[Mp4-tech] Question about QuickTime Pro

[Mody, Mihir]

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. 
Regards,
Mihir Mody,
Mulmedia codecs group,
Texas Instruments India, Ltd,
Email : 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
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