Lines Department - Reminiscences
Recollections of early link, cable and transmitter problems
by John Shelley and David Savage (ex. Designs Department)
The London Birmingham Radio Relay System (IJS)
The Sutton Coldfield station was opened in 1949 and was connected to London via an experimental 900MHz Radio Relay system supplied to the Post Office Radio Branch by the General Electric Co.Ltd. Originally, to meet BBC requirements, the specification called for simultaneous transmission in both directions between London and Birmingham. However it was not possible to provide the full two way installation in time for the opening of Sutton Coldfield and it was agreed that the BBC would, in the beginning, accept one channel provided that the direction of transmission should be 'capable of being reversed at will in less than ten seconds'. The BBC carried out acceptance tests from the old TV Switching Centre in BH Room G28. All went well until, at the end of the London to Birmingham tests we asked for the direction of transmission to be reversed (according to the specification, in under 10 seconds) .This proved to be a great mistake as the system collapsed and it was well over three hours before it was back in service! The 90OMHz links could not meet the BBC performance requirements and late in 1950 they were replaced with a 0.975in. coaxial cable system which provided more reliable simultaneous two-way transmission between Alexandra Palace, Broadcasting House London, Birmingham and Sutton Coldfield.
The Coronation (IJS)
During preparations for the TV coverage of the Coronation in 1953 it was obvious that any failure of either of the television links from London to Sutton Coldfield or Wenvoe (both provided by the Post Office Engineering Dept.) would be a major embarrassment and large areas of the country would lose their pictures. To guard against this eventuality it was thought that the Wenvoe transmissions might be received off-air at Sutton Coldfield (a distance of about lOO miles) and vice-versa thus providing an emergency link facility in either direction. The off-air reception used a screened and modified rebroadcast receiver (TV/REC3a) which I had already developed while in TV Transmission Section in Designs Dept and this was connected via highly selective filters to an aerial on the mast just below the main transmitting aerials. The technical problems of receiving a very weak signal in the close vicinity of a 50kW transmitter were severe and, in the case of Sutton Coldfield, involved restricting the definition of the transmitted 405-line signal to about half of normal -an acceptable situation in an emergency and better than a total loss of picture .
The installations at Sutton Coldfield and Wenvoe provided reasonably satisfactory reserve links and fortunately there were no network failures during Coronation Day.
There were lighter moments during this work:
The only time available for experimental reception tests was for short periods during morning Trade Transmissions and it was while making the necessary adjustments to restrict the transmitted bandwidth of the Sutton Coldfield transmitter that the EiC' s secretary came to ask if I could deal with a call from an agitated TV dealer. It appeared that, in anticipation of the forthcoming Coronation, he had taken a Phillip' s large-screen projection receiver to demonstrate at nearby Shugborough Hall (seat of the Earl's of Lichfield) and, having spent some considerable time setting up the receiver, the picture quality was so poor that he was about to be shown the door! I apologised and explained that we were having problems with the transmitter ( ! ) and suggested that he hold on and I would see what could be done. I returned quickly to the transmitter hall and restored everything to normal and, on going back to the telephone, the dealer was by then much relieved and he said that his potential customers were also more favourably impressed. I never found out whether or not he eventually made a sale!
The 'Rope' Effect (IJS)
In 1952 I was asked to look into reports of a strange disturbance on domestic receivers in the Holme Moss area. This took the form of a narrow unstable vertical stripe towards the left of the picture and because of it' s appearance it became known as the 'Rope' effect. Tests showed that this was the result of three factors. The time-of-rise of the trailing edge of the line sync. pulses from the Holme Moss high-level modulated transmitter was faster than had previously been encountered and the poor transient-response of the receivers at that time caused 10% transient variations (ringing) in the post-sync 'back-porch'. These two effects would not by themselves have been serious but most makes of receiver had a slow fly-back time and virtually no effective line-blanklng suppression with the result that a vertical striation ('rope'), occurring towards the end of each line-flyback, was clearly visible. A small off-tuning of the receiver reduced the effect but eventually changes in the blanking performance of receivers had to be made.
A problem with the Kirk 0' Shotts transmitter (IJS/DCS)
The Kirk 0' Shotts transmitter was opened in March 1952. During 1954 the network was extended northwards to an interim transmitter installation at Redmoss (Aberdeen) with a link consisting of direct reception of Kirk o'Shotts signals at the Post Office station at Craigowl feeding a Post Office micro-wave link on to Redmoss. Initial tests by Designs Department on this link indicated a severe loss of high-frequency-response which was traced to the output of the PO designed vestigial-sideband receiver at Craigowl. Further measurements led to the suspicion that there might be something wrong with the output from Kirk 0' Shotts and this was confirmed during reception tests at BBC Edinburgh; the 2.5 MHz bars in the centre of the Test Card were not visible and the local staff did not even realise that they should have been there! Following reception checks at other locations everything pointed to a serious transmitter problem and on-station tests were arranged for a Sunday morning (in those days this period was available for engineering purposes) .
Having made an early start we arrived at the station only to find the transmitter-hall floor covered with numerous valves! These had been removed from the vision transmitter and replaced with the set of valves used during the original 1952 acceptance tests. However this did not change the perfornance significantly and further investigation revealed that the transmitter was lined up more or less on the wrong sideband (upper instead of lower) which resulted in the severe limitation in the horizontal resolution of pictures viewed on vestigial-sideband receivers. It was not obvious how this could have come about although the on-station monitoring facilities were suspect. In order to radiate the correct lower sideband all the transmitter RF circuits had to be retuned. After this had been done further on-station tests, together With visits to Edinburgh confirmed that the Kirk 0' Shotts signal was then within specification. The high-frequency performance of the RBR + Microwave link to Redmoss was now acceptable and the 2.5 MHz bars were very visible on the Test card resulting in a significant improvement in the overall picture quality received in the Kirk o'Shotts service area.
The BH-Crystal Palace video links: a major advance in transmission engineering (IJS/DCS)
When the Crystal Palace transmitter was opened in 1956 the 405-line video feeds between BH and CP (a distance of 9 miles) were provided over two O.975in. coaxial cables without intermediate amplifiers. A 15MHz double sideband transmission system was developed in Designs Department; one cable was allocated as a permanent feed to CP while transmission over the other cable could be in either direction.
It was soon realised that additional circuits would be required and this was achieved by transmitting simultaneously over each cable a 3MHz video baseband signal as well as the 15MHz double-sideband signal. This was the very first time that this had been achieved with the performance of both the carrier and baseband systems meeting the BBC specifications for a short high quality link. (The major problems of transmitting video over a 9 mile long coaxial cable is the suppression of hum and low-frequency interference at the receiving end and this was overcome by using specially designed (P.O.) coaxial chokes consisting of coaxial cable wound on a high-permeability magnetic core) .
At a later stage, to accommodate two 625 line (5.5MHz) signals, the 15MHz carrier systems were replaced by vestigial-sideband systems using a carrier frequency of 10MHz. The bandwidth of the baseband channel was extended to accommodate a 5.5MHz video signal which was transmitted directly over the cable as before.