[Live-devel] Very large P-frames in a recording

[Roland]

Hi Mike

 

Perhaps the P-frame is really that big when sent from the source?

 

I've used Wireshark (http://www.wireshark.org/) to capture the raw network
packets, then go to "Statistics", "RTP", "Analyse Streams..." to get an
insight what the source is *really* sending... E.g. it turns out that my
source was sending flawless audio, but the video stream had corrupted
packets every now and then...

 

Roland

 

PS: One way to rule out the "mash-together" theory could be to have a look
at the big P-frame with a hex-editor. Look for sequences like 00 00 01 xx.
These are start markers for MPEG-2. Make a list of these markers and look
them up in the MPEG-2 spec in regards to what they mean (e.g. xx==b3 is
sequence_header, xx==00 is picture_start, xx==01-af is slice_start). If you
see multiple picture starts or the same slice code repeat, then indeed the
frame is mashed together.

 

From: live-devel-bounces at ns.live555.com
[mailto:live-devel-bounces at ns.live555.com] On Behalf Of
mamille1 at rockwellcollins.com
Sent: Wednesday, July 30, 2008 7:58 AM
To: live-devel at ns.live555.com
Subject: [Live-devel] Very large P-frames in a recording

 


All 

We're working with an embedded system that receives an MPEG-2 transport
stream from a TI DSP. We've built an application that uses the live555
libraries to write this stream out to disk. Every once in a while a very
large P-frame, on the order of four times the size of a typical P-frame in
the stream, is written out to the file. It's very intermittent -- it might
happen once out of 6,000 frames. Has anyone else seen this happen? We're
beginning to dig into this. We're concerned that our application might be
mashing together several P-frames' worth of data, or that we are not using
the live555 library correctly. 

I've reviewed the last several months of the mailing list, but didn't see
anything that seemed relevant. 

Thanks for your help. 

-=- Mike Miller
Rockwell Collins, Inc.
Cedar Rapids, IA 

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