[FFmpeg-devel] 5.1 AAC on 0.5 branch

[Reimar Döffinger]

On Mon, Mar 09, 2009 at 06:39:11PM -0400, Justin Ruggles wrote:
> Pascal Patry wrote:
> > I've been testing a lot of content on the 0.5 branch for regressions directly in FFplay. I 
> > found that AAC audio using multiple channels don't work properly.
> > 
> > FFplay correctly sets 'request_channels' to 2, but it seems to still get 5 channels from 
> > these streams.
> 
> request_channels is only a request, which the decoder does not have to
> follow. whatever channels is set to is what the decoder should output,
> and that is currently done correctly by the aac decoder. the issue is
> that ffplay does not do any post-decoding dowmixing. ffmpeg uses the
> audio_resample() API in libavcodec to do post-decoding (well, actually
> pre-encoding) downmixing when it is supported.

request_channels is specifically for the case where the codec has
special, especially input-dependent, information about how downmixing
(or even upmixing) should be done, if the decoder can't do it any better
than an external filter can, doing it in the decoder is only bloat.