[MPlayer-dev-eng] [PATCH] Automatic downmix

[Nicolas George]

Le nonidi 29 thermidor, an CCXVIII, ubitux a écrit :
> > On the other hand, it seems that libavcodec's FLAC decoder does not respect
> > the libavcodec policy about channels layout, and outputs in Xiph channel
> > order (FL FC FR RL RR LF).
> What you said is strange: I just encoded 6_Channel_ID.wav with flac, and
> tried to play the generated 6_Channel_ID.flac with my patched MPlayer, and
> everything went fine: channels assignment were good.

Sorry, my mistake: I assumed that FLAC, being issued by the same
organization as Vorbis, would follow the same channel layout, but I was
wrong: Vorbis follows its own channel layout (FL FC FR RL RR LF) while FLAC
follows the SMPTE/ITU-R recommendation (FL FR FC LF RL RR). Since ffmpeg
uses the same SMPTE/ITU-R layout, the current behaviour is correct.

Now, regarding your initial problem:

>>> Also, I note that the channels assignements was wrong if I followed the
>>> FLAC specs (are they wrong or is there a convertion somewhere?). So I used
>>> those two samples in order to get the right assignement:

The answer is: there is a conversion somewhere, namely in
libmpcodecs/ad_ffmpeg.c, a call to reorder_channel_nch. Apart from bugs, all
audio circulating inside mplayer, in particular through audio filters, is in
the ALSA channel order: FL FR RL RR FC LF. The correct arguments for pan
should therefore be:

# in    FL      FR      RL      RR       FC        LF
pan=2: 0.4:0 : 0:0.4 : 0.4:0 : 0:0.4 : 0.2:0.2 : 0.2:0.2
# out   L  R   L  R     L  R   L  R     L   R     L   R

(spacing added for better readability; where did my "Alternate syntax for -af
pan" patch disappear?)


Regards,

-- 
  Nicolas George
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