Subject: [AVT] Determination/indication of the number of H.263 macroblocks in a slice

From: Kris Huber <[email-masked]>
Date: Mon, 7 Jan 2002 19:46:56 -0700

Hello H.323 implementors and IETF AVT group,
 
I have a question about implementation of error resilience for video using
H.263's slice mode in the H.323 (or other RTP) environment.  Packets
typically consist of a slice, and when one is lost or too late, the lost
macroblocks should be concealed.  But knowing which macroblocks were lost
seems to require a bit more computation than you would think.  The address
of the last MB in the lost packet can be found quite easily by peeking at
the slice header inside the following packet's payload, but it seems that
VLC decoding of the entire H.263 slice preceeding the missing slice is
necessary to compute the address of the first MB of the missing slice.  Is
this true, or have I missed something?
 
Using the H.245 videoBadMBs command or videoNotDecodedMBs indication, the
range of MB addresses needs to be sent from the decoder to the encoder as
quickly as possible so it can code without dependencies an area that clears
the erroneous video.
One solution would be for the encoder to send the number of macroblocks in
each slice within the packet payload, but I think the payload is not
extensible in a standards-compliant way (is this correct?).
 
Is there another way that I may not be aware of to send the number of
macroblocks in each slice (H.245 messaging, perhaps, if not too slow?).
 
I believe the same thing is true of MPEG-4 video over H.323:  the packet
preceeding the lost packet must be decoded to know the address of the first
missing macroblock.  I suspect this is also true of MPEG-4 video over MPEG-4
systems as well.
 
Also, I vaguely recall reading of some standardization work to specify
packetization and codes for making low-latency reliable channels on IP
networks.  I'd be interested in a pointer to some information on the status
of that.
 
Thanks for any comments on this
 
Best regards,
Kris Huber
Senior Research Engineer
Sorenson Technologies, Inc.