THE TRANSMISSION GALLERY
TALES FROM A COLD FIELD
Being fairy-stories told to the author as a young engineer
© Ray Cooper, 2005 (2nd revised edition, Jan 2006)
What was that? Well, it stood for 'Television Outside Broadcast Facility'. In those distant days before satellite links, if you wanted to do an O.B. you had to provide a temporary land-based vision link to get the pictures back to the studios. Quite often you could get away with a single hop, so long as you could get your link aerials as high as possible.
Well, at the O.B. end you were limited to about eighty feet or so, if you used an Eagle Tower . These were basically zip-up telescopic towers mounted on a lorry, much in the style of a fire-brigade extending ladder. At the receive end, you could do much better - there were all these tall transmitting masts dotted round the country, many of them already connected to studio centres by cable or microwave.
The O.B. receive aerials at Sutton were 4 ft. parabolic dishes - two of them mounted on the corners of the maintenance platform at 600 feet, just below the F.M. cylinder. These dishes were mounted so that they could be rotated to point in any required direction under motor remote control. Of course, for a certain part of its rotation, a given aerial might be trying to look through the mast and so would be of no use: but the pair, between them, gave 360-degree coverage.
They worked in the 7 GHz band, and the masthead installation consisted of a balanced-diode mixer, a reflex klystron local oscillator and the front section of an I.F. strip. The reflex klystron could be tuned through the band, again under motor remote control, and the resulting I.F. signal was piped down the mast in co-ax to the O.B. room, where the rest of the link receiver and all of the control gear resided. From there it could be piped to Pebble Mill using one of the 'one-inch' Post Office tubes, reversed.
Mostly, all of this gear was set up and run by an engineer from Birmingham Comms. Department, who came up for the day (usually a keenly looked-forward-to visit, since the canteen at Sutton was then regarded as superior in quality to the studio one, and there were not so many annoying telephones ringing all day). But there was just one circumstance where we were allowed to get our hands on for ourselves.
This let-out came in the form of the Regional Contribution Studio at Nottingham . Which sounded very grand, and in these days has been expanded into something quite substantial: but in those days amounted to no more than a tiny studio with one camera channel, self-operated by the presenter that was going to do his piece. On arrival he would merely switch it on at a master switch on the wall, and go have a cup of tea whilst the gear warmed up. The studio was connected by landline to a fixed SHF dish mounted on a tower on the top of the nearby Technical College , and pointing at Sutton. It radiated on an O.B.-allocated frequency.
When the regional news programme (Midlands Today) wanted to do an insert from Nottingham , they would tip us the wink and somebody would shamble round to the O.B. room and switch the gear on. Probably it would not be already set up in the Nottingham direction, so he would have to pan the dish round onto the right bearing (Dymo label stuck onto the top of the bay) and tweak the klystron for the correct frequency (another Dymo label). If the Gods were with him, some sort of signal would be received, and the dish could then be nudged to and fro for the strongest signal. Then the klystron would be tweaked for best linearity of the video waveform. It's actually quite a way from Nottingham to Sutton, rather more in fact than is healthy for a link using four-foot dishes. So the pictures that you got would depend rather on atmospheric conditions. If propagation was odd, or there was heavy rain, the pictures might well not be usable, in which case the studio had to be rung with the bad news that it was all off. They would then hurriedly have to find something to plug the gap.
In later years, the studios were much expanded, and a permanent and decent link provided, using an intermediate point on Bardon Hill.
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