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Wrong UK 625 line TV system?

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Re: Wrong UK 625 line TV system?

Post by colly0410 » Thu Apr 10, 2014 5:09 pm

Mrhoover wrote:
colly0410 wrote:I remember being in a Boulogne TV shop in 1986, they had telly's showing BBC & French channels side by side, I tried to see if I could spot any difference between PAL-I & SECAM-L, they both looked the same = good.


Years ago I used to have off air French TV in Sussex from Boulogne.I found French TV always had what's best described as a "pinky blue" look,flesh tones rather pink and backgrounds rather blue.compared with UK TV.



Hugh


Can't remember the SECAM-L picture being blueish, mind you there were fluorescent lights blazing away in there so that could have masked the effect..

 

Re: Wrong UK 625 line TV system?

Post by Mrhoover » Thu Apr 10, 2014 5:39 pm

colly0410 wrote:Can't remember the SECAM-L picture being blueish, mind you there were fluorescent lights blazing away in there so that could have masked the effect..


You can to an extent see it on this Second network (Antenne 2,now France 2) news broadcast from 1981

http://www.youtube.com/watch?v=Dnwd2AtSnfo

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Fri Apr 11, 2014 5:23 am

The use of System I by Ireland was an interesting case. Presumably there was a desire for technical harmony with the UK, and RTE and other interested parties kept themselves apprised of the 625-line work that was being done by the BBC and others.

Interestingly, at the time of the 1961 Stockholm VHF-UHF meeting, although System I was nominated for use in Ireland, it was apparently not yet a final decision.

I have attached pertinent pages from the technical annex to the ITU Stockholm report.

ITU ST61 TA p.27.tif

ITU ST61 TA p.28.tif

ITU ST61 TA p.29.tif


System I was then listed only as a UHF system, not VHF, but the following footnote was included:

“Ireland is providing for the possibility of using a Standard similar to Standard I in Bands I and III.”

In the country-by-country UHF comments, the entry for Ireland was:

'No decision has been taken on the standards, including the number of lines, to be adopted for Bands IV and V in Ireland. But for planning purposes without prejudice to such decision, the parameters preferred are those shown in the Table under "Standard I".'

But at that stage, the UK decision was not yet final, as the pertinent UHF comment was:

"The parameters preferred for use in the planning of a 625-line system; the standards, including the number of lines, to be adopted in the United Kingdom for Bands IV and V, have not yet been decided."

I imagine that the Pilkington adjudication on the TAC recommendation was still awaited.

Notwithstanding the official uncertainties, it was evident that both Ireland and the UK preferred System I.

Cheers,

 

Re: Wrong UK 625 line TV system?

Post by Crusty Burke » Fri Apr 11, 2014 5:03 pm

RTE did not go on the air with 625 initially. Channel 7 from Kippure officially went on air on December 31st 1961 but Channel H on 625 lines from Kippure did not go on the air until the summer of 1962. Don't have the exact date.

Richard

 
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Re: Wrong UK 625 line TV system?

Post by Michael Watterson » Fri Apr 11, 2014 10:52 pm

There is an online RTE history archive but it's very sparse on detail :(

Thanks :)

Maybe they had a deadline to meet (before end of year) as I think the TV licence had been introduced August 1961 after nearly many years debate. So many people watching UK TV.

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Sat Apr 12, 2014 7:55 am

I haven’t been able to find the date of the initial Irish 625-line System I transmission; only that Kippure 625 transmitter was the third on-air after the Kippure and Truskmore 405 transmitters.

The previously mentioned ITU ST61 technical annex included a channel fit diagram for the various UHF 625-line systems, attached.

ITU ST61 p.31.tif
ITU ST61 p.31.tif (10.29 KiB) Viewed 2308 times


From that one would deduce that the letter allocations from G upwards were assigned on the basis of increasing overall vision bandwidth, with the full sideband as the tiebreaker. Thus I and K both had 6.75 MHz overall vision bandwidth, but K had a 6 MHz full sideband as compared with 5.5. MHz for I.

The previously mentioned channel edge overlaps for System L are also shown in the diagram.

ST61 also covers other aspects, such as protection ratios, including those for dissimilar systems in the same region, as would have applied in Ireland where Systems B and I were used side-by-side at VHF. The full document is available at: https://www.itu.int/ITU-R/terrestrial/b ... nnex_E.pdf.

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by Michael Watterson » Sat Apr 12, 2014 1:24 pm

Systems B & I or A & I?

