[MPlayer-dev-eng] [BUG] surround audio plugin broken

[D Richard Felker III]

On Tue, Jan 07, 2003 at 08:17:25AM +0800, Anders Johansson wrote:
> > Hi,
> > 
> > > > maybe we should change all codecs to output in their native formats only,
> > > > and left all kind of downsampling, float2int etc to libaf.
> > > > this way we lose conversions helped by the codecs (examples: liba52's
> > > > float2int, channels downsampling, mp3lib's half or quarter freq decoding,
> > > > mono decoding of stereo sterams, stereo decoding of mono streams).
> > > 
> > > This seems like something of a negative to me, especially with ac3
> > > decoding...but I'm not sure how it works now. If there are only 2
> > > output channels but the stream is 5.1 ac3, does liba52 really decode
> > > all the channels and then downmix them, or can it just skip the
> > > surround stuff and decode only l/r for better performance? If it can't
> > > do that, it seems like a really desirable feature, especially since
> > > ac3 decoding is so damn slow to begin with.
> > 
> > liba52 is very clever...
> > it downmixes channels in freq space (with coeffs) and the imdct only
> > the requested number of channels.
> 
> That means that if the highest frequency bins were set to 0 then
> downsampling could be run without filtering... It would probably be
> best to integrate that into liba52 however.

IMHO it would be nice to add one or more frequency-space audio formats
and modify the native codecs to output such formats. Then resampling
would become a non-issue since you can pick whatever sample rate you
like when you reverse the transform.

Rich