How would a re-engineered VHF TV band have worked?

[VinTech90]

Back in the 1970s there was much discussion on whether to re-engineer the UK's VHF allocation for 625 line working using eight channels (in all probability they would have used the Irish channels A through H, no idea if they'd have tried to persuade the ITU to allocate I and J), apparently this would have allowed for two complete networks, but I've not been able to work out exactly how this would have been done, has anybody ever seen some sort of preliminary proposal? I've tried to work it out digitally using a map of the British Isles but so far I've drawn a blank.

[Nuvistor]

Some light reading for you.
http://downloads.bbc.co.uk/rd/pubs/reports/1965-43.pdf

https://transdiffusion.org/2017/04/2...o-625-line-tv/

[G6Tanuki]

I can see the potential logic in something like this; it is after all how colo(u)r - albeit on 525 lines - happened in the US.

Rather than any sort of national networks, I would have done it by selling licenses to operate relatively low-power [5-10KW] stations on a city-by-city basis, with frequency-re-use and no guarantee of protection against interference caused by another station using the same frequency 50-100 miles away in the next town.

In many areas I reckon you'd have ended up with every channel occupied.

That was pretty much how I remember it being in the late-60s when I spent a few months in Dallas/Fort Worth with my parents.

[Nuvistor]

Didn’t the States have channel plans that protected the channels for around 100 miles or so, one reason they wanted to use UHF.

Cable TV allowed more channels to be used using extended frequency bands.
A very interesting set of posts from Synchrodyne of Cable TV tuners in his series of IF frequencies on V-Rat.
https://www.radios-tv.co.uk/communit...8/#post-119190

[G6Tanuki]

The IF considerations of fully-populating the VHF channels are interesting; assuming the usual 38-ish MHz IF then the image frequency [asuming the LO is tuned high of the signal frequency] always falls outside the TV frequency allocations.

On UHF here in the UK there was a rule that essentially barred the image-frequency from being used for a transmitter in the primary-coverage area of another transmitter. Early UHF tuners were not too good - certainly the valve ones had a reputation for deafness, and reliable/consistent signal-frequency selectivity [tracking!] was often rather questionable too.

Dual conversion would have helped...

[ I remember some of the US UHF ones used no RF amplification and fed straight into a diode mixer. Noise-figure? Who needs to worry about that??]

[VinTech90]

Originally Posted by Nuvistor

Some light reading for you.
http://downloads.bbc.co.uk/rd/pubs/reports/1965-43.pdf

https://transdiffusion.org/2017/04/2...o-625-line-tv/

I've read those before.

The R&D link is about the possibility of compressing a 625-line picture into the existing 5 mc/s channels rather than re-engineering the VHF band for 8 mc/s channels after 405 line closure.

[VinTech90]

Originally Posted by G6Tanuki

I can see the potential logic in something like this; it is after all how colo(u)r - albeit on 525 lines - happened in the US.

Rather than any sort of national networks, I would have done it by selling licenses to operate relatively low-power [5-10KW] stations on a city-by-city basis, with frequency-re-use and no guarantee of protection against interference caused by another station using the same frequency 50-100 miles away in the next town.

In many areas I reckon you'd have ended up with every channel occupied.

That was pretty much how I remember it being in the late-60s when I spent a few months in Dallas/Fort Worth with my parents.

That does actually sound rather a good idea.

Alas here the powers-that-be had a very fixed paternalistic view, demanding complete networks with over 95% population coverage.

The former USSR in the 1990s managed to mix the best (or possibly worst!) of both worlds; private broadcasters were allowed to pick blank channels in a given area so long as they didn't interfere with existing neighbouring stations, leading to fascinatingly weird situations where massive central masts would broadcast one or two VHF channels at powers of 100 kW or more and a further few channels of less than 10 kW.

The Minsk transmitter in Belarus poured out the two main networks at over 600 kW (!) on Channels R1 and R6 respectively. with two private channels squeezed in on R3 at 5 kW and R8 at 7.5 kW, with yet another transmitter on the roof of a factory transmitting 1 kW on R11.

[Synchrodyne]

Originally Posted by VinTech90

Back in the 1970s there was much discussion on whether to re-engineer the UK's VHF allocation for 625 line working using eight channels (in all probability they would have used the Irish channels A through H, no idea if they'd have tried to persuade the ITU to allocate I and J), apparently this would have allowed for two complete networks, but I've not been able to work out exactly how this would have been done, has anybody ever seen some sort of preliminary proposal? I've tried to work it out digitally using a map of the British Isles but so far I've drawn a blank.