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Sat Apr 12, 2014 4:01 pm

Would using 1.25 mhz vestigial sideband as used in systems H,I,K' & L give any advantage over using 0.75 mhz as other 626 systems use? I've noticed that the only difference between G & H is the vestigial sideband, same with K & K'..

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Sun Apr 13, 2014 4:39 am

As I understand it, the bigger vestigial sideband was advantageous, at least with the technology of the time.

The bigger vestigial sideband allowed for a gentler Nyquist slope in the receiver IF system. With the L-C filters of the time, a steeper Nyquist slope resulted in greater phase errors around the carrier frequency, i.e. at the LF end of the video spectrum. Such phase errors adversely affected picture quality. Apparently the BBC findings were that in perceptible picture quality increment terms, the best disposition of the final half megahertz of vision bandwidth available in an 8 MHz channel was towards a larger vestigial sideband rather than to extension of the video bandwidth.

A secondary benefit was that for a given oscillator drift, there would be less change in vision carrier amplitude relative to the desired -6 dB point. But that benefit would have fallen away once good AFC systems became the norm.

One may see that as the video bandwidth (full sideband) increases, so should the vestigial sideband in order to retain the same basic “shape” factor of the overall IF passband. Tighter shape factors will result in greater phase errors. But it would appear that the 0.75 MHz vestigial sideband chosen by NTSC (I) was then simply adopted for most other systems, even where they had higher video bandwidths. Thus it was for the Russian 625-line, British 405-line and CCIR 625-line cases. The French did differently with their 819-line system, which had a 2 MHz vestigial sideband, which suggests that they thought it through. But in that case, anything much less was probably difficult to achieve with typical L-C distributed selectivity IF strips of necessarily very wide bandwidth. Evidently in the mid-1950s, 9 MHz video bandwidth was the norm for French TV receivers, with some reaching 10 MHz. And video bandwidth was a often published parameter.

The European standard 8 MHz UHF channel provided an opportunity for rethink, hence the 1.25 MHz vestigial sidebands of Systems H, I K' and L. Of course, given that receivers operating in System H territories also needed to receive System B (or C in Belgium) VHF transmissions, one wonders whether many receivers were configured to use a gentler Nyquist slope on System H than on System B.

The arrival of SAW filters in domestic TV receivers from the late 1970s somewhat took away the original argument for wider vestigial sidebands. With SAW filters, amplitude and phase responses could be set largely independently. Thus there was some latitude in choosing the Nyquist slope, up to specific System limits. Evidently late in the analogue period there were SAW filters for multistandard TV receivers that had two Nyquist slopes, with one much steeper than it needed to be, I imagine not to compromise too much the video bandwidth looking from the carrier at the other (main) slope. As that approach was likely driven by the need for simplicity, and so avoiding the use of an internally switched SAWFs or multiple SAWFs, I should hazard a guess that receivers using dual-Nyquist SAWFs also used oscillator-high throughout, which meant that it was System L’ (Band I) that suffered in terms of the reduced video bandwidth available from the “other” Nyquist slope.

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Wed Apr 23, 2014 9:16 pm

I suppose all things being equal system K' (or is it K1?) would be the best system, it's got 1.25 mhz vestigial sideband, 6 mhz video bandwidth, negative video modulation & FM sound, only problem is it bursts out of the 8 mhz channel. However if you want to keep within the 8 mhz channel system I would be best. Looks like us brits got it right after all. Never mind Jack, your modified system N didn't curry much favour, but it got some very interesting comments, thanks for those...

 
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Re: Wrong UK 625 line TV system?

Post by Michael Watterson » Wed Apr 23, 2014 9:54 pm

Nowadays, and for several years since Synchronous modulators in ICs you only need a pilot carrier and no vestigial sideband. So in 8MHz you could have almost 6.5MHz video bandwidth, Zwei channel FM and Nicam.

Same is true of AM Radio. True SSB (no carrier) of course saves massive power but tuning and tuning stability is major issue. So reduced carrier allows AFC and automatic Synchronous detector lock.

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Thu Apr 24, 2014 12:15 pm

Good point Michael, as you say modern tellys would cope without a vestigial sideband but older ones with diode detectors wouldn't I presume. I've got an Elizebethan T12 potable from 1972 that worked OK just before digital switchover, I presume that's got a diode detector. I'll have to hook it up to the virgin cable box as that's got an RF output & see if it still works...

 
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Re: Wrong UK 625 line TV system?

