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)
The Coming of Stereo
Early experiments with stereo were made, of course, in the late 'fifties. They consisted of transmitting one channel using the TV sound channel, and the other channel on one of the radio services. Programme material mostly seemed to consist of ping-pong matches, and trains running across your mantelpiece. The entertainment value of this being low, and the shortcomings of the method apparent to all, it was something of a relief when the BBC announced in the early 'sixties that further experiments would be made using the Zenith-GE system (which is what the rest of the world were starting to use) on one of the FM services.
Mostly these experiments were made from the Wrotham transmitter. The advantage of doing this was that distances were short - both for engineers commuting between specialist London departments and the station site, and for the programme signals themselves.
It became apparent quite quickly that the system was a worker, but that programme distribution was going to be somewhat of a problem. The Post Office had provided a pair of programme landlines from Broadcasting House to Wrotham, guaranteed balanced both for frequency and phase response. (If the phase response of the two lines is not matched, mysterious shifts of stereo image result for listeners). The problem was that keeping the lines matched was likely to prove an expensive business: bearable for lines to Wrotham, but not so for a national network of programme distribution. The first step towards this network was announced in the mid 'sixties, when regular programmes were announced for Radio 3 only, initially from three stations, Wrotham, Sutton Coldfield and Holme Moss.
The ultimate solution to the programme distribution problem was to go digital, with the first 13-channel PCM system. But this was still a fair way ahead, with much development work to be completed first. Some stopgap system would have to be devised, and at minimum expense (a normal BBC criterion).
The slightly weird stopgap that resulted was made possible by an experimental VHF-FM receiver that had been devised by some department or other (Research?). This receiver used a pulse-counting discriminator rather than the more normal Foster-Seeley or Ratio Discriminator. In order to make a pulse-counter work efficiently, a low value of intermediate frequency is needed, and this unit used 1MHz. How would it be, somebody thought, if we sawed this receiver apart in the IF strip, moved the two halves miles apart, and joined them with a microwave link?
Well it wasn't quite as simple as that, of course, but the final solution consisted of a receiver site at Whipsnade, where the output of Wrotham was received off-air by the front half of this sundered receiver. This gave a 1MHz frequency-modulated sine wave output, which was fed into a Pye M715 microwave link. Its 7GHz output signal was beamed northwards to an intermediate site at Whichford Hill, where the SHF signal was transposed to a new frequency, amplified and shot on its way to Sutton Coldfield. Here the latter half of the receiver lay in wait to demodulate the output from a Pye link receiver. The onward path from Sutton to Holme Moss was accomplished in a similar manner.
It all worked surprisingly well. The only fly in the ointment was the intermediate site at Whichford Hill: in order to get an unobstructed path to Sutton, the receive dish at the latter had to be sited at a good height: halfway up the mast. The other snag was that one of the paths at Whichford passed through a belt of trees; no problem in the winter months, but this caused severe fading in the summer if the leaves got wet. Unfortunately the trees had a Preservation Order on them, and could not be lopped. Ultimately, when this link was re-engineered to handle the PCM distribution chain, Whichford had to go, and the link became four hops: Swains Lane (North London, where the PCM signal was modulated onto an SHF carrier) to Whipsnade, to Thorpe Lodge (north-east of Banbury), to Meriden (a Gas-board site, later used for BBC CWR local radio also) and so to Sutton. With this arrangement the receive dish at Sutton could be lowered to a more reasonable height.
Sutton was fortunate in its choice of the S.T. & C. FM drive. This unit proved very simple to modify for stereo operation: all that was involved being provision of an extra unbalanced 75ohm input for the stereo multiplex signal, and a changeover relay to select either that input or the original mono 600ohm balanced input. Hum was always more of a problem on an unbalanced input, but this was mostly removed by passing the signal through a massive Post-Office stop coil (basically, an inductor wound with co-ax cable around a magnetic core).
Stations that did not use the S.T. & C. drive were less fortunate. The Marconi FMQ drives that were used at some other stations proved quite impossible to modify for satisfactory operation. These stations had to be re-engineered with new drives - in the earlier days, BBC designed and built VRFM drives (Variable-Resistance Frequency Modulation) although these were less than totally satisfactory themselves, and in later years were again replaced with VIFM drives (Variable Inductance Frequency Modulation), also BBC designed and built. The performance of these last drives was so good that it was actually better than the measuring equipment being used to maintain it - what you were in fact measuring was the performance of the test gear. This situation had to be borne with for several years, until there was enough money in the piggy bank to replace the test gear too.
Maybe the re-engineered stations were not unfortunate - at least they got more up-to-date drives. Sutton had to soldier on with the original drives until its FM installation was re-engineered in the early 'eighties.
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