus MOS at the same bitrate. See concepts/polqa-mos-metric for the metric's 1–5 scale.

Not discussed in the MLow source

  • Opus's internal SILK/CELT hybrid mechanics.
  • Opus DTX, VBR/CBR modes.
  • WebRTC integration specifics.

Seen in

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