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Old 19th Apr 2009, 12:12 pm   #10
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: 625-Line Television Broadcast Standards

A sidebar issue here is the origin and timing of the various CCIR television transmission system letter designations, particularly for the diversity of 625 line variants. I see that the topic has already been discussed in other threads, such as: https://www.vintage-radio.net/forum/...ad.php?t=26968. In an effort to pinpoint the time when the letter designations were first used, I looked through the likely materials on hand. The best I could do was somewhere between 1966 and 1969.

Firstly, Hutson (1), in 1966, has a chapter on television signal standards, which goes into considerable detail, and covers most of them (although OIRT 625, Argentinean 625 and French 625 are not mentioned). Nowhere is there any reference to system letter designators. As one assumes that Mr. Hutson would have been “in the know” on such matters, it is reasonable to deduce that the CCIR had not assigned them at the time the book was written.

Secondly, Carnt and Townsend (2), in 1969, do refer to Systems G and I in their discussion of the PAL colour system. So by the time they were writing their book, the letter identifiers had been assigned.

Anyway, it seems that the letter identifiers were assigned after all of the various systems were in service, in which case they appear to be a combination of logic and arbitrariness. The UK’s pioneering efforts seem to have been recognized by the assignment of the letter A to the 405-line system. Then not surprisingly, the letter B went to CCIR’s own standard Gerber 625 line system, with the other early 625 line systems following as C (Belgian) and D (OIRT), although not including the Argentinean 625 line system, early though it was. Next were the two 819 line systems, E (French) and F (Belgian). So A for 406, B, C, D for 625, and E and F for 819, covering all of the early VHF systems used in Europe, does have a certain logic, although the order of the 625 line systems suggests a certain “political” input, given that the OIRT system was the first in service. Or one can impute the logic that for any given line standard, they are ranked in order of increasing video bandwidth, with N-type systems preceding P-type systems. That works for B, C and D, but fails for E and F, though.

The next group, G, H, I, K, K’ and L covers 625 line systems that were developed mostly with UHF transmissions in 8 MHz channels in mind, although some have also been used at VHF, as with System I in Ireland. G was essentially the same as B, H was G with the vestigial sideband extended from 0.75 to 1.25 MHz, I was the UK (1960 TAC Report) system with video bandwidth extended to 5.5 MHz and with a 1.25 MHz vestigial sideband, K was essentially the same as D, and L was the French P-type 625 line system with video bandwidth of 6 MHz and vestigial sideband of 1.25 MHz. So here the letter progression does follow the increasing video bandwidth rule from G through L. I suspect that K’ (D/K, but with a 1.25 MHz vestigial sideband) may have been a later addition that had to be shoehorned into the system, there being no free letters in the appropriate part of the system. Maybe the CCIR was not anticipating any further variants of the existing transmission standards when it developed its letter system.

Then came M (NTSC 525, NTSC here referring to its early 1940s work, not its later colour work) and N (Argentinean 625). The inferred rationale here was that the CCIR was a European organization, so it would list the European systems ahead of the others. Evidently it saw system N as being a rather ersatz 625 line variant, and more American than European, hence its inclusion in the American group, following the 525 line system. (Although compressing the Gerber system into a 6 MHz channel was surely a no greater act of cruelty than squeezing the 819 line system into a 7 MHz channel, as was done for System F.)

Clearly there is more than some guesswork in the above commentary, but it is difficult to make a logical country connection with the letters, such as I = Ireland. And perhaps the good folk of that country might have preferred E for Eire anyway, but the surely if the CCIR were open to such allocations, the French would have claimed F for the original 819 line system. (And reversing the E and F letter designations could have been justified on a video bandwidth basis.) G for Germany is another possibility, but even here, I think that Italy might have been earlier with the implementation of UHF TV broadcasting, and therefore use of System G.

Cheers,

(1) Television Receiver Theory, Part 1
G.H. Hutson
Edward Arnold, 1966
No SBN, ISBN or LCC

(2) Colour Television, Volume 2
PAL, SECAM and Other Systems
P.S. Carnt & G.B. Townsend
Iliffe, 1969
SBN 592 05946 4
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