Re: [Jack-Devel] Switch from CELT to Opus in JACK1/JACK2 sources

On 09/08/2012 01:36 AM, Jack O'Quin wrote:
> Some naive questions come to mind:
> 
>  * Why did Debian decide to stop packaging CELT?

- celt is no longer maintained and has been deprecated upstream.
- opus is the successor of CELT. CELT has been merged into Opus.

(CELT was likely dropped early to force people to adopt Opus)

>  * What are the advantages and disadvantages of CELT vs. Opus?

The problem we have is that libcelt comes with "--enable-custom-modes"
by default, but libopus does not -- Otherwise there are no disadvantages
of using Opus.

re "custom modes": https://wiki.xiph.org/OpusFAQ#What_is_Opus_Custom.3F
(they allow block-sizes other than multiples of 2.5 ms)

The main disadvantage of CELT is its limited bitrate range:
CELT: 32 kb/s to 128 kb/s
Opus:  6 kb/s to 510 kb/s

Opus adds support for VBR.  Bitrate, bandwidth and frame-sizes can call
be adjusted dynamically. The quality has apparently improved, too, but I
have not done direct comparisons nor measurements on that myself.
http://opus-codec.org/comparison/quality.svg
I only checked out the examples at http://opus-codec.org/examples/ --
well actually I attended Tim's presentation of it at FOMS earlier this
week where he played them.

>  * If JACK ends up packaging and distributing the library, anyway, is
> it an option to continue using CELT?

It is an option, but there is no gain - except that there'd be no new
code needed. Yet /upgrading/ from CELT to Opus is pretty much a
s/celt_/opus_/g and Opus performs much better (less CPU, better
compression or better quality at same bitrate).

robin

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