Home cinema - Different Broadcast resolutions

"Peter Hawkins" wrote in message
...
I capture a lot of Freeview stuff on a PC. Some of it I then edit and
burn to DVD to keep. I have noticed that some are 720x576 whilst some
are 704x576 which causes probs for authoring programs unless I
re-encode or patch the headers.

Why do they do this?

720x576 is the "slightly wider than 4:3 or 16:9" digital TV standard.
702x576 is the ACTUAL 4:3 or 16:9 digital standard that precisely matches
the existing analogue 52us active / 64us total line standard of 625. 704 is
the nearest MPEG2 compatible multiple to 702 (as MPEG2 is block based you
can only run at resolutions of a multiple of the block size)

720 is used internally by most bits of broadcast kit (but not all - some
early stuff still runs at 702x576 - annoying if you then DVE it down and
have to crop the edges to get rid of the black lines) The extra 9 samples
either side are there to allow for small timing errors, and ensure that
passing a signal between analogue and digital devices doesn't cause cropping
or "ringing" which might be the case if all kit worked at 702x576.

Some broadcasters allow 720x576 to continue through the broadcast chain all
the way to the final MPEG2 encoding. Others only MPEG2 encode the 704x576
portion (as this is a marginally smaller image) as this requires fewer
samples to be broadcast, and thus a slight reduction in transmission
bandwith, or a marginal improvement in picture quality.

544x576 is also used by some DVB-T and DSat broadcasters to save even more
bandwith. (ITV News Channel and QVC on Freeview were like this last time I
looked - which was a while ago). ITV1 on DSat is as well I think.

Steve