As far as I know, the non-use of the upper end (216 to 230 MHz) of Band III in the UK was a country decision, not an ITU decision. Certainly, the ITU 1961 Stockholm meeting showed Band III as covering the 162 to 230 MHz range, although the 162 to 174 MHz and 216 to 230 MHz parts had more limited use.

Originally Posted by G6Tanuki

In many areas I reckon you'd have ended up with every channel occupied.

That was pretty much how I remember it being in the late-60s when I spent a few months in Dallas/Fort Worth with my parents.

In 1985, the VHF channels used in the DFW area were 4, 5, 8, 11 and 13. Channel 2 was added in the early 1990s as I recall, and that six channel line up was still in use in 2002 when I left the area. As far as I know, that was the full FCC VHF allocation for DFW. Note that channels 4 and 5 were not actually adjacent, but separated by a 4 MHz gap. The potential VHF channel interference problems were shown in this chart:

Originally Posted by G6Tanuki

The IF considerations of fully-populating the VHF channels are interesting; assuming the usual 38-ish MHz IF then the image frequency [asuming the LO is tuned high of the signal frequency] always falls outside the TV frequency allocations.

On UHF here in the UK there was a rule that essentially barred the image-frequency from being used for a transmitter in the primary-coverage area of another transmitter. Early UHF tuners were not too good - certainly the valve ones had a reputation for deafness, and reliable/consistent signal-frequency selectivity [tracking!] was often rather questionable too.

In the USA, at the start of the UHF era, the FCC developed a list of channel assignments that needed to be avoided for interference reasons, this being known as the “UHF taboos”, as shown in this chart:





A similar chart was developed by the ITU at the 1961 Stockholm European UHF allocations meeting:

Originally Posted by G6Tanuki

[ I remember some of the US UHF ones used no RF amplification and fed straight into a diode mixer. Noise-figure? Who needs to worry about that??]

In respect of North American UHF TV tuners, no RF amplification, and bandpass tuning into the diode mixer, emerged as the standard format in the early 1950s, and remained so until 1970 or so, with just a change from valve to transistor oscillators. In the early days, the valve makers had developed triodes suitable for the UHF RF and mixer applications, but these were expensive and short-lived. On the other hand, the simple tuner with an essentially passive signal path, and feeding into the VHF tuner RF stage (typically a cascode) did an adequate job and met the oscillator radiation limits.

The change away from that form was driven in part by the advent of varactor tuning. Because it was somewhat lossy, an RF amplifier was needed, usually with single stage input tuning and then feeding into the bandpass ahead of the diode. At the back end, an extra gain stage (IF preamplifier) was needed, although less usually (e.g. Sylvania) a transistor mixer was used to provide the requisite gain. The extra gain was needed because with varactor type VHF tuners, it was inconvenient – in the absence of switched tuning - to feed the UHF tuner output into the RF amplifier. Rather it needed to go into the mixer. Given that the agc’d VHF RF stage was now bypassed, agc was typically applied to the UHF RF stage.

In Europe, the tighter limits on oscillator radiation indicated that an RF amplifier ahead of the mixer was needed. Also, the autodyne mixer was preferred because of its relatively low oscillator drive level. (probably valve count was also a factor here.) Thus the normal form was an aperiodic input to the RF amplifier with a bandpass interstage to the mixer. In the UK though, the decision to use a 39.5 MHz rather than the standard 38.9 MHz vision IF pushed up the image rejection requirements quite significantly, so that a tuned input was needed. Because such UHF tuners had some gain, their output was fed to the VHF tuner mixer, rather than to the RF stage. In valve form these UHF tuners had no agc, and that may have been a factor in the development of VHF frequency changer triode pentodes with pentodes of the semi-remote cutoff type, allowing some delayed agc relatively early in the UHF pathway.

Originally Posted by G6Tanuki

Dual conversion would have helped...

Indeed so. In fact the FCC sponsored a project for the development of a TV receiver whose immunity to interference problems would eliminate or at least reduce the need for the UHF taboos. The resultant design did use a dual conversion tuner. (1) Dual conversion had long been the norm for set-top cable converters, but although proposed here and there, it seems to have been avoided for off-air reception until the 1990s. In 1988, it was reckoned to increase the per set cost by $US3.50. (2)

(1) IEEE Paper 1982 November: “An Improved High-Performance TV Receiver”; Darrell L. Ash (RF Monolithics).
(2) IEEE Paper 1988 March: “Cableready Television Sets – The Myth Continues”; Earl Langenberg (American Television and Communications)



Cheers,

[marceljack]

Originally Posted by VinTech90

The R&D link is about the possibility of compressing a 625-line picture into the existing 5 mc/s channels rather than re-engineering the VHF band for 8 mc/s channels after 405 line closure.

