Wrong UK 625 line TV system?

[colly0410]

I'm assuming the UK was the first Country in the world to go all UHF in 1985 when 405/VHF shut down, although it went de-facto UHF in the early/mid 70's when most people went colour. I can't remember anyone using 405/VHF after mid 70's..

[dtvmcdonald]

The USA of course does not have "Band I" or "Band III" whatever those are,
but rather "low VHF" "high VHF" and "UHF". Not counting pre-1946 channels,
all are theoretically still in use, up to at least Ch. 51.

High VHF has proven to work just fine for digital. We have one station there,
and it is rock solid even on UHF only antennas. Low VHF is problematic
ONLY because of low power allocations, typically 10 kW. There are 39 licensed
low VHF stations, 16 of them on Channel 5, the most desirable. The highest power is only 34.5 kW ERP. That's a just a bit higher average power than was used for analog. A lot of these serve rural areas or oddball places like Key West FL
where the 1 kW power will blast the whole island and also get picked up
by the alligators on the mainland. With much more power both channels 4 and 5 would be very desirable, since they are not frequency adjacent.

Of course, in the USA it is generally considered that broadcast TV is
in a real death spiral.

[ntscuser]

In it's review of the then experimental 625-line receivers at the National Radio Show in the October 1961 issue, Wireless World bemoaned the fact that although the number of lines had been increased, nothing had been done to mitigate the effects of scanning frequency - the logic being that if you sat close enough to see the improvement in detail, you could also see the line structure. It goes on to say "Thus the only conclusion that emerges is that the field rate should be raised."

Later in the same article they describe a Ferguson 625-line prototype being demonstrated at the show with manual switching for 50 or 60 fps field rate.

[colly0410]

How a about a modified system I (I' ?) with positive modulation video & FM sound? That would have been be a super unique system. Any advantage over normal system I though? Would British set makers still have used mean level AGC on B/W sets? Would they have used mean level AGC on colour sets? Surely that would have been a step too far down the cheapskate route.

[Terrykc]

The USA of course does not have "Band I" or "Band III" whatever those are,
but rather "low VHF" "high VHF" and "UHF"...

What you call "low VHF" is actually Band I and your "high VHF" is Band III.

The actual bands vary with geography as can be seen here: http://www.pembers.freeserve.co.uk/Worl ... html#Bands although even that can vary within a region - Italy, for example, has/had a TV channel immediately below FM Band (Band II) but whether that is still in use since the European Band II was extended down to 87.5MHz, I'm not sure

[Synchrodyne]

It is possible that the US terminology “low band” and “high band” actually predated the “Band I” and “Band III” terminology.

The VHF low band/high band distinction may have come from the FCC’s mid-1945 spectrum reallocation, effective1946 March 01. Whether the FCC actually used those terms, or whether they had an informal start and later became official, I don’t know.

My understanding is that the “Band I”, etc., terminology came out of the 1947 Atlantic City meeting, but if so, I can’t find it in the ITU 1947 Atlantic City Annex, available here: http://www.itu.int/en/history/Pages/Ple ... 0201000019.

Very broadly, the global VHF broadcasting allocations developed at the 1947 meeting followed the FCC’s 1945 pattern.

Whilst one may find several references to early US VHF frequency allocations, the best I have seen is an article “What Ever Happened to Channel 1”, in “Radio-Electronics” for 1982 March, p.43ff, available here: http://www.americanradiohistory.com/Rad ... r_Page.htm.

This article also includes the history of the US 6 MHz TV channel, which evidently goes back to the original DSB version of the RMA 441/30 standard. That is pertinent here given the original question about 625/25 System N, with its 6 MHz channel. Evidently there was some angst about departing from the 6 MHz American channel for European 625/50, resulting in the Gerber compromise 7 MHz channel.

Some of the “fine detail” remains obscure, though. For example, when was the NTSC TV standard, as developed by NTSC I in 1941, adjusted in terms of its sound carrier characteristics. Originally it was specified with ±75 kHz deviation and 100 µs pre-emphasis, the same as then used for FM broadcasting. Then it was later changed to ±25 kHz deviation and 75 µs pre-emphasis. My working hypothesis is that the two parameter changes were concurrent, and became effective with the 1946 channel assignments, but I have no supporting data. In the case of FM broadcasting, there is empirical evidence that 100 µs pre-emphasis was retained for the 42 to 50 MHz band at least up until 1945, and perhaps until the end of transmissions in this band. Again the (unsupported) working hypothesis is that 75 µs was prescribed for the 88 to 108 MHz band from the start.

