[Mp4-tech] Re: [Audio] Re: SBR decoding

[Andreas Schneider]

Hello,
mp4-tech-bounces lists.mpegif.org wrote on 23.01.2024 06:23:43:
> Hi, Andreas
> 
> Thank you very much for your answer.  May I ask you my question in a
> slight different way: is it legal (according to MPEG-4 standard) to 
> have an AAC+ bit stream in which SBR does not appear all the time, such 
as
> 
> <<AAC>> ... <<AAC>> <<AAC><SBR>> ... <<AAC><SBR>> <<AAC>> ... 
> <<AAC>> <<AAC><SBR>> ... <<AAC><SBR>> 

no, such a bitstream would not be legal. At least this is the way I read 
section 4.5.2.8.2.2 "SBR extension payload for the audio object types AAC 
main, AAC SSR, AAC LC and AAC LTP".
Kind regards,
Andreas
> 
> If so, does the spec define the sampling frequency of such an AAC+ 
> bit stream, or it depends on decoder's implementation?
> 
> Thanks again.
> 
> Sincerely
> Yueshi
> 
> 
> On 1/18/07, Andreas Schneider <Andreas.Schneider codingtechnologies.com
> > wrote: 
> Hello Yueshi,
> 
> please see below.
> 
> "Yueshi Shen" < shenyueshi gmail.com> wrote on 18.01.2024 00:49:02:
> 
> > Dear Andreas and Devial,
> >
> > May I add one comment to Andreas' reply: in 14496-3:2005, subpart 4,
> > 4.5.2.8.2.1, page 105, it says:"Prior to SBR decoding, a SBR header 
> > part must be present. As long as no SBR header part is present, the
> > SBR decoder performs upsampling and delay adjustment only. In
> > continuous broadcast applications, SBR extension data elements with 
> > an SBR header part are typically sent twice per second. "
> >
> > Moreover, I wish to ask another question: If there's a situation
> > that an AAC+ decoder has already seen SBR headers, a new SBR comes 
> > along and such a SBR makes the decoder perform upsampling only, as
> > if no header has ever been seen.  This will be useful when we insert
> > blank audio frames into the middle of an AAC+ bitstream, as we know 
> > a blank AAC+ frame without SBR will be decoded at a lower sampling
> > frequency.  How can I construct such a "upsampling-only" SBR?
> 
> Such a situation can not really occur, at least not in a bitstream that 
is 
> fully standard compliant.
> If an SBR header was present once, then this very SBR header conveyed 
the
> settings that must be used to decode the SBR bitstream. These settings
> will stay in effect upto the point in time when a new - potentially 
> different header arrives.
> If the new header is valid, then the decoder will not switch to 
upsampling
> but uses this header instead.
> If the new header is not valid, the decoder may do anything, as there is
> no normative behaviour for such a case. 
> 
> Regards,
> 
> Andreas_______________________________________________
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--
Andreas Schneider
Senior Research Engineer
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