[Ffmpeg-devel] MP4: SBR in HE AAC - getting the right frequency?

Maxim limax
Tue Jan 10 11:10:16 CET 2006


Who is good enough in implementing retreival of the right frequency from MP4 
files with HE AAC? There is no checking for SBR currently implemented in 
Or, alternatively, one can use FAAD2 to get this information, since it is 
already implemented in it's libfaad/mp4.c file.

All I know about SBR, is that you have to check for additional info in MP4 
header on frequency or implicitly double all frequencies <= 24000 Hz if 
there's no explicit SBR info. All MP4 files coming from Nokia SymbianOS 
smartphones are with SBR, so converting them with ffmpeg produces twice 
longer sound with half frequency.

Currently I implemented dirty hack to double all frequencies that are 
<=24000. It works for me, but I don't think it is right for all MP4 files 
containing AAC.


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