[FFmpeg-devel] [PATCH] swresample/rematrix: handle 22.2 as a 9 channel layout

[Jan Ekström]

On Sat, Aug 15, 2020 at 12:29 PM Jan Ekström <jeebjp at gmail.com> wrote:
>
> On Fri, Aug 14, 2020 at 8:36 PM Michael Niedermayer
> <michael at niedermayer.cc> wrote:
> >
> > On Thu, Aug 13, 2020 at 01:26:58AM +0300, Jan Ekström wrote:
> > > This is as far as 22.2 follows the same channel order as
> > > WaveFormatExtensible's channel mask (and the AV_CH_* defines).
> > >
> > > After LFE2 the side channels would follow, but that offset of
> > > one stops us from utilizing them without further tweaks.
> > >
> > > This change was verified by using swresample to downmix to 5.1,
> > > and then feeding that to WASAPI.
> > > ---
> > >  libswresample/rematrix.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > probably ok
> >
>
> Thanks for taking a look. I originally planned on sending a link to a
> test sample I created somewhere during the time attempting to validate
> the AAC decoder, which I then also utilized to check the result of
> this:
> https://megumin.fushizen.eu/samples/22.2_input_test_sample.wav
>
> ...but alas then I never got to it afterwards.
>

Aand once again hit the send button a bit too soon.

For context, the reason why I started poking at this a bit more is
because a friendly person elsewhere who implemented 22.2 to X downmix
on his side pointed out that
https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2159-8-2019-PDF-E.pdf
contained the downmix matrix to 5.1 (TABLE11, System H is 22.2).

So I planned on taking the same shortcut of doing 22.2->5.1->X, since
5.1 was already implemented or could be pushed to most audio output
systems, but after looking at rematrix for a bit I got that no such
shortcuts would be possible.

So now the plan is to attempt to see if:
1) I can work around the +1 offset for side left/right, thus creating
the base level 10.1 that seems to be implemented in rematrix.
2) Mix top/bottom channels into their matching base level, adding
support for them being mixed in.
3) Mix LFE2 into LFE1.

If this would work, we could get a downmix going by means of 22.2->10.1->X .

Jan