[M4IF Technotes] Re: AAC Conformance

[Ralph Sperschneider]

Hi Anil,
I guess you can theoretically state any number for k. However, numbers below 8 
might become rather meaningless with respect to the audio quality, and anyone 
might be able to have a compliant decoder at an accuracy level of 1 bit ...
I guess a proper statement would be
"ISO/IEC 14496-3 compliant decoder fulfilling the "RMS/LSB Measurement" test at 
an accuracy level of 10 bit (16 bit is default)"
Best regards,
Ralph
Anil Kumar wrote:
> Hi Ralph,
> 	Regarding conformance, is there a minimum limit to the value 'k'. I mean, how low a value of k is acceptable?? Also, say if I design my decoder for a 10 bit accuracy, then can i claim my decoder to be "ISO/IEC 14496-4 compliant decoder upto 10 bit accuracy" ??
> 
> Thanks,
> Anil
> 
> -----Original Message-----
> From: Ralph Sperschneider [mailto:sps   iis.fhg.de]
> Sent: Wednesday, June 11, 2023 6:50 PM
> To: Alan Thomson; Anil Kumar
> Cc: technotes   lists.m4if.org
> Subject: Re: AAC Conformance
> 
> 
> Dear Anil, Alan,
> 
> for AAC, there is no such thing than "Fixed-point accuracy". However, there is 
> an accuracy measure k for the two conformance tests measurements (RMS and LSB 
> test). On default, k=16. If you are designing a decoder which is conform to a 
> higher or to a lower level, you might (higher) / must (lower) specify this level 
> in your decoder specification.
> 
> This variableness has been introduced lately for both, MPEG-2 and MPEG-4 AAC. 
> You will find the details in the appropriate conformance corrigenda:
> 
>   * ISO/IEC 13818-4:1998/Amd.1:1999/Cor.1
>      (N5029, not yet published)
> 
>   * ISO/IEC 14496-4:2000/Cor.2
>      (N5042, not yet published)
> 
> There, you will find the following text:
> 
> RMS/LSB Measurement
> To fulfill the “RMS/LSB Measurement” test at an accuracy level of “K bit”, an 
> ISO/IEC 14496-3 decoder shall provide an output waveform such that the RMS level 
> of the difference signal between the output of the decoder under test and the 
> supplied reference output is less than 2^-(K-1)/sqrt(12).  In addition, the 
> difference signal shall have a maximum absolute value of at most 2^-(K-2) 
> relative to full-scale. The “RMS/LSB Measurement” test shall be carried out for 
> an accuracy level of K=16 bit unless a different accuracy level is explicitly 
> stated.
> 
> Best regards,
> 
> Ralph
> 
> 
> Alan Thomson wrote:
> 
> 
>>Not really.
>> 
>>There are "fixed-point accuracy" criteria (which are much less strict 
>>than the full accuracy one) but I believe they only apply to CELP & 
>>TwinVQ, not AAC.
>> 
>>Alan.
>>
>>    -----Original Message-----
>>    *From:* Anil Kumar [mailto:anil.kumar   ittiam.com]
>>    *Sent:* 10 June 2023 13:54
>>    *To:* technotes   lists.m4if.org
>>    *Subject:* [M4IF Technotes] AAC Conformance
>>
>>    Hi all,
>>        Is there any such thing as 'Limited Accuracy' conformance for
>>    AAC decoder similar to that of MP3 decoder conformance? If so, is it
>>    the same as MP3 Limited Accuracy conformance?
>>     
>>    Thanks in advance,
>>    Anil
> 
> 

-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
FhG IIS                         | Fax:   +49 9131 776 398
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