Anyway, ±25 kHz deviation and 75 µs pre-emphasis were in place by the time that System N started in Argentina in 1951. I suspect that European ideas to use a 6 MHz channel for 625/25 included ±50 kHz deviation and 50 µs pre-emphasis, as derived from the Russian work and as eventually used in the Gerber standard. What was the Japanese thinking during the 525/30 vs. 625/25 and 6 MHz vs. 7 MHz channel debate is unknown, but it may be noted that whilst Japan followed NTSC precepts for TV, it chose the European 50 µs pre-emphasis number for its FM broadcasting.

Also, somewhere along the way, the NTSC nominal vision bandwidth moved up to 4.2 MHz from the 4 MHz noted in the original NTSC I documents. This seems to have happened before NTSC II determined the colour system parameters. But on the other hand, the tabulation in Wireless World 1952 August still shows 4 MHz. By deduction, it looks as if System N started with 4 MHz and moved up to 4.2 MHz to stay in step with NTSC.

Cheers,

Steve

[Synchrodyne]

How a about a modified system I (I' ?) with positive modulation video & FM sound? That would have been be a super unique system. Any advantage over normal system I though?

I suspect that there would have been a conflict over the use of intercarrier sound, which the setmakers would no doubt have wanted in the interests of cheapness. With conventional positive modulation systems, the near-zero excursions of the vision carrier during sync pulses would have made intercarrier sound very difficult, at least with the technology of the day. (It is of course quite doable with PLL fully synchronous vision demodulation, as Motorola showed.) Moving the sync pulse level up enough to allow intercarrier sound probably would have been resisted by the broadcasters as forcing inefficient use of the available modulation range. Just how far up one would need to go is unknown. Whilst 10% peak white was acceptable for negative modulation systems, where the occurrence of 10% would be somewhat random, 10% sync tip may not have been for positive systems, where minimum modulation was a regular occurrence. Just consider how many receivers with conventional intercarrier sound produce caption buzz because of the peak white events. System L was modified to a 5% minimum sync level to accommodate use of the quasi-split sound (QSS) technique for the NICAM carrier, but I suspect that it [the NICAM carrier] was more robust than FM in rejecting vision carrier interference and anyway, QSS itself offered a marked improvement.

Split sound with FM was doable, but was more exacting in its requirements than split sound with AM. It was usually reserved for higher quality applications, for example, see the D.C. Read TV tuner design published in Wireless World from 1975 October. But even then, it could be tripped up by incidental phase modulation (ICPM) at the transmitter end. That’s why, for example, early “Component TV” tuners, such as those from Sony and Luxman, which had split sound also had a backup intercarrier sound system for use when one encountered a transmitter with ICPM.

Would British set makers still have used mean level AGC on B/W sets?

Do you even need to ask.....

Would they have used mean level AGC on colour sets? Surely that would have been a step too far down the cheapskate route.

Surely some would have tried, aided and abetted by the IC makers. For example, the Philips TDA2542 IC, the positive/AM counterpart to the negative/FM TDA2540/TDA2541 IF/demodulator/afc/agc/video noise inversion combination, had mean-level agc. However, the video-to-agc section connection was external to the IC, so an enterprising designer could have inserted a gating circuit, at the cost of a transistor or two. Or maybe a jungle IC could have been used to provide the agc. This also applied to the TDA2549 multistandard version in the TDA254x series. By the way, the TDA2540/TDA2541 did not have line-gated agc, but rather noise-gated sync tip agc. I imagine this was in deference to VCR applications, where the customary line flyback pulse was not readily available. Although that said, inclusion of a sync separator and agc gating pulse generator within the IC does not strike me as having been too difficult, and it was done by others not too long afterwards.

Cheers,

Steve

[Synchrodyne]

Some information on African TV channel allocations and numbering is provided in the document available here:

http://www.itu.int/pub/R-ACT-RRC.11-1989

Scroll down to chapter 3 (p.59) of the .pdf.

It does not answer the questions as to why Band I was not used in South Afrcia, but it does show that Band I channels were assigned for Africa generally.

Cheers,

Steve

[Synchrodyne]

It was originally planned to extend 625-line transmissions to Bands I and III in Britain once 405-lines ended, thus allowing a fifth and sixth national channel. That idea seems to have fallen by the wayside. I presume vertical polarity would have been retained to maximise compatibility with existing aerial installations.

That was mentioned in Wireless World 1967 April.

It would have been an interesting exercise, with three Band I and five or six Band III channels (8 MHz wide), depending upon whether the allowed upper limit was 216 or 222 MHz.

