Subject: question on video transcoder or gateway

From: wmshen <[email-masked]>
Date: Wed, 16 Feb 2000 11:08:28 +0800

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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From: Abdul Sadka <[email-masked]>
Date: Wed, 16 Feb 2000 10:08:21 +0000 (GMT)

Video transcoding between two DCT-based compression algorithms consists of
syntactical mapping of output video parameters from one bitstream to
another. For the case of ITU-T H.261 and H.263, the layering structure and
the syntax are very similar except for few differences in half-pixel
accuracy and motion compensation that make the parameter-mapping a bit
more complicated. For instance, H.261 uses one MV predictor to compensate
for motion in the current MB as opposed to 3 MV predictors in H.263
whereby one median predictor in both x and y dimensions is considered for
motion compensation. This will induce certain complications in the
associated video transcoding algorithm.

However, the main focus in heterogeneous video transcoding today is on how
to map the output bitstreams of 2 video coders which have completely
different layering structures and syntactical properties. For example, it
is not very straightforward to transcode from MPEG-4 to H.263 since MPEG-4
employs the arbitrary shape VOP structure as opposed to the full picture
MB-Grid in H.263. To achieve this mapping, you will need to map the VOP
defined in MPEG-4 onto its precise location in the picture area of H.263
and extrapolate the pixels that are off the contour of the VOP but
inside the tightest box that encloses the arbirary shape object.

Perhaps, to have an introductory step into the area, you could refer to
the following publication:

S. Dogan, A. H. Sadka, A.M. Kondoz
"Efficient MPEG-4/H.263 Video Transcoder for Interoperability of
Heterogeneous Multimedia Networks" 
IEE Electronic Letters, Vol. 35, No. 11, pp 863-864, 27th May 1999.

Regards.
_________________________________________
      Dr Abdul H. Sadka (Lecturer)       |
Multimedia Communications Research Group |
Centre for Communication Systems Research|
Uni. of Surrey, GU2 5XH, Surrey, England |
Tel:         +44 (01483) 259490          |
Fax:         +44 (01483) 259504          |
WWW: www.ee.surrey.ac.uk/Personal/A.Sadka|
_________________________________________|