[Mp4-tech] [HE-AAC]understanding of AAC+ conformance test

Mon Apr 14 11:54:22 EDT 2008

Hi Andy,
There are two aspects involved here:
One is that the AAC conformance spec allows you to define against which criteria you want to test your decoder. This can be done by selecting the parameter 'k' in the equations that are used to determine the conformance thresholds. So one way to approach this is to set the k-parameter to 15, which results in conformance thresholds that can be met with 16 bit PCM output. The value of k that was used for the conformance test must be mentioned when claiming AAC conformance (, unless you set k to the default value of 16, in which case 16 bit PCM output won't suffice for conformance testing).
The other aspect is that the conformance point, i.e. the signal that is considered for conformance testing is the actual output of the AAC decoder after the overlap-add of the inverse transform but before a quantization to 16 bit PCM. MPEG added a clarifying statement regarding the location of the conformance point to the AAC conformance standard in the same document that specifies SBR conformance (ISO/IEC 14496-4:2004/FDAM 8:2004).
Best,
Andreas
Andreas Schneider
Senior Research Engineer
Dolby Germany GmbH
Deutschherrnstr. 15 - 19            
90429 Nuernberg, Germany           
http://www.dolby.com
+ 49 911 928 91 - 26 (phone)
+ 49 911 928 91 - 99 (fax)
mailto:Andreas.Schneider dolby.com
HRB 17557, Amtsgericht Nürnberg,
GF: Dipl.Ing. Martin Dietz (Managing Director)   
> -----Original Message-----
> From: Andy Quan [mailto:zhengyuan.quan gmail.com] 
> Sent: Donnerstag, 10. April 2008 04:50
> To: Schneider, Andreas
> Cc: mp4-tech lists.mpegif.org
> Subject: Re: [Mp4-tech] [HE-AAC]understanding of AAC+ conformance test
> 
> Hey Andreas,
> The explanation is crystal clear. Thank you.
> And here comes another question. As I remember, some early 
> discussion of LC decoder in this forum says that there is no 
> way for a 16-bit output PCM to pass RMS conformance test in 
> LC suite as the reference is an independent 24-bit sound 
> file. If this is true, how can we judge the compliance of 
> this kind of LC decoder? As we all know, most LC decoders 
> available today provide 16-bit output rather than 24-bit ones.
> 
> On 4/8/08, Schneider, Andreas <andreas.schneider dolby.com> wrote:
> > Hi Andy,
> >
> > If you want to call your decoder ISO compliant, it must 
> produce an output signal that satisfies the criteria for max. 
> diff and RMS as defined by ISO. The conformance document 
> actually tells you to apply these thresholds in the column 
> "test procedure" of the table that defines the conformance bitstreams.
> > So any decoder that claims that it is fully ISO compliant 
> effectively claims that it passes all these conformance tests.
> >
> > Andreas
> >
> > Andreas Schneider
> > Senior Research Engineer
> >
> > Dolby Germany GmbH
> > Deutschherrnstr. 15 - 19
> > 90429 Nuernberg, Germany
> > http://www.dolby.com
> >
> > + 49 911 928 91 - 26 (phone)
> > + 49 911 928 91 - 99 (fax)
> > mailto:Andreas.Schneider dolby.com
> >
> > HRB 17557, Amtsgericht Nürnberg,
> > GF: Dipl.Ing. Martin Dietz (Managing Director)
> >
> > > -----Original Message-----
> > > From: mp4-tech-bounces lists.mpegif.org 
> > > [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Andy Quan
> > > Sent: Dienstag, 8. April 2008 15:36
> > > To: mp4-tech lists.mpegif.org
> > > Subject: [Mp4-tech] [HE-AAC]understanding of AAC+ conformance test
> > >
> > > I am recently working on AAC+ ISO conformance test(SBR,
> > > 14496-4) for a fixed point decoder. One thing that keeps 
> puzzling me 
> > > is what the significance of passing conformance test is.
> > >
> > > As the whole suite of conformance test compasses many 
> different test 
> > > streams with different accuracy criteria of both MaxDiff and RMS 
> > > threshold, failure in any of them leads to being "not passed" in 
> > > this "conformance test". However some of these test cases are not 
> > > meant for syntax checking but for computational accuracy. 
> As we all 
> > > know, fixed point decoder prioritizes performance and 
> usually puts 
> > > computational accuracy a little behind.
> > >
> > > In such cases, i mean if it can not pass all CT tests, 
> can we still 
> > > OFFICIALLY call it "ISO compliant decoder"? (I would rather take 
> > > "compliant" as "compliant syntax parsing") I find a lot of 
> > > commercial and embedded dedicated AAC+ codecs claim themselves 
> > > "fully compliant to ISO". Does this mean that they can 
> pass this ISO 
> > > conformance test suite?
> > >
> > > Any one got comment? Thanks in advance.
> > >
> > > --
> > > Regards,
> > > Andy Quan
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> 
> --
> Regards,
> Andy Quan
>