[MPlayer-dev-eng] [RFC] 6-channel AAC and channel reordering

[Corey Hickey]

Giacomo Comes wrote:
> I have been working on extending the patches submitted some time ago
> by Alexander in order to support in the correct order 4/5/6 channels sound stream.
> 6 channels is easy because you can only have 3f+2r+lfe.
> With 5 channels there are more combinations: 3f+2r, 2f+2r+lfe, 3f+1r+lfe
> and with 4 channels: 2f+2r, 3f+1r, 3f+lfe, 2f+1r+lfe
> 
> Currently there is no way in mplayer/mencoder (and in libavcodec) to specify the number of
> front/rear/lfe channels, you can only specify the sum of them with the option -channels.
> For this reason the patches currently assume the following configurations:
> 6ch 3f+2r+lfe
> 5ch 3f+2r
> 4ch 2f+2r
> because this is what you get with libavcodec ac3 encoder.

I agree. Those seem to be the most common configurations, and for now I
don't think it's worth supporting others. It's definitely possible to
remix odd configurations into the closest common configuration, but,
without an abundance of samples or demand, I think it would complicate
the current effort. Perhaps in the future.

> The changes to ad_faad.c, ad_pcm.c, ae_lavc.c, ae_pcm.c and ao_pcm.c
> are just extension of the previous patches. Assuming that the original patches
> were correct, they should also be. The changes to ae_faac.c instead are completly 
> new, check them carefully.

Thanks. I'll take a look.

> I was planning to submit the patch before rc1 was released, but I didn't have the time.
> Also as Corey has noticed, channel reordering introduces some overhead.

Yes, but not a lot.

> May be we can add an option like -nochanreorder the tell mplayer/mencoder to not
> do channels reordering (keeping maximum speed in case it is required).

Perhaps.

> Corey, can you integrate the ideas of this patch in yours?

Yes, one way or another. If I can see how, I want to compare the
efficiency of doing the reordering within each ae_*.c vs. adding
af_channels to the end of the filter chain just like I'm currently doing
at the beginning.

...but not until I get home from work today.

Thanks again,
Corey