[CELT-dev] PLC Question & OPUS Migration?

Benjamin M. Schwartz bmschwar at fas.harvard.edu
Wed Jun 22 08:38:15 PDT 2011

On 06/22/2011 10:09 AM, Bob Bang wrote:
> My main applications for the codec
> will use a fixed frame and packet size, so I will not take much advantage
> of the dynamic nature of the codec. But are there any other benefits in
> terms of audio quality/latency/error robustness that OPUS will have over CELT?

If you are using frame sizes smaller than 10 ms, Opus is essentially
identical to CELT.  Opus also generally has about 2.5ms longer latency,
i.e. 15 ms latency on 10ms frames, instead of 12.5 ms with CELT*.

For speech, at rates below about 24 kbps (or 48 for stereo) and packet
sizes of 10 ms or greater, Opus should sound better than CELT.  Currently
you must tell the encoder whether you are coding speech or music, but the
developers are working on adding automatic content type detection to the
encoder.  The encoder would then dynamically switch between speech and
music coding modes while maintaining constant frame size and duration.

Also, even at fixed size and duration you could still benefit from Opus's
dynamic lowpass, which allows the encoder to choose which frequencies to
code for each frame.  This feature is also optionally available in CELT
starting in libcelt 0.11.3.  The encoder currently only uses this with VBR
mode, but a future encoder may also employ it in CBR.


*: If you use the libcelt API with the libopus codebase, you should still
get the lower CELT-style latency (2.5ms lookahead).

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