[Mp4-tech] Re:[Audio] LOAS/LATM streams

[Ralph Sperschneider]

Bes Julien wrote:
> Dear all,
>  
> I was looking for MPEG4 PES streams, and I finally found some on the 
> MPEGIF forum (http://www.m4if.org/resources.php -> MPEG-4 Audio 
> Conformance test sequences for download (ftp link!) 
> <ftp://mpaudconf:adif2mp4@ftp.iis.fraunhofer.de/> [MPEG, 03]).
>  
> This discovery has revealed some questions to me:
>  
> - In the standard, it is specified that LOAS sync word is 0x2B7, 
> stored on 11 bits. On the streams I downloaded, I couldn't find such 
> synch word, all of these streams beginning with 2E 6D ... How can I 
> recognize the LOAS sync word?
Dear Julien,
the file does not start with the sync word. Hence, the LOAS sync word 
needs to be recognized by a sync seeking process. Although this is might 
come unexpected, it is rather common in case of a streaming format. With 
regard to these sequences, the sync routine can be rather easy (always 
skip the first x bytes :-) ).
> What are these streams made of? What is their audio content (AAC, 
> AAC+SBR,...)?
The file content can be estimated based on the file name, see 
ftp://mpaudconf:adif2mp4@ftp.iis.fraunhofer.de/mpeg4audio-conformance/doc/fileNameConventions.html 
for details
>  
> - Moreover, I have read in the mailing list archives that LOAS/LATM 
> format was not compatible with ADTS header. Does it mandatory to have 
> AAC raw data blocks encapsulated in LOAS/LATM transport layer?
This question I do not understand. LOAS and ADTS are different streaming 
formats. How can they be more or less compatible? Anyway, the support of 
certain streaming formats or file formats usually depends on the 
application.
Best regards,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
Fraunhofer IIS                  | Fax:   +49 9131 776 398
Am Wolfsmantel 33               | mailto:ralph.sperschneider iis.fraunhofer.de
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