[FFmpeg-devel] Decoder-independent multichannel downmixing

[Justin Ruggles]

James Lee wrote:
> I originally posted this as a question to ffmpeg-users ("Error
> converting 6 channel FLAC to stereo"), but it appears that this is a
> feature which simply isn't currently supported so I thought I'd post
> here to ask whether this is being worked on and offer to do so myself
> if not.
> [...]
> 2) Is channel ordering a worry here?  The way I envisaged it, the
> resample filter would assume standard wav channel ordering (i.e. FL,
> FR, FC, LFE, BL, BR) and it would be up to the decoder to output in
> this order irrespective of the internal channel ordering of that
> codec.  Is this how things work?  If not then where in the decoding
> process does this happen (eg. for normal 5.1 FLAC/AC3/DTS --> 5.1 WAV
> conversions)?

Channel ordering is one big issue.  Currently, all decoders send output
in native channel order for that codec.  I agree that requiring decoders
to output in WAVE/SMTPE channel order would be best, but it can be
tricky.  Especially with raw audio, the channel order can depend on the
container, so the PCM decoder will not know if or how to rearrange the
channels without access to that information.

I have tried in the past, but it didn't get anywhere.  I believe
Benjamin has also attempted this, and he may have a good solution.

See:
http://wiki.multimedia.cx/index.php?title=FFmpeg_audio_API

-Justin