(06-06-2021, 12:40 AM)Synchrodyne Wrote: [ -> ]It is interesting to note that the NTSC 6 MHz channel was not an ab initio choice for the 525/60 system. Rather it was an imposed initial condition, carried over from the RMA 441/60 system. Within the available bandwidth, the tradeoff between line-count and horizontal definition was chosen to be somewhat in favour of line count, enough lines to ensure flatness of field according to Fink, who was evidently a major proponent of the 525 number. Interestingly though, Fink was later on record as saying that the 6 MHz channel was not a good choice; rather 8 MHz would have been better.
That was in Donald Fink’s 1999 paper, “Two NTSCs”. Here is the pertinent paragraph:
“The RMA Allocations Committee had the burden of setting the width of the television channel. The figure they chose, 6 MHz, was ambitious at the time but it has since turned out to be one of confinement. If they had known what the future had in store they should, in this author's opinion, have opted for an 8-MHz channel. But that is hindsight!”
In the same paper, Fink also said:
“How do the NTSC television standards stand up in present perspective? One of the best informed modern judgments was issued in 1974 by Raymond Wilmotte, a distinguished consulting engineer commissioned by the Chief Engineer of the FCC to write a survey report "Technological boundaries of television," . Writes Wilmotte: "One of the principal findings of the (present) study is that in spite of the relatively infant state of the art at the time that the NTSC standards were adopted, these standards have stood the test of time.... With the technological revolution that has taken place since those days, it is indeed remarkable to be able to state today that these standards (monochrome and color) can hardly be faulted."
and:
“On the international scene, the judgment is also largely favorable. A group of eminent television engineers from many countries, under CCIR auspices, considered what the preferred standards would be, given a free choice. Their verdict: 600 lines, 60 fields per second, 6-MHz video bandwidth. The NTSC values are 525 lines, 60 fields per second, 4 MHz. The match is imperfect, particularly with respect to video bandwidth. But the verdict for 60 fields, a vote against the 50-field standard (used everywhere in the world except North America, Japan, some Latin American countries, and a few others) is a significant endorsement.”
In the early days, tying the field frequency rate to the utility frequency seemed to be an imperative. But this requirement fell away by the early 1950s. For example, in 1953, Japan adopted 525/60 nationwide despite having both 50 and 60 Hz utility grids, quite neatly separated geographically.
Anyway, allowing that in 1944 the need to tie field frequency to utility frequency was more-or-less a given, then one could say that the Russians got it right when it came to optimal exploitation of analogue possibilities.
And perhaps just “how right” the choice was might have been less readily visible had someone (in this case the French) ventured a step or two further out, in fact one might say “a bridge too far”, along the line-count and bandwidth vector.
Against that, one could also say that the position of 625 lines as something of a “goldilocks” system was somewhat marred by the multiplicity of variants. In approximate chronological order the major variants were:
Russian, 8 MHz channel, 6 MHz vision bandwidth, later known as systems D and K, also Francophone African K1 with 1.25 MHz vestigial sideband.
Latin American, 6 MHz channel, 4 (later 4.2) MHz vision bandwidth, basically 625/50 crammed into an NTSC channel, originally 1949 for the Torino, Italy experimental transmitter, in “production” 1951 in Argentina. Later known as system N. It would appear that the main reasons for its choice in the 50 Hz Latin American countries was to allow uniform American-pattern channeling across both the 50 and 60 Hz territories.
Western European, 7 MHz channel, 5 MHz vision bandwidth, Gerber compromise. Not sure of first experimental use date; promulgated by CCIR in 1950. Later known as systems B and G, also H with 1.25 MHz vestigial sideband.
Belgian, essentially the CCIR system but with positive vision modulation and AM sound, pre-emphasized, later system C. Belgium required that its transmitters could handle both 625/50 and 819/50 programme material, with more-or-less instant changeover. Clearly a modified 819/50 system was required to fit the 7 MHz channel, and whilst this might have been aligned with the CCIR 625/50 system, apparently that would not have been an even-handed approach in respect of the two constituencies involved. So both systems had to be different to their respective prototypes.
French, 8 MHz channel, 6 MHz vision bandwidth and 1.25 MHz vestigial sideband, positive vision modulation and AM sound, not pre-emphasized, system L. The latter two parameters were chosen to match the existing 819/50 system so simplify design of dual-standard receivers. (cf. the Francophone Africa system K1, which did not have the same constraint and so was negative/FM.)
UK & Ireland, 8 MHz channel, 5.5 MHz vision bandwidth and 1.25 MHz vestigial sideband, system I. This was developed by the UK TAC as its view of the optimal use of an 8 MHz channel. Hitherto the working assumption was that the UK would use the Russian parameters.
That leaves out minor variants such as B1 and I1. (L’ or L1 was not a formal CCIR designation.)
Cheers,
Steve