[Synchrodyne]

As has been observed, the original question is potentially very broad, and a comprehensive answer could go very deep. I think that most of what needs to be said has already been said, so what follows is simply a somewhat different perspective on the analogue case.

Viewed from within the set of analogue TV systems, the various systems, 405, 525, 625 and so on, can look quite different. Viewed from without, they look more like variations on a theme, as follows.

Of the systems that were used for regular broadcasting from more than just one transmitter:

They all had the same basic scanning system with 2:1 interlace.

They had from the start, or were changed relatively early in their lives to have, the same 4:3 aspect ratio.

The number of horizontal lines varied, with 405, 525, 625 and 819 being used.

The field frequency was either 50 or 60 Hz, usually tied to the mains supply frequency in the countries in which they were used, but not exclusively so.

The same basic video waveform was used with time and amplitude separation of the picture and synchronizing information. The form of the synchronizing signals varied somewhat amongst the various systems, and sometimes within subsets with the same line count.

The bandwidth allowed for the video signal varied with the line counts and field rates. The more lines per picture and the more fields per second, the higher was the required bandwidth for a given horizontal definition. The “hard” limit on vision bandwidth was imposed by the transmission system.

Each of the basic systems had an associated transmission system, in some cases, as with 625 lines, several associated transmission systems.

For all transmission systems within the set:

They used channels of defined bandwidth and pattern within the VHF and UHF bands.

They used separate carriers for vision and sound. The relative positioning of those two carriers varied with transmission system. The majority had vision carrier low, sound carrier high, but a minority were the other way around. The latter group included all 405-line and some 625- and 819-line transmissions.

Vision was transmitted by amplitude modulation (AM) with preservation of the DC component.

They had from the start, or were changed relatively early in their lives to have vestigial (asymmetric) sideband AM. Whether the lower or upper sideband was vestigial was determined by whether the vision carrier was low or high within the channel.

Inclusion of the DC component allowed the vision modulation to be in either a positive (modulation depth increasing with picture brightness) or negative (modulation depth decreasing with increasing picture brightness) sense. Most transmission systems were negative but some were positive. The latter group included all 405-line, all 819-line and some 625-line transmission systems.

Vision bandwidth, being that allowed for the full sideband, varied by line-count and within a set of given line-count systems. Those actually used were as follows:

405 lines: 3.0 MHz.
525 lines Originally 4.0 MHz, later 4.2 MHz.
625 lines: 4.0 (later 4.2), 5.0, 5.5 and 6.0 MHz.
819 lines: 5.0 and 10.4 MHz.

The sound could be transmitted by either amplitude modulation (AM) or frequency modulation (FM). FM was the majority choice. Audio pre-emphasis (50, 75 or 100 microseconds) was always used with FM, and sometimes (50 microseconds) with AM. In practice, FM sound was always associated with negative vision modulation systems, and AM sound with positive vision modulation systems.

The power ratios of the vision and sound carriers varied amongst the systems, and different broadcasting authorities used different ratios for nominally the same system.

Both horizontal and vertical polarizations were used for transmitting aerials, and later circular polarization was also used. In the UK, vertical polarization was primary for the 405-line system, with horizontal also used to obtain orthogonal separation of co-channel transmitters. Elsewhere, horizontal was primary, with vertical used in some countries, but not all, to obtain orthogonal separation.

To that list one could add the various colour systems that were "piggybacked" on to some of the existing monochrome systems. NTSC, SECAM and PAL were also variations on a theme, with subvariants to suit specific monochrome transmission systems. Later the various means of transmitting stereo and multi-channel sound were also “adds-on” of distinct varieties rather than being variations on a theme.

In respect of the 405-line system, I think that it was Schoenberg of EMI who decided to step up to what was an unprecedented line count at the time. This had about the same line frequency as the RCA 343-line, 60 Hz experimental system. To the extent that line frequency was a better indicator than line count of the degree-of-difficulty, then EMI was moving to the edge of, but not going beyond, what was known to be possible at the time.

The 525-line system was the outcome of the deliberations of the first NTSC in 1941. (The first NTSC was distinct from the better-known 2nd NTSC that developed the eponymous colour system in 1953, although many members worked on both.) The first NTSC considered all possibilities, the only “given” being that the channel width was fixed at 6 MHz. Also, the 60 Hz power supply frequency was a persuasive and non-variable factor. On line count, the initial leaning was towards 441, as had been used for the RMA experimental system. But Donald Fink, a major and influential member of the first NTSC, proposed 525 lines on the basis that as there was some flexibility in the trade-off between vertical and horizontal definition, the line count should be sufficient to ensure flatness of field and avoidance of “lininess”, which 441 lines did not do. NTSC chose negative vision modulation with vestigial lower sideband and vision carrier at the lower end of the channel, FM sound, the use of equalizing pulses in the synchronizing signal, and horizontal transmitter polarization, imputing advantages – not necessarily major – to all and so to some extent established these as norms.

Then circa 1944 the Russians adapted the NTSC system to suit a 50 Hz field frequency, which meant 625 lines for approximately the same line frequency. With what turned out to be very good foresight, they chose an 8 MHz channel, with 6 MHz vision bandwidth, apparently in order to match 16 mm film horizontal definition. Regular broadcasting started in 1948.

The French went for very high definition with their 819-line system in 1949, but eventually moved to 625 lines. The 819-line system used positive modulation and AM sound, and originally had vision carrier-high channels. Why this was so I am not sure. But the existing 441-line single transmitter service was that way, so perhaps it was done for commonality.

The Western Europeans picked up on the Russian work, but did not like the 8 MHz channel. Evidently there was some debate, with some suggesting that that the American 6 MHz channel, as had been used for an American-supplied 625-line experimental transmitter in Torino, would be appropriate. Eventually a compromise 7 MHz channel was agreed later in 1950. This turned out to be lacking in foresight. By the time that European UHF TV allocations were being planned in the second half of the 1950s, the 8 MHz channel was adopted, and this allowed the France and the UK to develop improved 625-line systems that exploited this channel width. Discussion of the multitude of 625-line variants and why they were the way they were would I think be too much of a digression.

Both the 525- and 625-line systems lasted until the digital era, so could be seen as “Goldilocks” choices. If one looks at the progression of analogue TV systems, using line frequency as a single parameter measuring their development state, then the major points on the curve were:

343/60 and 405/50 (441/50 would also fit here)

441/60 – essentially experimental (the 50 Hz counterpart would have been 525/50)

525/60 and 625/50

819/50.

The plateau was reached at 525/60 and 625/50. 819/50 was an isolated peak on the plateau, a peak too far one might say.

The UK was an early mover and thus locked in to a system that was still on the upward part of the learning curve. But its later entry to 625/50 meant that it could use a refined version.

So we could say that 405 was an early system, about the best that could have been done at the time, but short of what soon became practicable.

625 was a stasis system which, although not the best that could possibly be done in the analogue era, was about the optimum choice, representing the best trade-off amongst all competing factors until digital arrived.


Cheers,