Down the Tubes
The year 1986 was notable for the FIFA World Cup in Mexico in the summer, during which Maradona claimed the 'Hand of God' was responsible for the hand goal which put England out of the contest. Earlier in that year the Chernobyl Power station had exploded causing a major nuclear disaster.
The World Cup was already underway when we commenced the day shift one morning in the BBC Communications Centre in Bristol. This technical area at the time was a hub for the BBC's vision, sound, and data circuits between London, Cardiff, Plymouth and Southampton. In particular, BBC1 and BBC2 was received was London and fed to Cardiff, Plymouth and the Mendip transmitter station. Additionally, the Centre enabled Points West opt-outs on both television networks by switching the local studio into circuit to Mendip.
Part-way through the morning the BBC's transmitters in Southwest England lost their line feeds and went to their reserve feeds. Plymouth phoned to complain of the loss of both television networks, and Wenvoe Monitoring and Information Centre (MIC) confirmed that Caradon Hill, Huntshaw Cross and Stockland Hill were on RBS (Re Broadcast Standby). Enquiries revealed that all vision circuits between Plymouth BBC and the BT Repeater Station at Forder Battery (outside Plymouth) had been lost. A digger working on the new Plymouth Parkway - a new dual carriageway carrying the the A38 through Plymouth to the Saltash Bridge - had sliced through the vision tubes and completely cut BBC Plymouth off from the BBC network. Coincidentally Television South West, the regional ITV franchise, had escaped this fate by the IBA self-providing all its inter-regional circuits and was therefore not reliant on BT.
On the BBC's instructions, BT Forder Battery plugged the incoming BBC1 and BBC2 circuits into the amplifiers feeding Caradon Hill and Stockland Hill, an action which bypassed BBC Plymouth and restored programmes to the transmitters.
Regional Communications Structure
In those days each BBC Region had a Communications Manager responsible for the provision of temporary and permanent circuits, of any description, for use by the BBC in the Region. Reporting to him were Assistant to Comms Manager (formerly Communications Systems Engineer) and Communications Planning and Operations Manager. The latter was responsible for the provision of Outside Broadcast circuits from BT or the mobile Comms department (Radio Links) and the staffing of the Communications Centre.
The A to CM possessed expert knowledge of all regional permanent circuits, had good professional relationships with BT counterparts throughout the Region, and also had a grasp of BT's networks.
The A to CM in Bristol was an experienced engineer, having worked in the South and West for many years. When this report reached him he remembered the 405 line days before Plymouth Broadcasting House was extended for colour operation. At one time, he recalled, the original BBC-GPO cable containing a vision tube and audio pairs went in an entirely different route than later cables. A phone call to BT confirmed that this cable was still terminated in both BT Forder Battery and BBC BH Plymouth although unused and unconnected. He gathered together two pairs of vision equalisers - a Cauer and a Bode - and left for Plymouth.
BBC Vision Equalisers
The staple diet the BBC used as equalisers were the Bode and the Cauer. Fundamentally these equalisers were based on the Bridged-T design which preserved the source and load impedances.
The Bridged-T attenuator and fader were first designed in the early days of radio transmission, where the load impedance was built around 600 ohms. This figure is derived from the characteristic impedance of an infinitely long transmission line i.e. two wires run parallel to each other on telephone poles. It was found in the early days of the GPO that such a line had a characteristic impedance of 600 Ohms at around 800 Hertz.
The Bode and the Cauer (the latter known coloquially as 'Fairy fingers' and based on the Zobel network) were versatile pieces of equipment. With some knowledge of the effect of various frequencies on the vision signal, by looking at a 'Pulse and Bar' test signal, fine correction could be applied to bring vision signals into spec.
Back in business
The old vision tube was re-terminated at both ends and equalised, and plugged to a Protection Circuit to Bristol. BT locked this circuit off so that it could not be automatically switched away to make good an existing vision circuit. BBC Plymouth were once again re-connected with the BBC Network via Bristol Comms Centre. But what about their opt-outs?
BT confirmed that this arrangement was going to last for quite a period since they had to wait for the roadworks to end before re-routing and reinstating the axed tubes.