Band I would not, I think, have accommodated a whole network, so that Band III at least would to some extent have been shared by both networks. Perhaps it would have been one network making maximum use of the three Band I channels, with some use of Band III, and the other using Band III alone, or Band III with spot use of Band I for lower-powered transmitters where possible. Or perhaps the Band I channels could have been shared amongst both networks. If say channels 1 and 3 (or A and C if they were lettered) were assigned to London, could they have been re-used for central Scotland, perhaps with reverse program allocations? The geographical distance separation then would be comparable to what was used here in NZ for Band I co-channel main transmitter separations (albeit with polarization differentiation as well), but of course the NZ terrain has quite a few non-negligible “spiky bits” at strategically convenient points that help with reducing co-channel separation.

I suspect that maintaining location polarity parity and perhaps proximate channel frequency parity with the 405-line networks would have been a significant extra complication in the channel assignment process.

I wonder when that plan for re-engineering Bands I and III was abandoned.

Cheers,

Steve

[Terrykc]

Whilst one may find several references to early US VHF frequency allocations, the best I have seen is an article “What Ever Happened to Channel 1”, in “Radio-Electronics” for 1982 March, p.43ff, available here: http://www.americanradiohistory.com/Rad ... r_Page.htm.

A clear, concise summary of the pertinent information in that article can be found here:

http://www.pembers.freeserve.co.uk/Worl ... ml#Missing

[ntscuser]

I wonder when that plan for re-engineering Bands I and III was abandoned.

I doubt if it was ever formally abandoned but it was effectively dead and buried when it was decided to cram Channel 5 into what little was left of the lower UHF spectrum. Band III has of course since been reactivated for digital radio coverage, vertical polarity and all.

The aforementioned Wireless World report on the 1961 National Radio Show explained that the 625-line signal for the demonstration stands was piped using the continental 'Gerber' 7MHz bandwidth system on Band III VHF and upconverted where necessary to UHF on the exhibition stands themselves. (Some UK tuners apparently had a wafer which could be reversed to permit reception of 625-lines on Band III). It was thus not a true demonstration of the 8MHz bandwidth system which Britain eventually adopted. There were also allegations (but no evidence) that the 405-line signal had been artificially degraded to make 625-line pictures look much better when compared side-by-side.

[Synchrodyne]

The Pye dual-standard receiver described in the same WW 1961 October article had a 4.25 MHz video bandwidth, as determined by the IF bandpass. That was a long way short of the Gerber 5.0 MHz transmitted bandwidth, let alone the 5.5. MHz of the TAC system, as System I was then known.

Assuming that Pye video bandwidth was representative, then it was a strange way to demonstrate the advantages of a new system.

One can see what was Pye’s game, though. Its IF strip had a basic IF bandpass of the double-Nyquist type, and that was used for 625. For 405, the sound trap was switched in to truncate the bandwidth. Doing it properly, with above 5 MHz video bandwidth, would have required a switchable 405 Nyquist slope filter, as 34.65 MHz would have been on the flat part of the basic curve. So the corner cutting with 625 started early. Moving the IF up to 39.5 MHz helped somewhat, as then the double-Nyquist approach would have provided a 4.85 MHz video bandwidth on 625.

Cheers,

Steve

[Till Eulenspiegel]

About the same time Pye introduced the V700D rival firm Ekco introduced the T398 dual standard receiver. It would be interesting to find out how the Ekco designers approached the bandwidth requirements of the two systems in the T398.

It comes as a surprise to learn that the Pye V700D had such a restricted 625 vision bandwidth considering the posturing of company chief C. O. Stanley against the 405 line system. You would have thought the debut receiver would have the very best possible characteristics possible to promote the potential of the 625 system.

Goes to prove that cost cutting and the attitude of "what we can get away with" was alive and well even in 1961.

Till Eulenspiegel.

[Terrykc]

I well remember my disappointment when I saw the first 625-line pictures at the Radio Show at Earl's Court.

It is important to realise what a huge event the Radio Show was: large stands for all the major manufacturers, stage shows and live radio broadcasts from the BBC and, to ensure that there were plenty of TV pictures in those days of all day test cards, two complete TV studios, one for Band I and one for Band III output!

So, fully expecting to find a third studio for 625-line programmes, I headed off to Earl's Court ...

What did I find? A tropical fish tank with a fixed vidicon camera aimed at it ...

[Cathovisor]

What did I find? A tropical fish tank with a fixed vidicon camera aimed at it ...

A Vidicon?! So not even a real camera then!

[Terrykc]

A Vidicon?! So not even a real camera then!

