[M4IF Technotes] Re: AAC Conformance

[Ralph Sperschneider]

Ben Waggoner wrote:
> Ralph,
> 
>     Do I read this as saying that some AAC implementations could
> meaningfully decode a file to PCM at more than 16-bits, ala Dolby Digital?

Ben,
I am not involved into the details of AC3 (Dolby Digital), but for AAC this is 
definitely true. The accuracy depends on the rounding schemes used in the 
decoder. There are several points in the decoding algorithm where you most 
likely want to round the result of an operation. The most obvious one is the 
IMDCT filterbank. It usually puts out float values, which you will round to 
integer values in order to put them into a wave file. All reference waveforms 
are provided with 24 bit resolution, so you can easily test your decoder up to 
an accuracy of 23 bit.
Best regards,
Ralph
> 
> 
> Ben Waggoner <http://www.benwaggoner.com>
> Compressed Video Consulting, Training, and Encoding
> 
> My Book:            <http://www.benwaggoner.com/books.htm>
> Cleaner e-book:     <http://www.cmpbooks.com/cleaner>
> 
> Compression Classes at Stanford  June 30-July 4 and Aug 11-15
> <http://www.digitalmediaacademy.org/compression.html>
> 
> Compression Class in New York City, July 13
> <http://www.manhattanvideoworkshop.com/courses.html>
> 
> 
> 
> 
> on 6/11/03 6:20, Ralph Sperschneider at sps   iis.fhg.de wrote:
> 
>>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 ³RMS/LSB Measurement² test at an accuracy level of ³K bit², 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 ³RMS/LSB Measurement² test shall be carried out
>>for 
>>an accuracy level of K=16 bit unless a different accuracy level is explicitly
>>stated.
> 
> 
> 
> 
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-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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