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Old 13th Jun 2012, 12:12 am   #3
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Video modulation question.

It was mostly by happenstance, I think, that we ended up with positive/AM and negative/FM systems, and no positive/FM or negative/AM systems. (Although some pre-WWII 441-line broadcasts might have been negative/AM?)

The EMI 405-line system pretty much predated FM broadcasting anyway, and no doubt Mr. Blumlein had good reasons for choosing positive vision modulation at the time.

NTSC (I) considered the vision and sound modulation separately and independently, and chose negative for vision and FM for sound. There is no suggestion in the deliberations that the choice of one affected the choice of the other. So positive vision with FM sound and negative vision with AM sound were both technical possibilities.

The NTSC work in turn informed the initial Russian work on the 625 line system, which was thus also negative/FM, as were most of the subsequent 625-line variants, including the Western European Gerber, Argentinean and UK TAC.

France followed the EMI 405-line precedent of positive/AM with its 819 line system, also eschewing the use of equalizing pulses. Perhaps similarity to its 441-line system, also positive/AM, was a factor in its choice. Or dare I say it, not following the American precedent, which thus outruled the use of negative vision modulation and FM sound, whether together or individually.

The Belgian situation seemed to have been determined more by political than technical considerations, but the French choice seems to have been influential as Wallonia was receiving French 819 transmissions from Lille (whose transmitter was allegedly sited to give good coverage into Belgium) before the start of Belgian TV. Thus the Belgian 819-line system was essentially the French positive/AM system shoehorned into a Gerber 7 MHz channel, albeit with the addition of equalizing pulses. And the Belgian 625-line system was a positive/AM version of the Gerber. If these choices were made to allow some simplification of multistandard receivers, then that was negated by the fact that most receivers also covered the Gerber negative/FM system, to allow reception of Dutch broadcasts in Flanders. Still, once it was determined that a less bandwidth-hungry version of the 819-line system would be used in Wallonia, thus making it different to the neighbouring French system, maybe it was thought of as some form of equality that the 625-line system for Flanders had to be somewhat different to the neighbouring Dutch system. Both Belgian positive/AM systems used pre-emphasis in the sound channel, unlike the other positive/AM systems. It is interesting to note that NTSC (I) had envisaged the use of pre-emphasis regardless of whether AM or FM was chosen. Tibbs and Johnstone ascribe some benefit to the use of pre-emphasis with AM, although less than for the FM case, not surprising given its rectangular rather than triangular noise distribution.

Thus largely because of the NTSC (I) choice, negative/FM became the majority type, with positive/AM in the minority. France chose positive/AM for its 625-line system to align with its existing 819-line system and so simplify dual-standard receiver design. But for its outré mer territories, it chose the negative/FM System K’.

Interestingly, and has been mentioned in other threads, Carnt and Townsend, in their seminal work on colour television, came down emphatically in favour of positive/AM over negative/FM for NTSC colour systems (and presumably this applied equally to the SECAM and PAL derivatives). On the other hand the NTSC (I) choices were more in the nature of one being a bit better than the other overall, rather than major differences.

Cheers,
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