Put it this way, it was very small and a vidicon was the only thing that would have fitted inside, as far as I know ...

[Cathovisor]

When was this, Terry? I'm just wondering if there was the possibility of a Plumbicon by then: I know Kingswood Warren was evaluating them in late '63.

[Terrykc]

When was this, Terry?

A very good question!

It would have been the first year that any dual standard sets were demonstrating 625-line capability at Earl's Court.

I did consider the possibility of a plumbicon but was convinced it was earlier. The 1963 show was only about 8 months before the BBC2 launch so I would have expected more than a few tropical fish swimming around (in black & white!) to attract the punters - the Radio Show was still firmly aimed at the public in 1963, remember!

[Synchrodyne]

Wireless World 1961 October, attached, mentioned the fish tank as the live 625 feed in its Radio Show Review.

The use of reduced video bandwidth by the setmakers was not a new idea, though, as shown in the attached correspondence from WW 1952 October and November. There the BBC was (incorrectly, and one imagines deliberately so) blamed for short-changing on the transmitted bandwidth.

WW 195211 p.457 TV Bandwidth.gif (11.33 KiB) Viewed 4288 times

Cheers,

Steve

[Synchrodyne]

Here is the IF bandpass curve for the 1961 Pye dual-standardreceiver. I had already attached this in the TV IF thread, but it is easier to repost here than to redirect.

Cheers,

Steve

[colly0410]

I'm thinking of some of the advantages of band 1: I think it would be cheaper to transmit for a given area coverage; so many watts ERP (or fractions of a watt ERP) per square mile covered, they would bend round hills better, less relay/gap filler stations needed & if the TX's are put on high ground with a tall mast the signal will go far. Think how far Holme Moss went on channel 2, it was coast to coast, I suppose it would have gone a bit further if it had used channel 1.... Then the disadvantages: Ignition/electrical interference, co-channel interference when sporadic E, F2, TE skip & tropo kicks in, large receiving aerials catching the wind & people saying they're ugly. (I think they're aesthetically pleasing but I'm biased. ) Anyone think of any other advantages/disadvantages of band 1?

[ntscuser]

Anyone think of any other advantages/disadvantages of band 1?

It would only be able to carry a single digital multiplex which - in this multi-channel obsessed age - wouldn't be acceptable to many viewers even though that still represents more channels than many of had to choose from for most of our lives.

[Synchrodyne]

The paucity of Band I channels was a drawback.

Use of the Band I channels for the main transmitters of a network, with Band III channels for the intermediate and gap-filling transmitters probably allowed the maximizing of Band I benefits.

But where Band I channels were also used for the intermediate and gap-filling transmitters, then the problems became apparent. Mutual interference problems limited the coverage of the additional transmitters, in turn requiring more of them, in turn exacerbating the channel-sharing problems, and so on.

One might say that the greatest strength of Band I – its long reach - was also its greatest weakness.

Cheers,

Steve

[colly0410]

Been reading a book written by by a lady who lived in Port Stanley in the Falkland islands during the Argentine invasion. She says they started a TV service & rented out sets at next to no charge, she made one payment & that was that, they carried on TX'ing till they surrendered, they never asked for the sets back. The TV had a rabbit ears set top aerial & got a good picture. I presume they used standard system N Argentine spec TX's & sets & used band 1 to get maximum coverage for least power. I wonder what happened to the TV's? I understand they use system I at UHF now (unless they've gone digital) & relay British Forces TV..

[Synchrodyne]

I imagine that System N was used. It seems highly unlikely that the Argentineans would have done other than use their established system. As I recall, they changed the rule-of-the-road during their occupation, so that things would be done as they were on the mainland.

Given the distances from anywhere else with TV broadcasting, there was probably a free choice of channels. Band III would have required higher radiated power than Band I, but then aerial gain was relatively easy to obtain at these frequencies.

I’d pick channels A4, A5 and A6 as having been the best option. The very low end of Band I seems to suffer from problems that can cause significant frequency response lumpiness, particularly when relatively simple aerials are used. I saw an extreme case of this – really serious suckout at the sound carrier – with channel Aus2 in Sydney. But I have also seen it with channel NZ1 in Wellington and to a lesser extent with NZ2 in Auckland. In Dallas I had a multi-element array in the roofspace, but channel A2 was never as “clean” as channels A4 and A5, even though there was no shortage of signal. (The transmitters were more-or-less co-sited on an antenna farm over at Cedar Hill.)

Who knows, there might be a few System N TVs lurking in the UK, brought back by folk who spent time in the Falklands.

Cheers,

Steve