question on video transcoder or gateway

[Gary Sullivan]

Weiming Shen et al,

The only real way to translate in either direction in general is to decode
(including IDCT)
and then re-encode.  This is because H.261 cannot represent the half-pel
motion
used in H.263, and H.263 cannot represent the loop filter in H.261. Unless
you can
somehow stop the original encoder from using features that aren't found in
both standards,
you're stuck doing real transcoding.  The re-encoding complexity
can be reduced significantly by re-using the motion vectors and
prediction mode selections from the incoming video rather than doing a new
motion
estimation in the re-encoder.  Since motion estimation and mode selection
can take
a lot of processing power, getting rid of some of that can help.

For best performance it would be good to have the re-encoder do a local
refinement of
the incoming motion vectors and to try different prediction modes.

There are standards for gateways and multipoint conferencing units.  I don't
think they
give much advice in them about this issue.  I don't recall which standards
those are,
but if you look at a list of the H-series standards list at www.itu.int
<http://www.itu.int>  you'll probably
spot them.

-Gary


-----Original Message-----
From: wmshen [mailto:wmshen at PUBLIC.WH.HB.CN]
Sent: Tuesday, February 15, 2000 7:04 PM
To: ITU-SG16 at MAILBAG.INTEL.COM
Subject: question on video transcoder or gateway


Hello, all experts,

I am a Ph.D student working on video communication. I am wondering about how
the video transcoder/gateway works between H.261 and H.263, and between IP
network and PSTN or ISDN, in paticular i wonder whether there is a method by
which bitstreams can be translated directly between H.261 and H.263 format.

Who knows the technique details on above topics or where may i get some
useful reference infomation?

Any help will be approciated.


Weiming Shen


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