[Mp4-tech] Re: Could 16-bit AAC decoder's PCM output pass RMS test of AAC LC conformance test

[Ralph Sperschneider]

Lijie Tang wrote:
> Hi, audio experts
> 
>  
> 
> I’m implementing the AAC LC conformance test on my 16-bit AAC LC 
> decoder. My decoder failed in case “al00_08.mp4” and “al00_08.mp4”. I’m 
> surprised to find that even FAAD float point LC decoder and ISO mpeg2 
> reference float point decoder failed in these two cases (both decoders 
> output 16-bit wav). Then I try to convert the reference 24-bit wav 
> “al00_08.wav” to 16-bit wav with rounding then test the RMS level of 
> this 16-bit wav, it still failed. I’m not quite sure if the failure was 
> caused by incorrect RMS calculation or the 16 bit AAC LC decoder could 
> never pass the RMS test.
> 
>  
> 
> My question is:
> 
> Does anyone know if a 16-bit AAC LC decoder has the possibility to pass 
> RMS test defined in ISO 14496-4? Is there any 16-bit third party LC 
> decoder that has passed the conformance test?
> 
> Or maybe my RMS calculation is incorrect. Is there any reliable RMS 
> testing tools?
> 
>  
> 
> Here is detailed info of my RMS test implementation:
> 
> RMS threshold for 16 bit is: 8.809666e-006
> 
> RMS level of FAAD 16-bit float point decoder is: 8.821491e-006
> 
> RMS level of ISO mpeg2 reference 16-bit float point decoder is: 
> 8.821491e-006
> 
> RMS level of 16-bit wav derived from reference 24-bit wav is: 8.821491e-006
> 
>  
> 
> Aac stream and reference 24-bit wav are all from ISO Conformance test CD.
> 
>  
> 
> Thanks.
> 
>  
> 
> Lijie

Dear Lijie,
the conformance criterion cannot be fulfilled if the decoder just outputs 16 bit 
per sample. To pass that test, at least 17 relevant bits need to be present, 
which usually requires to write 24 bit per sample. Therefore all reference test 
sequences are provided with 24 bit per sample.
Note: The criterion is designed to test the decoder, not the subsequent quantizer.
Hope this helps,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
Fraunhofer IIS                  | Fax:   +49 9131 776 398
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