I was very surprised by reading this report which envisaged to use a different 625 lines standard than the one used in UHF just to keep the existing 5 MHz VHF channeling !
We had the same problem to solve in France when converting the existing 819 lines VHF network with 14 MHz channel width, but as far as I know nobody thought of a different 625 lines system in VHF than in UHF.
It would probably have been possible to place two 625 lines 7 MHz channels in the space of one 14 MHz 819 lines channel, but it would have required a different sound-vision spacing than in UHF, thus complicating the receivers.
So a new so called L' 8 MHz channeling was defined for VHF (only used by Canal+ in most parts of France).
To be noted that TMC (Tele Monte Carlo) converted their 819 lines transmitter to 625 lines before the L' channeling was defined, so they kept their channel 10 (syterm E) with 11,15 MHz sound-vision spacing during many years. A waste of almost 5 MHz !

[Mr Hoover]

Originally Posted by marceljack

I was very surprised by reading this report which envisaged to use a different 625 lines standard than the one used in UHF just to keep the existing 5 MHz VHF channeling !
We had the same problem to solve in France when converting the existing 819 lines VHF network with 14 MHz channel width, but as far as I know nobody thought of a different 625 lines system in VHF than in UHF.
It would probably have been possible to place two 625 lines 7 MHz channels in the space of one 14 MHz 819 lines channel, but it would have required a different sound-vision spacing than in UHF, thus complicating the receivers.
So a new so called L' 8 MHz channeling was defined for VHF (only used by Canal+ in most parts of France).
To be noted that TMC (Tele Monte Carlo) converted their 819 lines transmitter to 625 lines before the L' channeling was defined, so they kept their channel 10 (syterm E) with 11,15 MHz sound-vision spacing during many years. A waste of almost 5 MHz !

There was also the compressed 819 line system used by Belgium and Luxembourg,
System F with +5.5MHz AM audio.
France also had some 819 line Uhf relays (I remember receiving Cherboug Ch65 in
Southern England, not sure if this used +5.5 or 6.5 audio.

[marceljack]

There were a few UHF retransmitters of the first programme in 819 lines, using normal french L standard (+6,5 MHz sound to vision spacing).
This was "provisional" transmissions awaiting the conversion of the 1st programme to 625 lines SECAM.
In fact with AM sound intercarrier was not possible so it was not too difficult to adapt the receiver to various sound-vision spacings by reducing the vision bandwith (for Belgian and Luxembourg channels this bandwidth reduction was done by special "barrettes" on the turret VHF tuner).
As far as I remember only 3 VHF channels were using this 7 MHz bandwdth with positive modulation and AM sound (Luxembourg E7, Belgium E8 and E10).

[Graham G3ZVT]

Originally Posted by marceljack

To be noted that TMC (Tele Monte Carlo) converted their 819 lines transmitter to 625 lines before the L' channeling was defined, so they kept their channel 10 (syterm E) with 11,15 MHz sound-vision spacing during many years. A waste of almost 5 MHz !

Originally the vision transmitter from Alexander Palace used DSB, and the wider allocation for Channel 1 was a lasting legacy.

[marceljack]

Originally Posted by Graham G3ZVT

Originally the vision transmitter from Alexander Palace used DSB, and the wider allocation for Channel 1 was a lasting legacy.

This was the same for the 441 lines transmitter of the Eiffel Tower (sound at 42 MHz, vision at 46 MHz) but most receivers processed it as a VSB signal.

[Mr Hoover]

Originally Posted by marceljack

There were a few UHF retransmitters of the first programme in 819 lines, using normal french L standard (+6,5 MHz sound to vision spacing).
This was "provisional" transmissions awaiting the conversion of the 1st programme to 625 lines SECAM.
In fact with AM sound intercarrier was not possible so it was not too difficult to adapt the receiver to various sound-vision spacings by reducing the vision bandwith (for Belgian and Luxembourg channels this bandwidth reduction was done by special "barrettes" on the turret VHF tuner).
As far as I remember only 3 VHF channels were using this 7 MHz bandwdth with positive modulation and AM sound (Luxembourg E7, Belgium E8 and E10).

Also on band 1, Belgium E2 Aalter/Ruislede (closed late 70's) and E3 Liege.They
changed to the usual system B on Vhf about the time E2 closed.

[SteveCG]

I imagine if you lived in East Anglia during the 1960s then the DX TV hobby would have been quite interesting...

[VinTech90]

While all this has been very interesting, I think I might not have clarified exactly what I meant.

