[Mp4-tech] [Audio] Non-sbr streams in HE-AAC decoder

[Andreas Schneider]

Hello,
> According to HE-AAC spec, the max AAC sampling frequency for non-SBR
> streams (AAC only streams) for levels 2/3 decoder is 48kHz .
> (Table 1.8A of N5570 doc)
> 
> 1) Is my understanding right? Why is such a restriction imposed? 

Yes, your understanding is correct. This restriction has been imposed to 
limit the requirements of such a decoder.
Decoding an AAC bitstream with a sampling rate of 96 kHz requires 
(approx.) twice the computational ressources than decoding a 48 kHz AAC 
bitstream. So by restricting what the decoder has to support to 48 kHz, a 
decoder manufacturer may use a slower and potentially cheaper chip while 
still being compliant to HE-AAC Profile and AAC Profile Level 2.
> 
> 2) If an HEAAC decoder, decodes AAC-only streams with sampling freq 
> greater than 48kHz, will the decoder be considered not compliant to 
> HEAAC spec? 

No, the decoder will still be compliant to Level 2 of the HE-AAC Profile.
The standard has the following statement:
"[...]Hence, a decoder supporting the High Efficiency AAC Profile at a 
given level can decode an AAC Profile stream of the same or a lower level.
"
Hope that helps,
Andreas
> 
> Regards, 
> Hari
> 
> _______________________________________________
> NOTE: Please use clear subject lines for your posts. Include [audio,
> [video], [systems], [general] or another apppropriate identifier to 
> indicate the type of question you have.
> 
> Note: Conduct on the mailing list is subject to the Antitrust 
> guidelines found at http://www.mpegif.org/public/documents/vault/mp-
> out-30042-Antitrust.php

--
Andreas Schneider, Research Engineer
Coding Technologies GmbH
Deutschherrnstr. 15-19
90429 Nuernberg, Germany
phone: +49 (0) 911 92891 -26
fax: +49 (0) 911 92891 -99
mailto:snd CodingTechnologies.com