[M4IF Technotes] Re: Question on AAC object type

[Ralph Sperschneider]

Hans-Juergen Bardenhagen wrote:
> On Wednesday, 25.06.03, 10:56 Anil wrote:
> 
> 
>>1. Can I get an ADTS or ADIF as the transport layer for an MPEG-4
>>stream?? (i guess it is yes, but just to confirm).
> 
> 
> Not quite, because ADTS and ADIF are the header types of an AAC file  
> (not a "transport layer" in the usual sense like e.g. HTTP or RTSP) that  
> has not been multiplexed to a MP4 container yet. Inside this container  
> the raw AAC bitstream does not have any headers anymore, because these  
> informations (MPEG-x version, bitrate etc.) are handled by the MP4 file  
> header then.

Maby some more information:
- adts ( audio data transport stream ) is intended for transport. It was 
originally considered as become a "layer 4" (beside layer[123] as specified in 
ISO/IEC13818-3).
- adif ( audio data interchange format ) is intended for storage (it has just 
one header at the beginning, followed by the raw_data_stream())
Both formats are specified in MPEG-2 and MPEG-4. There are some extensions in 
MPEG-4 to allow the signalling of AAC LTP in addition to AAC Main, AAC LC, AAC 
SSR and to distinguish between MPEG-2 and MPEG-4. However, these are AAC 
specific formats.
For MPEG-4 Audio transport it is recommended to use LOAS/LATM (low overhead 
audio stream, low overhead audio transport multiplex).
All these formats are specified in ISO/IEC14496-3.
Best regards,
Ralph
>>2. What is the object type index for AAC LC in case of MPEG-2 and
>>MPEG-4. Does this change depending on the transport layer(ADTS or ADIF
>>or GA). 3. How do I distinguish b/w MPEG-2 and MPEG-4 in case of ADIF/
>>ADTS (Is it the bitstream_type and adts_id) 4. Where can I get a
>>complete documentation of all these things??
> 
> 
> A short overview of the ADTS (and ADIF) header structure is available in  
> the Wiki of Audiocoding.com:
> 
> http://www.audiocoding.com/wiki/index.php?page=ADTS
> 
> If you need more informations than this, you can download the  
> specifications of MPEG-2 and/or MPEG-4 AAC from the MPEG homepage or the  
> MPEG Audio Subgroup (links are mentioned in the Wiki, too). This also  
> refers to the question of the MP4 file format, by the way.
> 
> ZZee ya, Hans-Jürgen
> 
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Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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