Post by Michael Watterson » Thu Apr 24, 2014 1:50 pm

Modulators in Setboxes are invariably full double sideband (DSB)AM (no filters) and you thus get interference on adjacent channel on the VSB side.

A diode detector absolutely needs VSB as a minimum. DSB of course works.

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Thu Apr 24, 2014 3:13 pm

I've had personal experience of video interference: When channel 5 started we were on Sutton Coldfield, the video output was on factory default ch 36, when Lichfield fired up on ch 37 they clashed with each other, I turned the video modulator down a bit & all was well, although we couldn't watch ch 5 because of co-channel interference from Emily Moor. Then Waltham ch 5 fired up on ch 35 & that clashed, so I turned it down a bit more & it then clashed with Nottingham ch 5 on ch 34. I ended up going the other way but if I went too far I clashed with Sutton BBC 2 on ch 40, so I had to tune to about ch 38 1/2 & all was well. The telly then went bang & we got a new one with a SCART socket so no probs after that..

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Fri Apr 25, 2014 2:16 am

Receivers with very narrow bandwidth PLL fully synchronous demodulation might work with a single sideband vision transmission with pilot carrier, but even then, some care would be needed with the IF passband to obtain on the one hand, a sharp cutoff on the non-sideband side of the carrier, whilst on the other preserving the amplitude and phase linearity down to virtually DC on the sideband side of the carrier. It is not like voice-quality HF SSB reception, where audio frequencies below around 300 Hz may be ignored. With HF SSB broadcast relays, 100 Hz was the customary lower limit (90 Hz for the BBC), as compared with around 50 Hz for normal broadcast practice, and I suspect that some phase errors in that vicinity would have gone unnoticed. And very narrow bandwidth PLLs for vision demodulation have the disadvantage that any incidental phase modulation is carried over to baseband.

That said, whether SSB transmission with preservation of amplitude and phase linearity would be feasible is another question. My understanding is that the original choice of VSB rather than SSB had as much to do with the transmission end as the receiving end. VSB allowed the transmitter sideband filter to be reasonably removed from the carrier frequency.

PLL fully synchronous vision demodulation became more common in the later analogue era, but by no means universal. The probably more common quasi-synchronous (QS) variety, as conventionally implemented, might not have been very comfortable with SSB signals, the more so those with reduced carriers. Any sideband asymmetry in the signal presented to the carrier channel limiter results in demodulation errors (distortion), a facets that is seldom noted in the literature. The IF passband Nyquist slope thus in and of itself causes errors in conventional QS demodulation. And the errors would be worse for a fully SSB signal with zero sideband on one side. With a VSB signal, one could extract the vision carrier using a narrow bandwidth (i.e. less than the twice the VSB bandwidth) SAWF ahead of the main SAWF (and I understand that this was done in some BBC receivers), but that would not work for SSB, unless the filter could have say a 50 Hz or lower bandwidth. It may also be seen that the Nyquist error will be reduced with a gentler Nyquist slope, so that receivers using QS demodulation would ideally have an IF bandpass that fully utilized the transmitted VSB, even though with SAWFs, a steeper-than-needed Nyquist slope was feasible without compromising phase linearity.

As an aside, the Nyquist slope error was responsible for some of the intercarrier sound buzz with both rectifying and QS vision demodulators, and from that source it could not be removed by hard limiting in the sound IF channel. (There was though a work around back in the rectifying demodulator days, e.g. see the B&O 3000 circuit.) Nyquist slope buzz became intolerable in the (modern) stereo and multichannel sound era, hence QSS demodulation for the sound channel. (I say the modern era, because the French had bilingual sound transmissions with System E in Algeria in the second half of the 1950s.)

Regarding System I, by design I think it made the best use of an 8 MHz channel when following the established wisdom re guard bands; one assumes that the use of established guard bands was taken as a given in the investigative work. The French “cribbed” an extra 0.5 MHz by overlapping the guard bands. Likely this was based upon their experience with System E channelling. Whether negative vision modulation was a better choice for System I is I think very debatable, and probably a topic where if one asked 12 experts, one might receive 13 answers. In that era though, the body of opinion seemed to favour negative, so System I was ‘going with the flow” as it were. System L was counter-current simply to make easier the design of dual-standard receivers for Systems E and L.

