[Mp4-tech] Question about QuickTime Pro

[Dave Singer]

At 2:54 PM +0530 8/20/04, Mody, Mihir wrote:
>Hi Ryan,
>
>As Sync atom is not mandatory for video stream in 3gp file, the best 
>way to locate information is find I frame information from video 
>elementary stream it self.  This will avoid verifying validity 
>of sync data given movie layer.

The sync atom is only optional in a 3GP file when all samples are 
sync samples (e.g. for I-frame-only coding).  It's not required to 
include the sync atom in this case.  Otherwise, it is required.  So, 
in the case where some samples are not sync samples, it is needed, or 
else the file mis-informs the reader that all samples are sync 
samples.
The best way to find sync points in any ISO family file is to use the 
sync sample table.  Whether QuickTime is strictly compliant here is 
debatable, perhaps, in the absence of a strict ISO file specification 
for storage of 263 in ISO family files.  However, QuickTime is not 
lying;  these samples are indeed effective sync points in the file.
If you choose to ignore what the file format is telling you, and try 
to answer the question another way, you will get a different answer. 
"Where are the sync points in this video?" -> the sync sample table 
answers the question.  "Where are the I-frames in this 263 stream?" 
-> scan the stream.
The user interface offers the possibility to manage the "key frame" 
rate (yet another term), not specifically the I-frame rate, so the 
file you get does correspond to the UI (I claim).
>
>Regards,
>Mihir Mody,
>Mulmedia codecs group,
>Texas Instruments India, Ltd,
>Email : <mailto:mihir ti.com>mihir ti.com
>Phone : +91-80-25099307
>
>-----Original Message-----
>From: mp4-tech-bounces lists.mpegif.org 
>[mailto:mp4-tech-bounces lists.mpegif.org]On Behalf Of Dave Singer
>Sent: Friday, August 20, 2023 1:04 PM
>To: Ryan Lei; mp4-tech lists.mpegif.org
>Subject: Re: [Mp4-tech] Question about QuickTime Pro
>
>At 4:57 PM -0400 8/19/04, Ryan Lei wrote:
>
>>Sorry for the mass email. I have this question for the QuickTime expert.
>>
>I am using QuickTime Pro to create 3gp files with H.263 elementary 
>streams in it. When I do transcoding
>I can specify the frame rate and I frames interval. After I create 
>the 3gp file and open it in the analyzer, I can
>See there is a SyncSampleBox, which tells me which frames are I 
>frame. However, when I extract the H.263
>Elementary stream out from the 3gp file, and open the elementary 
>stream in the analyzer, it tells me only
>The first frame is the I frame, all other frames are all P frames. 
>If I open the stream in a Hex Editor and check
>the frame start code of H.263 stream, it actually shows that there 
>is only 1 I frame. Can somebody tell me what's
>going on.
>
>
>Well, this isn't really an MPEG-4 question at all, since it's a QT 
>and H.263 question.
>
>The frames you are looking at are independently decodable (i.e. they 
>do not use prediction), and are thus valid 'sync' points in the 
>stream.  For various reasons, they are coded using P-frame syntax. 
>Hope this helps.
>
>>
>Ryan Lei
>ATI Technologies Inc.
>Handheld Products Team
>33 Commerce Valley Dr. East
>1-905-8822600 ext 2712 (o)
><>www.discover.uottawa.ca/~leizj/
>
>
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>
>
>--
>David Singer
>Apple Computer/QuickTime

-- 
David Singer
Apple Computer/QuickTime
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