[FFmpeg-devel] [PATCH] aacdec: add support for dual mono in Japanese DTV

[Akihiro TSUKADA]

(2012年08月24日 11:26), Michael Niedermayer wrote:
>>> so there is no way to distinguish dual mono from stereo and the user
>>> has to set this for each program she wants to watch manually ?
>>
>> There is, at the level of MPEG TS system.
>>> so there is no way to distinguish dual mono from stereo and the user
>>> has to set this for each program she wants to watch manually ?
>>
>> There is, at the level of MPEG TS system.
>> Each audio PES is tagged by PMT and
>> EIT describes the mapping from tag to encoding parameters
>> including channel configuration.
>> So those parameters are defined per program/show and
>> can change over time. (PMT is usually fixed and does not change).
>> TV receivers track EIT of the current station (TS, programID) and
>> output either channel (defaulting to the left) or
>> switch among them on user's request.
>
> Is this information available to the TS demuxer ? If so it could
> maybe be passed to the decoder via side data on AVPackets, this would
> avoid the need to manually set it

TS demuxer cannot know at packet accurate level when a program/show
starts or ends, nor at which AAC frame dual mono starts or ends.
Thus, decoder is expected to detect dual mono and not do decode
both channels without the (inaccurate) designation from demuxer.

It may be possible that demuxer can automatically select
which dual mono channel to decode, for example, based on the user's
language preference, but still it is required that users can change
the selected channel when they want it.
Every Japanese TV has a "switch audio" button,
and people occasionally choose to change  audio to English version.
---------
  akihiro