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[mpeg2] kfir latency revisited
- To: mpeg2@linuxtv.org
- Subject: [mpeg2] kfir latency revisited
- From: Bram Stolk <bram@sara.nl>
- Date: Thu, 8 Aug 2002 14:01:51 +0200
- Content-transfer-encoding: 7bit
- Content-type: text/plain; charset=US-ASCII
- Organization: Stichting Academisch Rekencentrum Amsterdam
- Sender: mpeg2-bounce@linuxtv.org
Hello,
I am trying to determine the latency of the BMK kfir card.
Rob Morris mentioned previously on this list, that the latency
is at least as long as the time between two I frames.
With GOPChop, I see that by default, the card generates GOPs
of 15 frames, like this:
IBBPBBPBBPBBPBB
So I expect a 15 frame delay.
However, experiments suggest a shorter delay.
What I did was modify the mpeg2dec.c sources to use a very small
buffer. Then I start mpeg2dec to read directly from /dev/video0.
The delay I see is approximately one third of a second.
However, as the stream is composed of 25 frames / second, and
the GOP size is 15, I would expect a delay of 15/25 = 0.6 sec.
Are the I/B/P frames 'half-pictures' instead of full pictures
consisting of only even or uneven scanlines?
This would explain the 1/3rd sec latency.
Also, I cannot get mplayer to read directly from /dev/video0
as mplayer first seeks to EOF to check filesize, which is not
appropriate for a device node.
Thanks,
Bram Stolk
--
------------------------------------------------------------------------------
Bram Stolk, VR Specialist.
SARA Academic Computing Services Amsterdam, PO Box 94613, 1090 GP AMSTERDAM
email: bram@sara.nl Phone +31-20-5923059 Fax +31-20-6683167
"I heard if you play the NT-4.0-CD backwards, you get a satanic message."
"Thats nothing, if you play it forward, it installs NT-4.0"
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