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Old 6th Jan 2009, 4:30 am   #1
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
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Default 625-Line Television Broadcast Standards

Has anyone here ever come across a book or magazine article that provides a comprehensive account of the multiplicity of 625-line television broadcast standards that were developed, and the reasons, technical and otherwise, for such diversity?

Whilst one can piece together parts of the story from various sources, there are still some apparent gaps.

What might be called the “parent” 625-line broadcast standard, namely the Gerber standard, later CCIR System B, was promulgated by the CCIR in July, 1950. It was evidently derived largely from the US NTCS 525-line standard. However, I suspect that the Gerber standard had a long gestation period, and that its likely key parameters were known well in advance of 1950.

But contraindicating this “parent system” position is the fact that as far as I know, the Russian 625-line service was started before the Gerber standard was released, perhaps circa 1948. Assuming that the OIRT standard (later CCIR System D) was used from the start, one can wonder why the Russians chose 6 MHz video bandwidth and +6.5 MHz sound carrier spacing in an 8 MHz channel. Was it a case of “oneupmanship” over the anticipated Western European standard, or at the time, was the CCIR still deliberating over video bandwidth, etc., only later on opting for the 7 MHz channel parameters? Still, given the later choice of 8 MHz channeling for Western European UHF services, one might say that the Russians were a little prescient.

Then why did Belgium choose to do differently in 1953, with its own variants of the 625 and 819 line systems, particularly as it was one of the six countries (the others being Denmark, Italy, Netherlands, Sweden and Switzerland) that were heavily involved in the development of the Gerber system. That Belgium needed both 625 and 819 line systems was surely a political rather than a technical requirement, and given that, then “squeezing” the 819 line system into a standard 7 MHz channel was an understandable decision, even if technically questionable. Similarly understandable was the desire, to the extent possible, to have common parameters for both systems. This could have been done by using the Gerber system for 625 lines and developing a companion 819 line variant with negative vision modulation and FM sound. But instead, it was elected to use positive vision modulation and AM sound, which required a new 625-line variant as well as a new one for 819 lines. In the thread “Experimental 1029 line TV c.1949”, at https://www.vintage-radio.net/forum/...ad.php?t=14741, ORTF & Co. suggests that there may have been US (RCA?) patent and royalty issues with the use of negative vision modulation and FM sound, which is why in 1948 France chose its own 819-line system with positive vision modulation and AM sound (System E). Perhaps the Belgian choice, like that in France, was made in order to avoid royalty payments? But then I understand that most early Belgian receivers were designed for four systems, namely B (Gerber), C (Belgian 625), E (French 819) and F (Belgian 819). So from the receiver design viewpoint, the benefits of the similarities of Systems C and F were not fully realized. As a digression, was it perhaps this early need for multistandard receivers that launched Belgian company Barco along the pathway to particular expertise in this field?

An interesting facet of the Belgian 625- and 819-line systems was that the AM sound channel had 50 microsecond pre-emphasis, something not used with the UK 405-line and French 819-line systems. Insofar as, unlike FM, AM does not have a triangular noise spectrum, there is no pressing need for pre-emphasis. Perhaps, though, it was thought that the required de-emphasis roll-off in receivers would mitigate the effects of any line frequency components that found their way into the sound channel, particularly in the 625-line case. More recently, though, 75 microsecond pre-emphasis has found its way into MF radio broadcasting, as part of the NRSC standard developed in the USA, and also used elsewhere, such as by the ABC in Australia.

When was it decided that the European UHF channels would uniformly be 8 MHz wide, regardless of system? I suspect that this would have happened somewhere in the 2nd half of the 1950s, which was why both BBC and RTF were both working within that channel width for their “new” 625-line UHF systems.

I recall reading somewhere many years ago that when the BBC was working on the development of what became System I – in the late 1950s I think – it determined that optimum use of the 8 MHz channel was obtained with 5.5 MHz video bandwidth, not 6.0 MHz as with the OIRT System D, but with the vestigial sideband extended from 0.75 to 1.25 MHz, and with +5.5 MHz sound carrier spacing. My memory is hazy here, but as I recall the rationale was that extended vestigial sideband made it easier to design IF filters (back in the days of distributed selectivity with LC filters) without too much phase distortion of the lower video frequencies, the benefits of which were greater than going from 5.5 to 6.0 MHz video bandwidth . On the other hand, in approximately the same time period, RTF, in the case of System L, felt that both 6.0 MHz video bandwidth and a 1.25 MHz vestigial sideband could be accommodated within an 8 MHz channel. Which came first, System I or System L? And was System H ahead of both?

Also in the above-mentioned thread, ORTF & Co. states that RTF chose positive vision modulation and AM sound for System L in order to maintain commonality with System E, and so ease receiver design. This seems to be reinforced by the fact that for its overseas territories, where System E had not been used, France chose System K1 essentially the “inverse” of System L, with negative vision modulation and FM sound.

Finally - for now, anyway - when was 625-line System N introduced? I would guess that Argentina was the first country to use this system, which “squeezed” a 625 line signal into a 6 MHz NTSC channel, I would imagine to ensure that the whole of South America (or nearly so, anyway) adhered to a standard channeling system. But the 6 MHz channel was perhaps a little ironic considering that in 1950, Channel E1, 41 to 47 MHz, was deemed unsuitable for 625-line television.

Cheers,
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