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Old 16th Jan 2009, 12:31 pm   #6
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: 625-Line Television Broadcast Standards

Thanks to all for your responses.

ppppenguin: thanks - yes, I should probably have entitled the thread “625-line Television Broadcast Transmission Standards”, but I am not sure if I can amend it now. Initially anyway, I was focusing on the basic monochrome systems; the permutations and combinations start building up when one adds the various colour systems, and then the diversity of stereo sound systems.

I have used Alan Pemberton’s excellent site at: http://www.pembers.freeserve.co.uk/W...rds/index.html as my primary reference, and I imagine that it is not going to be so easy to find answers to any questions that are unanswered therein. Still, it’s always worth a try.

Ian – G4JQT: that BBC is full of fascinating information. It’s a pity that it doesn’t go back to the late 1950s, as I suspect that it was then that the BBC decided upon the 625-line system parameters that became System I. In that connection, though, I have since found this comment in the Volume 1 of the book “Colour Television” by Carnt & Townsend (*):

“For 625 lines the suggested chrominance bandwidths are the same as for the American system. If an 8 Mc/s channel is available for each station, as in the U.S.S.R., the optimum utilization of the extra megacycle of bandwidth is probably to increase the luminance pass-band by 0.5 megacycle and use the other 0.5 megacycle to increase the vestigial sideband of the vision carrier to 1.25 Mc/s. The latter reduces the effects of vestigial sideband distortion, particularly on negative modulation. If the sub-carrier frequency is left unaltered, the extra luminance bandwidth can be used to increase the chrominance bandwidth and to ease the problem of providing a sharp luminance and chrominance cut-off at the sound channel frequency.”

Also, later in the same book is the interesting comment, supported by a preceding detailed analysis:

“It would appear that while some schools of thought may regard the difference between N and P monochrome systems as being marginal, or even in favour of N systems, there is overwhelming evidence in favour of positive modulation, A.M. sound systems for N.T.S.C. use.”

Cheers,


(*) Colour Television
Volume 1, Principles and Practice
P.S. Carnt & G.B. Townsend
Iliffe, 1961, 2nd Impression 1968
No ISBN, SBN or LCC
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