[FFmpeg-trac] #929(avcodec:new): AAC 5.1 channel decoding issue

[FFmpeg]

#929: AAC 5.1 channel decoding issue
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             Reporter:  kaijun61    |                    Owner:
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  avcodec
              Version:  git-master  |               Resolution:
             Keywords:  AAC         |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
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Comment (by cehoyos):

 Replying to [comment:10 kaijun61]:
 > I assume ffmpeg application decodes aac, then downmixes it. If I just
 use aac codec as a plugin. it can't do downmixing. As you said, I need
 down-mixing routines to implement it externally.

 Don't you think that down-mixing will be generally needed, i.e. users
 might want a particular down-mixing that is not supported by any codec,
 how should this work without your application supporting FFmpeg's down-
 mixing routines?

 > Any particular reason not to implement it internally unlike other
 codecs?
 Yes, many:
 * This work has to be duplicated for every codec that supports
 multichannel encoding (and I suspect that for some lossless codecs, this
 will be difficult) and development time unfortunately is a limiting
 factor.
 * The general down-mixing code can easily be optimized for different CPUs,
 optimizing it codec-wise is significantly more difficult.
 * Assuming the down-mixing does not come for free, it means a performance
 penalty for users who do not want down-mixing.

 I suspect that all three points may not be relevant for AAC, but I do not
 know, otoh, I cannot believe that AAC - and TrueHD, AC-3 and DTS - are the
 only codecs for which GStreamer-users expect down-mixing. (Does GStreamer
 really not support down-mixing?)

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Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/929#comment:11>
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