[FFmpeg-devel] RFC: ADTS for AAC
Thu Dec 18 00:52:12 CET 2008
The FFmpeg ADTS situation is a mess right now. This message covers a
lot of different issues so I've broken this up into sections with
1. Broken ADTS muxer (Issue 751)
When channel_configuration == 0, the ADTS muxer omits the necessary
PCE. There are a few ways to get the PCE to the ADTS muxer, we could
try to pull the start and end offsets of the PCE from the MP4 demuxer.
We could read in the PCE and write it back out. All the
AudioSpecificConfig/GASpecificConfig is private in aac.c and contains
some initialization crap. It needs some refactoring. Possibly to be
moved to another file depending on how we wand the dependency chain
for ADTS muxing to work.
2. Stop gap for broken muxer
Until someone fixes ADTS muxing should we fail or spit out a warning
when someone tries to write out an ADTS file with
channel_configuration = 0?
3. PCE equivalence
In some cases the PCE is equivalent to a channel_configuration > 0 and
can be converted. Should we attempt to do this at all? In my opinion
it is certainly more idomatic ADTS and makes operations like cutting
and joining simpler. It could be implemented as a bitstream filter I
4. Shared code with AC3
As far as ADTS decoding goes, I was told that should go directly in
aac_parser.c and aac.c. But there is some code from ac3_parser.c/h
that needs to be moved up to aac_ac3_parser.c/h. Should that be done
in an earlier patch?
5. Header representation
As far as representing the the data in the ADTS header AC3 uses a
struct called AC3HeaderInfo. AAC could use an AAC version of that or
it could use an extended version of MPEG4AudioConfig from
6. Broken old files created by FFmpeg
In regard to the way we handle the broken files that our ADTS muxer
has been spitting out up until this point. The specification says that
we should ignore all data before the first PCE (aka ignore all data)
when in MPEG4 mode. However, mp4audec_mc from 14496-5 decodes it
7. Other notes
In general, I question the attitude that ADTS is the one true AAC
elementary format. Eventually I'd like to see FFmpeg be able convert
LATM, ADTS, ADIF, and MP4.
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