[Mp4-tech] MPEG4 Configuration packets

[Mohammed Tahir]

Hi Folks,
I'm trying to decode an mpeg4 stream from an IP camera. At the moment we
have an embedded mpeg4 encoder and decoder working. The configuration
packet is being sent separately - that' how the encoder is configured.
On the decoder side, I check the incoming RTP packet's first 4 bytes are
00 00 01 B6 and that the 2 upper bits of the 5th byte are 00 before
allowing the packets through to the decoder. This I'm told is the check
for an I-frame. I believe this is known as the VOP_START_CODE. This all
works fine. I would like to know what the remaining combination of upper
bits of that last byte refer to i.e. the 01, 10 and 11 combination.
The problem arises when I attach an IP camera to our decoder. It appears
that the mpeg4 encoder in this camera sends a configuration packet every
I-frame i.e. the configuration data is sent in-band. The routine that
detects the I-frame stops working because the location of the
VOP_START_CODE has moved. I have no idea where it is now. Also, I can't
quite decode the stream - the picture looks a bit of a mess. From what I
can find out from the information in the VOL, the decoder can pick up
the resolution of the incoming stream but not much more. The decoder
doesn't know, for example, whether it's PAL or NTSC. Is there a way of
finding out where the parameters are located in the incoming RTP packets
so that I could modify them? For example, is there s flag that says the
configuration data is in-band or out-band?
Also, during an RTSP session, there is a string called the 'config' e.g.
m=video 49170/2 RTP/AVP 98
a=rtpmap:98 MP4V-ES/90000
a=fmtp:98 profile-level-id=1;
config=000001B001000001B5090000010000000120008440FA282C2090A21F
Can this play a part as I believe this is user data? Is this in a
standard format.
I need to somehow get this IP camera to work with our decoder and I
don't where in the RTP packet the information I need is located.
Thanks
Mohammed
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