RE: H.261 video problems - fixed

Mark,

> - with modern video endpoints at least. Any "good" encoder should
> generate packets where the media payload starts with a GOB/Slice
> header anyway and in this case the extra information in the rtp
> header is mostly redundant

True, but impossible with plain H.263 (still used by most endpoints out there,
and all 3G mobile phones) unless you can accept packets of any size, even
bigger than the MTU and thus fragmented by TCP/IP.

H.263+/Slice, MPEG4 and H.264 certainly do provide for what you described
above.

> The most important way (IMHO) to cope with packet loss on the
> internet is for the decoder / receiver to implement an algorithm
> that uses flow control messages to dynamically lower the bit rate
> that the far end (encoder) is transmitting at in response to packet
> loss / network congestion. This makes a HUGE difference in real
> world performance.

Yes, I agree.

> The Tandbergs & Polycoms (and Codians!) of this world all do both
> of the above.

So does my humble video softphone! :)

> Another point is that for compression efficiency I don't think the
> H.263+ annexes are relevant any more - since all the major vendors
> have moved to H.264 which is much better and much cleaner to
> implement.

And jumped over MPEG-4 for probably good reasons. Except that it is MPEG-4
being available in 3G mobile phones today (with baseline H.263).

> The bits of H.263+ that are really useful are the parts that
> introduce new features : e.g. the custom picture formats (which
> everyone supports) - for instance encoding at the native resolution
> of the camera (perhaps 352x240) rather than scaling / clipping and
> encoding at CIF is a big win. Also the use of H.263+ (and ++)
> annexes N+W or U+W to implement 60 fields per second interlaced CIF
> (all the main vendors support this).

I have seen some Tandbergs (the point of my earlier reference) that only
support Modified Quantization in their H.263+ implementation.

Thanks for your comments. Coming from an expert (you really coded H.261, H.263
and H.264 all alone?), it is a treat.

Guilhem.


                
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