[Mp4-tech] [AAC Audio] [ADTS]

[Alan Thomson]

Hi Will - 
Just guessing, but looks like your test data is ADTS but isn't AAC.
It's an older MPEG format. I don't have copies of 11172-3 and
13818-3 to hand, but I think the answer will be there.
Channel config of 0 is legal and means the info is elsewhere 
(e.g. program_config_element) or implicit.
cheers,
Alan.
IndigoVision Ltd.
> -----Original Message-----
> From: Will Bolam [mailto:w.bolam vqual.biz]
> Sent: 10 February 2024 17:25
> To: mp4-tech lists.mpegif.org
> Subject: [Mp4-tech] [AAC Audio] [ADTS]
> 
> 
> hi all,
> 
> i've recently been looking at implementing an AAC decoder, 
> and i now have
> this working (based on the ISO reference code), i'm currently 
> adding ADTS
> support. i've got it working and decoding the test streams from here:
> ftp://mpaudconf:adif2mp4@ftp.iis.fhg.de/g...io-conformance/ (the adts
> streams are in the mpeg2 aac folder)
> 
> however i am unable to decode any of the various test streams 
> we have from
> customers. the test streams i have look like ADTS - they have 
> the ADTS sync
> word (12 1's) every so often in the file (including at the 
> start), but the
> rest of the ADTS header doesn't seem right - the layer field 
> seems always to
> be 2, when all specs i've seen say this must always be 0.
> 
> example header that doesn't work is:
> 
> 1111 1111 1111 - syncword
> 
> 1 - mpeg 2/4 flag
> 
> 10 - layer
> 
> 0 - protection absent
> 
> 11 - profile obj type
> 
> 0001 - sampling frequency index
> 
> 0 - private bit
> 
> 000 - channel config (Q. - is 0 valid for ADTS channel 
> config? how else
> would it be calculated?)
> 
> 0000 - 4 copyright flags
> 
> 0010 1000 1111 1 - frame length
> 
> 1000 1010 101 - buffer fullness
> 
> 01 - num raw data blocks
> 
> does anyone know whether that is or isn't a valid ADTS 
> header? are there any
> other audio file formats that use 12 1's as a sync word?
> 
> any help much appreciated!
> 
> regards,
> 
> will
> 
> 
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