[matroska-devel] Re: AAC profiles /matroska codec IDs

Christian HJ Wiesner chris at wiesneronline.net
Thu Feb 20 00:24:16 CET 2003


Roberto José de Amorim wrote:

> --- Christian HJ Wiesner <chris at matroska.org> escreveu: 
>  
>
>>Seems i have to create a few more codec IDs than the existing
>>2 :-( ??
>>    
>>
>
>That would be a wise idea. :)
>
>  
>
>>Right now we have :
>>
>>10. 'A_MPEG2_AAC' ; Audio, AAC, MPEG2 standard
>>
>>11. 'A_MPEG4_AAC' ; Audio, AAC , MPEG4 standard
>>
>>Of course Ivan Dimkovic was the one to clarify here, but i
>>doubt he 
>>would want to support matroska development with respect to
>>AAC audio, 
>>maybe he proves me wrong one day :D .
>>    
>>
>
>I don't know if he would _want_, my guess is that he doesn't 
>care.
>
>  
>
>>I guess the interesting question for us was the compatibiity
>>with 
>>existing en/decoders. For example it is said in the thread
>>that MPEG4 
>>AAC LC and MPEG2 AAC LC only differ by a few bytes in the AAC
>>header ( 
>>dont aks me though what the sense is behind differentiating
>>here 8)
>>    
>>
>
>Because one is for MPEG4 systems, the other is for MPEG2 
>systems. If it wasn't for this bit, MPEG4 extensions (LTP, SBR,
>LD...) wouldn't be possible.
>
>Actually, there are some decoders that only support MPEG2
>or MPEG4 LC. That even makes sense with MPEG2 AAC, since,
>in the possibility the decoder is old, it wouldn't recognize 
>the MPEG4 bit and would interpret it as corrupt frame. But
>supporting only MPEG4 AAC is indeed stupid. 
><cough>envivio<cough>
>
>  
>
>>and maybe we strip the AAC from the MP4 container in any
>>case, so 
>>we dont have to care at all ?
>>    
>>
>
>You don't have to only if you use exclusively your own 
>homebrewn MP4 demuxer, that would, say, always output MPEG4
>AAC headers. MP4creator, the most widely (the only?) MP4
>demuxer outputs MPEG2 or MPEG4 AAC files based on the object 
>descriptors inside the MP4 stream.
>
>  
>
>>The more i read that the more i get the intention we had to 
>>differentiate between
>>
>>AAC LC
>>AAC main profile
>>AAC SSR
>>AAc V2
>>AAC V3
>>    
>>
>
>SSR is pointless. Nobody uses it.
>AAC V2 and V3 makes no sense. These versionings are only used 
>by MPEG to define what's been added/changed in the standard.
>For example, the biggest novelty in V3 will be SBR.
>MPEG4 AAC LC V2 will be bit-identical to MPEG4 AAC LC V3.
>Same thing with LTP, Main, LD...
>
>  
>
>>and not care at all about the differences between MPEG4 and
>>MPEG2, as 
>>those seem to be onyl container related ? What do you all
>>think ?
>>    
>>
>
>These are indeed container related, but, as I said, there 
>are some stupid players that make a big deal about these
>differences.
>
>This is what I suggest:
>
>10. 'A_MPEG2_AAC_LC' ; Audio, AAC LC, MPEG2 standard
>11. 'A_MPEG4_AAC_LC' ; Audio, AAC LC, MPEG4 standard
>12. 'A_MPEG4_AAC_SBR' ; Audio, AAC SBR, MPEG4 standard
>13. 'A_MPEG_AAC_Other' ; Audio, AAC other profiles, both 
>standards.
>
>The reason is that MOST players support LC only (QuickTime, 
>Envivio, Ligos, Dicas MPEGable, Expanium, Philips...)
>Besides, most encoders output LC only (Quicktime, Ligos, 
>Dicas, Philips),and the ones that don't (FAAC, Psytel) output
>LC as default.
>
>SBR has it's own personal profile because it's "The Next Big
>Thing (tm)", and it's expected to be widely supported.
>And SBR is MPEG4-only, so no need to worry about 
>A_MPEG2_AAC_SBR
>
>I put "everything else" in it's own ID because these profiles
>(Main, LTP, LD) aren't widely used and there's only one 
>available decoder for them: FAAD.
>
>  
>
>>Roberto, would you care to clarify for our list ( cc :
>>rjamorim ) ?
>>    
>>
>
>What?
>
>Hope this helped.
>
>Kind regards;
>
>Roberto.
>
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>  
>


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