[speex-dev] Speex modes

[Jean-Marc Valin]

> I'm about finished developing a QuickTime component that supports Speex 
> (on
> MacOS X and Windows).. As it is now the user can set complexity
> (SPEEX_SET_COMPLEXITY) and quality (SPEEX_SET_QUALITY /
> SPEEX_SET_VBR_QUALITY) and to wether to use VBR or not. Will these 
> options
> make it possible to produce all combinations of bitrates/qualities? Or
> should I also use SPEEX_SET_MODE/SPEEX_SET_LOW_MODE/SPEEX_SET_HIGH_MODE 
> to
> accomplish this?

The first thing to know that setting quality from 0-10 is in fact a more
user-friendly of setting the mode. That being said, for narrowband
encoding, all modes are available with at least one quality setting
(sometimes two quality settings point to the same mode because there are
less than 10 modes). For wideband encoding, not all possible mode
combination (one mode for the low-band, one for the high-band) are
available with the 10 quality settings, but those that aren't available
are mostly useless anyway (e.g. a combination that gives you very good
quality above 4 kHz, but very poor below that is useless). 

In most cases I would suggest not making the modes directly available,
unless maybe for "expert users". The only other place where it can be
useful is that modes have a specific bit-rate/quality associated to
them, while the mapping between the "quality settings" and the modes are
not guarantied to remain the same in the future. That being said, you
probably better keep what you have now. Hope this helps.

        Jean-Marc


-- 
Jean-Marc Valin, M.Sc.A.
LABORIUS (http://www.gel.usherb.ca/laborius)
Université de Sherbrooke, Québec, Canada


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