[Mp4-tech] MP4 file parser

[MRUKANT POPAT]

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hi Dave & community,
Thanx for your reply.
The 14496-1 spec says following about stsc atom:-The table is compactly coded. 
Each entry gives the index of the first chunk of a run of chunks with the same 
characteristics; by subtracting one entry here from the previous one, you can 
compute how many chunks are in this run. You can convert this to a sample count 
by multiplying by the appropriate samples-per-chunk.
query 1:-What does it mean by similar 
characteristics among chunks. How is it decided. 
query 2:-I am sending mp4dump of an mpeg-4 file. The stsc atom inside it 
compactly codes the table in stsc atom, in the sense each entry gives the index 
of the first chunk of a run of chunks with the same characteristics. Now looking 
at this file I concluded that->  1) There are three groups of chunk
  2) First group (say g1) having only 1 chunk
  3) Second group(say g2) having 352-1=351 chunks
  4) Third group (say g3) having 353-352=1 chunk. Thus total number of chunks is 1+351+1=353
  5) Now number of samples in all chunks can be found by samples-per-chunk in 
     stsc, so,      
     5.1) In g1 number of samples is 1*1=1
     5.2) In g2 number of samples is 351*2=702
     5.3) In g3 number of samples is 1*1=1
     5.4) Thus, total number of samples in file is 1+702+1=704
     query 2.1:- But the stsz atom which gives size of all the samples, lists only 655 sample size. whereas according to me it should list 704 samples. 
                 WHAT COULD BE THE REASON FOR THIS?
                 Also the stco atom which gives offset of all the chunks, lists 654 chunk offsets. whereas according to me it should list 353 chunks. AM I RIGHT. IF WRONG, WHERE AM I WRONG?
regards,
MRUKANT
On Mon, 23 Aug 2023 Dave Singer wrote :
>At 6:52 AM +0000 8/21/04, MRUKANT POPAT wrote:
>>hi all,
>>I have some queries on MPEG-4 file parser.
>>I have read chapter 13 of 14496-1 which describes MPEG-4 file format.
>>I also came across 14496-12 (ISO Base Media file format).
>>I have certain queries after reading both documents, which I hope you
>>guys can help me solve.
>>
>>query1:- I have understood that ISO Base Media file format is superset
>>for all media file formats. Am I right? If not how does it differ from
>>other media file format?
>
>This is such a general question I am not sure how to answer it.
>
>>
>>query2:- If I implement parser for MPEG-4 files, does it have to look
>>for atoms mentioned in 14496-12 (ISO Base Media file format) or the
>>atoms mentioned in chapter 13 of 14496-1.
>
>If you are looking at a full spec. in chapter 13, that pre-dates the making of part 12.  I['d suggest lookign instead at parts 12 and 14. Yes, you should look at all atoms mentioned in parts 12 and 14.
>
>>
>>regards,
>>MRUKANT
>>
>>
>>
>><http://clients.rediff.com/signature/track_sig.asp>
>>
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>
>-- David Singer
>Apple Computer/QuickTime