On the bandwidth issue, here is an interesting quote from Carnt & Townsend, Volume I, from Chapter 4, “Transmitter Coding”:

“For 625 lines the suggested chrominance bandwidths are the same as for the American system. If an 8 Mc/s channel is available for each station, as in the U.S.S.R., the optimum utilisation of the extra megacycle of bandwidth is probably to increase the luminance pass-band by 0.5 megacycle and use the other 0.5 megacycle to increase the vestigial sideband of the vision carrier to 1.25 Mc/s. The latter reduces the effects of vestigial sideband distortion, particularly on negative modulation. If the sub-carrier frequency is left unaltered, the extra luminance bandwidth can be used to increase the chrominance bandwidth and to ease the problem of providing a sharp luminance and chrominance cutoff at the sound channel frequency.”

Looking at the colour side of things, the chrominance subcarrier needed to be of as high a frequency as possible to minimize its visibility in the luminance channel, and with System M and B the tradeoff had been sideband asymmetry in the chrominance channel. But the subcarrier frequency chosen for System B was evidently sees as high enough to be satisfactory from the (in)visibility viewpoint, such that the additional vision bandwidth available with System I could be allocated to avoiding significant chrominance sideband asymmetry.

Thus one might say that System I was better optimized for future colour transmissions than was System B. On the other hand, System N was going in the other direction, as it would have required a lower, and thus one assumes more visible subcarrier frequency, and with no relief from sideband asymmetry.

Had “quantity” won the argument over “quality”, with a modified (positive/AM) version of System N being chosen for the UK circa 1960, one wonders whether inverted channels (vision above sound) would also have been chosen to align with System A and so minimize dual-standard receiver complexity. Receiver simplicity would have argued argue for a common sound IF (38.15 MHz) and thus a vision IF of 33.65 MHz for 625 lines, which one hopes would have been a satisfactory choice, although at least at UHF, channel assignments could have been planned around it. Also would have been the question as to whether to use the European 625-line waveform (5 equalizing pulses) or that of System N (6 equalizing pulses.)

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by Michael Watterson » Fri Apr 25, 2014 1:02 pm

Actual SSB is very difficult. But a 200KHz BW receive bandwidth beyond carrier and ample carrier for the detector to lock avoids the issue of a true SSB system. Think of it more as an absolutely minimalist VSB with reduced carrier rather than true SSB. True SSB is a nightmare.

Good point on the Colour Subcarrier.

Had anyone seriously considered colour at all 1945 to 1948 during 625 Line development and trials in Russia and Europe? Hard to believe that they had not as colour was already in Cinema and "Colour" TV test transmissions in Germany in I think 1924! (no idea what the system was). Field Sequential is really a throwback to mechanical TV and no coincidence it's essentially what single chip DLP uses to display colour (typically 4 segment, one clear, 3 x dichroic filter, wheel at multiple of signal field rate).

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Fri Nov 14, 2014 1:32 am

As recorded upthread, the conventional wisdom re the origins of System N is that it was developed for Argentina and proposed for use in Japan, although which came first is unclear.

However, I have recently come across a couple of small items in Wireless World 1950 February which suggest that the idea of accommodating 625/50 in a 6 MHz NTSC channel may go back before those two events.

First is this, referring to Italian TV experiments.

WW 195002 p.45 Italian TV.jpg


It is noted that the American equipment was essentially built to American standards except that it was 625/50 rather than 525/60. The transmitter frequencies used, 83.25 MHz vision and 87.75 MHz sound correspond to 525/60 practice, with 4.5 MHz spacing between the two. In fact, they corresponded to channel A6. One wonders whether this choice was connected with the unique VHF channelling system that Italy adopted for System B. Italy did seem to be influenced by American practice; as recorded in the TV IF thread, very early on it standardized 45.75 MHz (vision) as its TV IF, this being the same as the US standard.

Anyway, that Italian experiment probably conformed to what later became System N. Assuming that no changes were made other than those needed for 625/50, then the chances are that the sound transmitter operated with ±25 kHz deviation and 75 µs pre-emphasis.

Second is this:

WW 195202 p.60 International TV.jpg


This shows that the optimum video bandwidth for 625/50 had yet to be determined, but that the choice was between 4.25 and 5 MHz. The 4.25 number more-or-less corresponds to Systems M an N, which are usually quoted as 4.2 MHz. System M started out (according to the NTSC (1) proceedings) at 4 MHz, but somewhere along the line moved out to 4.2 MHz. (Previously I thought that that may have happened with the arrival of colour per NTSC (2), but it seems that the change was earlier than that.) Anyway, the implication is that the debate at the time was between 4.25 and 5 MHz and therefore between 6 and 7 MHz channels. There was no mention of 6 MHz video bandwidth and 8 MHz channelling, as adopted by the USSR. Perhaps this had already been dismissed as too costly in bandwidth terms, or perhaps the cold war politics of the time intruded. Had Europe opted for the 6 MHz channel, I suspect that it would have made some changes as compared with American practice, such as ±50 kHz deviation and 50 µs pre-emphasis for the sound channel (which numbers appear to have come from Russian practice).

