[M4IF Technotes] questions about mpeg-4 file format

[Dave Singer]

Richard has it perfect for the media data.  The access units are 
stored in the mdat in the placesyou compute from the chunk offsets 
and sample sizes, and the sample sizes give you, um, the sizes.
The SL headers are not stored in the file.  There is a pre-defined 
value of the SLConfig (2) that is used in the file format.  It is 
expected that any hinter/streamer that delivers the data using SL 
headers will replace this with an appropriate config that matches 
what they do.  The pre-defined value means essentially 'you're in a 
file, the info that would normally be in SL headers is in the 
meta-data'.
At 09:10 -0600 8/29/02, Richard Shields wrote:
>Hi xin chen,
>
>The mdat atom can contain any data in any order you see fit.  There 
>is no "framing" data stored in the mdat and thus it has no 
>structure.  It is the meta data in the moov atom that gives the data 
>in the mdat atom structure but only indirectly.  Typically you would 
>write video or audio samples consecutively in decode order into the 
>mdat but there are no rules stating that you have too.  You have 
>both audio and video tracks it's advantageous to write them in an 
>interleaved fashion.  The moov atom will indicate to the player how 
>to locate the samples.
>
>SL Layer... I'll let the more educated among us to answer this one :-)
>
>-Richard
>
>On Thursday, August 29, 2002, at 06:11 AM, xin chen wrote:
>
>>Hi All,
>>I read the related part in ISO/IEC 14496-1, and
>>quicktime file format, but they are all concentrate on
>>the meta-data part, that is the structure of moov
>>atom. I want to know more about how the samples are
>>stored (in mdat atom, right). Where to get documents?
>>
>>And about the SL layer, since the SL packet hearder is
>>composed according to the SLconfigdescriptor, which is
>>stored in ESDescriptor, my question is: the SL packet
>>hearder are generated only when the mpeg4 file is
>>transmitted, or is it stored already in the .mp4
>>(Because on the figure 1 of ISO/IEC 14496-1, the
>>terminal get a file from storage medium will need to
>>go through deliver layer and sync layer for decoding
>>too.)? I think it's a waste to store the same inf.
>>twice.
>>
>>Thank you very much.
>>
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-- 
David Singer
Apple Computer/QuickTime