Supposing the Irish channel spacing were adopted, which I see as quite likely given other System I countries such as South Africa also used (most) of the Irish channels (specifically D to J). How then would this have been implemented? Would British VHF sites have had their frequencies moved to the closest System I frequency? For example IF's vision channel occupied B9's audio channel; in that case would Winter Hill and Stockland Hill, for example, output on IF? Where would this leave the slightly awkward stations on peninsulas jutting out into the Irish Sea such as Arfon? Because B10 is overlapped by IF and IG; given the smaller number of channels available would they have had to find a semi-decent UHF channel for these sites?

The documentation available claims two full networks could have been fitted into channels A through H, so I'm a little puzzled how they might have achieved it.

[dave walsh]

I'd always thought that 625 lines was really an economic/political move to capture European markets. It's been pointed out here that colour was achieved with less lines elsewhere. We had to wait a very long time for colour in Britain, at 625 lines, although JLB was showing live colour TV events to Cinemas [via telephone lines "cable"] in 1935/6, It's "Back To The Future Now" with optical distribution from poles to homes. 1936 was when the De La Warr [Futurist] Pavillion opened here in Bexhill. John Logie spent the last seven years of his life in the town but died in 1946 when he was just about to demonstrate a new [all electronic] colour tv system in London.

It does make me wonder how things might have turned out in an earlier 405 line colour world? It was mentioned in the Canterbury to Ally Pally edition of Great British Railway Journeys Railway [Weds 6/12 BBC4] that many people in the "Establishment" simply thought Radio would always be superior to this new fangled "tele vision" run by "fools". They may have been the "influencers" of their day, holding things back and regarding all mediums as transitory....not worthy of archiving!


Coincidentally I mentioned the BBC 1966 Colour TV Trials recently. It was in Colin B's recent thread titled "Older DAB sets not working?" so it's not at all obvious [Modern Tech 3/12 post 41*]

Dave W

[Graham G3ZVT]

I had a vivid dream a few years ago. In it I had rediscovered the HMV 1871 we had when I was growing up. I switched it on and was amazed that most, if not all 13 channels had a 405 line station, and all of them in colour(!) Some seemed experimental, weird test cards and suchlike, and some showed OUesque academic programming.

Anyway, make of that what you will.

I've always thought the decision to abandon VHF and build an all UHF network a very brave one when you consider the number of transmitter sites involved.
UHF TV seemed very cutting-edge to me, and this was only reinforced when I joined the trade. It was a minor miracle to keep a PC86 oscillating on BBC2 near the top end of the band (ch62 Winter Hill).

It was the right decision though, who needs all that co-channel Sp-E? (all right, don't all shout out at once).

[G6Tanuki]

Originally Posted by Graham G3ZVT

I've always thought the decision to abandon VHF and build an all UHF network a very brave one when you consider the number of transmitter sites involved.
UHF TV seemed very cutting-edge to me, and this was only reinforced when I joined the trade. It was a minor miracle to keep a PC86 oscillating on BBC2 near the top end of the band (ch62 Winter Hill)..

The initial takeup of 625-line UHF TV in the UK was both slow and problematic; problematic because of the lacklustre performance of the first generation of valve tuners (made even more significant when the poorer coverage of UHF became apparent - 'fringe' reception with early UHF tuners was never a fun experience), and slow because at first the only offering on UHF was BBC2 whose content was perceived as 'highbrow' and lacking in mass audience-appeal. Dual-standard TVs were not the best things in the world.

As noted upthread, I still think the UK went down the wrong path with the idea of enforced 'national networks' on both VHF and UHF TV [and also on the first iteration of FM radio] - a much more relaxed, regionalised/localised model would surely have created more happy viewers/listeners along with more and varied content-providers.

Imagine the VHF 405-line TV equivalent of the 1960s pop pirates! If you're a pirate TV station running on Band-III from Manchester you could happily camp out on the London ITV Band-III channel with little risk of interference. And add in something like the US "Stratovision" with aircraft-mounted transmitters [100 Watts at 45000 feet gives you a brilliant line-of-sight coverage!]

[VinTech90]

Originally Posted by G6Tanuki

As noted upthread, I still think the UK went down the wrong path with the idea of enforced 'national networks' on both VHF and UHF TV [and also on the first iteration of FM radio] - a much more relaxed, regionalised/localised model would surely have created more happy viewers/listeners along with more and varied content-providers.]

I suspect it was very much a holdover from the 1920s; the fledgling BBC made the decision that the transmitters owned by its parent companies would be forced to keep well apart from each other and operate in separate cities instead of being allowed to compete for listeners. By the mid-1920s they seemed to relish wasting the UK's wavelength allocations on tiny relay stations when these could have been allocated to competing stations. For some reason Bradford and Leeds had separate transmitters yet both relayed the same programme! I find it hard to believe that Bradford listeners would not have been able to hear the Leeds station and vice versa.