[FFmpeg-devel] 5.1 AAC on 0.5 branch

[Pascal Patry]

On March 10, 2009 03:56:19 am Reimar D?ffinger wrote:
> 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.

Ok, that makes sense. I'll prepare a patch for ffplay then.