Anyway, it seems evident that System N (or something like it) was considered and rejected by the various Western European bodies in the lead up to the decision (at the 1950 July CCIR meeting, I think) that resulted in System B. Thus System N was probably already “on the shelf” when the Argentinean and Japanese deliberations started. More significantly, given though that the Europeans had already rejected System N in 1950, presumably as being not good enough, a UK decision to adopt it around a decade later would have been perverse, to say the least.

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Thu Jul 16, 2015 10:01 pm

When South Africa started their TV service in 1976 they had the choice of any TV system in the world with no pre-conditions. However they chose system I, so presumably that was the best system for their needs. Interesting that they didn't use band 1..

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Fri Jul 17, 2015 5:01 am

Yes, that was an interesting case. With the use of Bands IV/V as well as Band III, 8 MHz channelling may have been a predetermined parameter, in which case logically the choice would have come down to Systems D/K, I, K’ and L. L would have been an outside runner in any case, even though positive/AM was arguably a better choice for colour than negative/FM. K’ was the French Outré-Mer standard, and was negative/FM despite system L.

At the time Systems D/K, K’ and L were strongly associated with SECAM colour; I am not sure if there were any PAL variants in use in 1976. On the other hand System I was associated with PAL, so that may have been a factor in the RSA choice.

To my eye, System I – in the UK and South Africa - always looked just a bit better than System B/G. Whether that was real or expectation fulfilment I don’t know.

Why the non-use of Band I in RSA I don’t know. Possibly it was because of potential long-distance propagation and consequent interference problems, although with the longish intercity distances, that seems less likely. More likely is that the Band I frequencies were otherwise occupied. With the benefit of hindsight, it is a question that I could have asked the Sentech folks when I called them to obtain information on the local TV and FM frequencies – it must have been late 1996 or early 1997. I did learn that FM polarization had been changed from horizontal to vertical at some time in the past in order to better address the needs of car radios. Cape Town had quite a few low=power UHF TV repeaters because of its mountainous topography. There was one – my local transmitter - at the western end of Table Mountain, adjacent to the cable-car top station, just under the ledge, and visible from the viewing platform.

South Africa did use an upwardly extended Band III though, to 254 MHz, I think, which allowed 10 8 MHz channels. With the solid state devices then current, it wouldn’t have been a problem. (That said, I don’t know what kind of VHF tuner devices were used in South African TV sets; perhaps mosfets, but there may have been some of those silly Philips bipolar jobs that just about overloaded without any signal.) But back in the valve days, I think that 254 MHz would have been a bit beyond what say an ECC189/ECF86 combination could have managed in a noise-free way.

Also, the standard vision IF was 38.9 MHz, the CCIR number, and not the British 39.5 MHz. That would have made it easier to obtain good image rejection.

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Fri Jul 17, 2015 9:42 pm

Maybe South Africa didn't use band 1 as they'd heard or seen what problems with co-channel & impulse interference other Countries had (especially if they'd seen UK 405 lines/BBC1 in the Nottingham area in mid-summer) & thought better of it..

 
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Re: Wrong UK 625 line TV system?

Post by Terrykc » Fri Jul 17, 2015 10:16 pm

Synchrodyne wrote:... South Africa did use an upwardly extended Band III though, to 254 MHz, I think, which allowed 10 8 MHz channels ...

You are right about 254MHz, Steve, but not the 10 channels.

Alan Pemberton's excellent resource (backed up in the Library as its web future is uncertain) shows that, for some reason, channel 12 was not used. http://www.pembers.freeserve.co.uk/Worl ... html#I-RSA

Interesting, though, that the numbering range is from 4 to 13, suggesting that there were originally up to 3 channels planned for Band I but not implemented.

 
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Re: Wrong UK 625 line TV system?

Post by colly0410 » Mon Jul 20, 2015 9:38 pm

Apart from South Africa (& Malta & Gibraltar I think) did any other countries NEVER use band 1 for TV?... Wondering if any Countries are still using band 1?

