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)
Some stations get their pictures off a microwave link, but Sutton has always used some form of cable. For the 1949 opening, the Post Office (as it then was) provided no less than two different routes from a main London exchange (MUSeum, I think, but may be wrong) to the main Birmingham exchange in Steelhouse Lane . One route used a UHF radio link, the other a co-axial landline. In any event, the links from Alexandra Palace to London , and from Birmingham to Sutton, were by co-ax cable.
The intermediate link used for the opening was apparently the UHF link, specially developed by the Post Office, using multiple hops on 900 MHz and designed to be reversible, so that programmes originating in the Midlands could be sent to London . Reversing was meant to be done in a matter of seconds, but legend states that the first time it was tried, it took hours to get it back. There was a tower on the London exchange roof, linking via intermediate repeaters at Ivinghoe, Charwelton and Rowley Regis, to a similar tower on the Steelhouse Lane exchange roof. Quite large parabolic dishes were used, of openwork mesh construction.
The other method was by co-axial landline cable connecting the London and Birmingham exchanges. The cables used were a special type incorporating two 0.975inch diameter co-axes, and multiple twisted-pair circuits for sound programme, control lines and telephones. The entire cable was about 2.5inches in diameter, and lead-sheathed. Initially, the vision signal was modulated onto a lowish frequency RF carrier, instead of plain video, to ease equalisation problems.
At Sutton, the cable emerged from a hole in the ground into the L.T. (Lines Termination) room, having made its way from the main Birmingham exchange in Steelhouse Lane via various intermediate telephone exchanges. The vision signal was demodulated with Post Office owned equipment and then handed on to BBC equipment. One co-ax circuit ('tube' in Post Office parlance) was used as the main feed, the other one carrying a standby signal.
The UHF link was never completely successful, seemingly. It had design problems, which meant that its performance was never quite up to BBC expectations. Quite early on, it was relegated to absolutely-last-ditch standby duties, and not long after that quietly switched off for good. The towers (well, at least the one on the Birmingham exchange) remained in place for many years, presumably having been diverted to other duties by that time. The Birmingham one remained until the new concrete Post Office Tower was completed adjacent to the exchange in the early 'sixties.
At some date in the early 'sixties, equalisation techniques had advanced sufficiently to enable the Post Office carrier gear to be removed, and vision signal distribution was thereafter done using a plain baseband video signal.
When BBC2 arrived in 1964, a new vision cable was required. This was smaller in diameter than the original, and contained four 0.375 inch diameter co-axes, rated as being colour-capable. Spare circuits therefore became available.
When colour came to BBC1 in 1969, the old vision circuit remained in use, but had to be re-equalised for colour service. This was done using tandemed equalisers: there was hardly any chroma visible on the signal coming direct from the tube, only a few percent of normal: but by using modern equaliser designs, this could be jacked back to 100% without wrecking the noise performance of the link. The circuit from Birmingham to Sutton was connected up as one continuous length: no repeaters or amplifiers, so effectively it was about a nine-mile cable that was being equalised. The benefit was that both of the 'one-inch' tubes now had no associated Post-Office equipment between Pebble Mill studios and Sutton, and so were reversible under BBC control.
One of the tubes was permanently allocated for BBC1. The other on, being reversible, could be used either as a standby for BBC1, or as a return path to the studios for OB contributions. However, there was always the possibility of losing it all. This happened on at least one occasion, when a construction worker put his digger through the whole cable. A cable of this complexity takes some time to repair, and to form an interim feed Birmingham Comms. Department hurriedly rigged up an OB link transmitter on the roof of the studios, which was picked up at Sutton by the usual OB link receivers (see TOBF).
At most transmitting sites, useful standbys are provided by using off-air reception of a nearby main transmitter. This option didn't work at Sutton - it was the only nearby main transmitter. It got excellent signals from Holme Moss, but since this was further up the distribution chain it didn't help if the failure was between London and Birmingham . (The reverse path, Sutton to the Moss, was also excellent and much more useful to our northern cousins). Neither Alexandra/Crystal Palace nor Wenvoe gave usable signals. Oxford , which could have been of use, was in 405-line days only a low-power station. However in later years, when UHF TV started, Oxford became a high-power site and the BBC2, and later BBC1, pictures were, if not perfect, at least usable. The only snag was that Oxford , in those days, re-radiated Sutton Coldfield.
There were ways round this problem: means existed to induce Oxford to use Crystal Palace as its main feed instead of Sutton. Since such methods are probably still in use, and may have slight security implications in view of the number of malcontents who seek to disrupt broadcasts, I shall not enlarge on them. Additionally, similar facilities enabled Sutton to rebroadcast FM radio signals from Oxford if needed.
Much later - around 1990, in fact - British Telecom tried to re-negotiate line rental fees across the country. Video private circuits have never been cheap, but their proposed increase amounted to a factor of about three. This was a lot for the BBC to swallow. Fortunately, the monopoly stranglehold on permanent point-to-point links that the P.O./B.T. had enjoyed for years was at an end, so the BBC went round all potential providers with the shopping bag. In the end, they decided on Energis.
Energis were at the time a subsidiary of National Grid. There had been all sorts of uninformed chatter in the press that Energis were going to 'send TV pictures down the mains'. The reality was rather more prosaic and believable - they were going to use the earth-cables running from top to top of electricity pylons across the country as support for a fibre-optic data highway: the fibre was wrapped in a very slow spiral round the earth cable. There was a suitable Grid power line handily close to the site. Transmission down the fibre was done digitally of course, and there was plenty of room for at least two TV channels and the PCM sound distribution system. All of the necessary decoding gear was provided by Energis and installed in the L.T. room. Switching the PCM distribution to another supplier meant that the BBC-provided PCM SHF link became redundant, but it was not scrapped at that point since it might have further uses.
The TOBF >