[Mp4-tech] [MPEG-4 Audio] Size mismatch when AAC-LC test vector is passed through HE-AAC decoder

[Andreas Schneider]

Hello Veenit,
the size-mismatch you found doesn't seem to be related to the SBR decoder. 
If I build the reference decoder without SBR the size of the output is not 
affected. So apparently the reference wave-files provided by ISO have been 
created with a decoder that removes the first two frames, though, frankly, 
I have no idea why one would want to do that. Maybe other audio experts 
can shed more light on this.
Best,
Andreas
mp4-tech-bounces lists.mpegif.org wrote on 18.05.2024 08:47:55:
> Hello Everyone,
> 
> I am working on the HEAAC profile (AAC-LC + SBR) of the MPEG-4 
> standard. I am using the Reference Code for the decoder provided by 
> ISO for this purpose. The version of the Reference Code is: ISO/IEC 
> 14496-5:2001/Amd.6:2005. The decoder that I am using is located in 
> the folder "mp4AudVm_Rewrite".
> To test the conformance of the "AAC-LC" part of the decoder, I am 
> passing a reference vector for the AAC-LC AOT that is given by ISO, 
> through the HEAAC profile decoder. It generates an output file whose
> size is larger than the size of the corresponding reference wav file
> that is provided by ISO. On viewing the data of the two files, I 
> find that the test data exactly matches the reference data after a 
> certain number of bytes are offseted from the start of the test 
> file. My queries are:
> 1. Why would the HE-AAC decoder generate a test output whose size is
> larger than the reference output, for the AAC-LC test vector. (the 
> size is not exactly double so this eliminates the possibility of 
> this happening due to SBR tool being enabled in the decoder ( since 
> SBR operates at twice the sampling freq of AAC))?
> 2. How can I eliminate this offset being written by the HEAAC 
> decoder. On removal of these offset bytes at the start of my test 
> output, my test output would be bit-match with my reference output.
> Thanking everyone in advance. Awaiting an early reply.
> 
> Regards,
> Veenit Vora.
> Patni Computer Systems Ltd.,
> Unit No. 19, SDF 7,
> SEEPZ, Andheri (E),
> Mumbai - 400096.
> Tel: +91-22-28291454. Ext: 5937
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--
Andreas Schneider
Senior Research Engineer
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