[Mp4-tech] Looking for general information on AAC

[Andreas Schneider]

Hello James,
mp4-tech-bounces lists.mpegif.org wrote on 17.08.2023 18:09:20:
> To all,
> In brief:  I'm looking to obtain more information on HE-AAC 
> (reference code ISO/IEC 14496-5:2001/Amd.6:2005), the extend of what
> is contained there and what I can expect it to do. Right now I'm 
> running the conformance bitstreams from the publicly available standard: 

> ISO/IEC 14496-4:2004/Amd.8:2005(Information technology -- Coding of 
> audio-visual objects -- Part 4: Conformance testing -- Amendment 8: 
> High Efficiency Advanced Audio Coding, audio BIFS, and structured 
> audio conformance). 
> 
> Some of the conformance test streams are not passing, and some do 
> not s"line up":
> For example(1), I'm not sure why this .mp4 has four corresponding .
> wav files associated with it:
> sbr_conftool al_sbr_cm_48_4.mp4 al_sbr_hq_cm_48_4_f00.wav
> 
>  al_sbr_hq_cm_48_4_f00.wav
>  al_sbr_hq_cm_48_4_f01.wav
>  al_sbr_hq_cm_48_4_f02.wav
>  al_sbr_hq_cm_48_4_f03.wav

The bitstream you mention contains 4 channels. Each of these wave-files 
contains the decoded output of one of these.
> 
> (2) With this one I don't get a CONFORMANCE PASSED message.
> 
> sbr_conftool al_sbr_sr_16_2_fsaac08.mp4 al_sbr_hq_sr_16_2_fsaac08.wav
> 
> (Informative) QMF-TEST: Max Diff: 0.040 RMS Diff: 0.009
> ===  SBR-TEST: Max Diff: 0.072 RMS Diff: 0.011

This conformance bitstream has no hard conformance criterion.
> 
> More information that would be helpful would be: what the file name 
> letter combinations symbolize, what the directory SBR_CONFORMANCE_3 with 
the 
> _lp_ files is, does the ISO/IEC conformance standard that ISO sells 
> have documentation that the publicly available one does not have.

Yes, all your above questions are answered in ISO/IEC 
14496-4:2004/Amd.8:2005.
> 
> Also, I am under the impression that in order to have an official 
> conforming decoder, one must pass an SBR and an AAC conformance 
> test.  If I am correct and the aforementioned constitutes the SBR 
> conformance test, does anyone know where to find the plain AAC 
> conformance bitstreams.

You are correct. They AAC-LC conformance bitstreams are here:
http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_14496-4_2004_Conformance_Testing/
There name is al*
> 
>  Any documentation on any of  this would be invaluable.

The documentation for AAC-LC conformance is available from ISO as well. It 
is part of the 2nd edition of MPEG-4 conformance ISO/IEC 14496-4:2004.
Best,
Andreas
> Thanks
> 
> James
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--
Andreas Schneider
Senior Research Engineer
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