 
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Re: Wrong UK 625 line TV system?

Post by Terrykc » Mon Jul 20, 2015 10:01 pm

Without being answering the precise question posed, I think it all comes down to history.

In the beginning there was only one band and that was Band I, so any country starting a TV service had no option but to use it, although - of course - the frequency range of Band I was liable to variation, depending on country ...!

Then came Band III. This was capable of satisfying demand for increased coverage and/or multiple channels.

Finally, along came Bands IV and V. This opened up the available range considerably. Countries that used Band I had long been plagued with problems caused, principally, by Sporadic E propagation, so newcomers now had the choice of avoiding Band I completely whilst early adopters were usually still tied to it by a consumer base which still used a proportion of VHF only receivers - look at how long it took for a switch to 625-line UHF in the UK to finally force users to stop using aging 405-line VHF sets, for example.

It is many years since the UK abandoned both VHF bands but many other countries have had to wait for the switch to digital broadcasting to take advantage of abandoning Band I at least ...

So, the answer really seems to boil down to countries, in the main, that only started TV broadcasting after it was no longer necessary to use Band I for coverage in the first place.

 
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Re: Wrong UK 625 line TV system?

Post by Synchrodyne » Wed Jul 29, 2015 5:31 am

The other side of that is that in some situations, the Band I channels were highly prized.

Here in NZ, for example, the first national TV network was set up in the early 1960s, by which time the Band I potential problems would have been known, yet all seven of the main transmitters were allocated Band I channels, of which there were three. Specifically, Wellington was assigned channel NZ1 (44 to 51 MHz) because it was thought that the signals would more readily bend around the many hills. At times the Wellington transmitter did cause interference to the Southland (Hedgehope) transmitter, also on channel TV1, but I think that was the only notable case. Not only that, but when the second network (TV2) was established, one of the seven main transmitters was also fitted into Band I.

In the USA, the low band channels seemed to have been preferred where they were available. In the late 1980s, when the Dallas-Fort Worth PBS operation wanted a second transmitter, it acquired the rights to channel A2, assigned to the nearby town of Denton, but not actually used, rather than taking a UHF channel. And when, in the early 2000s, it wanted to divest itself of that channel, it had not problem finding a taker notwithstanding that the analogue channels were by then on borrowed time.

Possibly the UK Band I experience was at the other end of the scale. The large number of transmitters and therefore the concomitant extensive channel sharing would not have helped. In a sense the BBC had to “chase its own tail” once its original idea to use some Band III channels was scuttled, or at least postponed for a decade. There was more channel-sharing by medium-power transmitters than originally planned, and gap-filling stations probably often had smaller coverage than otherwise possible in order to minimize co-channel interference, in turn requiring additional gap-gap-fillers and so more channel sharing. The predominant use of vertical polarization would not have helped, in that typical domestic receiving aerials have not much horizontal directionality when vertically oriented.

From my recollection of South African topography and the disposition of its population centres, I’d say that it was a place where on balance the Band I channels would have been satisfactory and beneficial. But for whatever reasons they were not used.

As already said, the channel numbering implies that three Band I channels (1 through 3, say 44 to 68 MHz) were at least notionally allocated. It is possible that RSA decided to align its channel numbering and frequency assignments with that already established for the French Outré Mer territories, which used System K’ in 8 MHz channels. As far as I know, in Africa only the Band III channels were used, but I have a vague notion that French territories elsewhere used the Band I channels.

The numbering might also have been to obtain approximate alignment with European practice, so that imported subassemblies used in locally manufactured TV receivers could be used without modification. By way of an example, the NZ-manufactured Philips K9 receiver (the most popular of the early colour receivers here) came with a European-style tuning tray, with each of the six presets marked channels 2 through 12. The resultant errors vis-à-vis the NZ channel numbering were deemed to be tolerable. (The K9 had one of those VHF tuners that self-flipped between Band I and Band III as the tuning control moved from E4 to E5. Performance-wise, that VHF tuner was an awful design, but during the 1970s the Europeans mostly seem to have clung to bipolar circuitry, instead of following the American example and moving up to the much better dual-gate mosfets.)

Cheers,

Steve

 
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Re: Wrong UK 625 line TV system?

Post by ntscuser » Fri Jul 31, 2015 11:21 am

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.

I also vaguely remember that many people thought that when when we did eventually get a fifth channel, albeit on UHF, that it should have been digital from the outset.

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