Throughout its history the BBC had used a number of different technologies to exchange programmes between its various stations and transmitters. Because of the prohibitive costs of manufacturing and building its own infrastructure, the BBC has usually relied upon the facilities provided by the General Post Office (GPO).
The first system
The first complete SB scheme was centred round a 'central exchange' in the BBC's London control room from which open-wire trunk telephone lines radiated to the various provincial stations. The signals were carried overhead on poles similar to the circuits used for ordinary telephony. This type of circuit could be adapted by the use of special apparatus at the receiving end to give excellent quality of reproduction, but from its very nature it was exposed to interruption and interference from bad weather, road accidents, and many other causes.
In order to secure better quality, the exchange was sub-divided, with sub-relay stations installed at Leeds (opening 1 November 1925) and Gloucester to repeat, amplify, and purify transmissions between London and the provinces. From Glasgow, programmes were then fed onwards to Edinburgh, Dundee and Aberdeen:
By the reorganisation of the land-line system for simultaneous broadcasting, carried out in 1924–25, Leeds became the pivot controlling all transmissions relayed between the South and the North. Hitherto land-line connections between one station and another had been operated through London, except for a small switchboard at Glasgow for linking up Scottish stations. The chief function of Leeds and the Gloucester repeater station, subsequently added as pivotal points, was to improve the quality of all items received from London to the same excellence as when they left London, distortion and other faults being corrected and weak signals amplified before they are passed on.
All main Post Office cable routes soon included specially designed broadcast circuits carefully shielded against inductive interference from neighbouring telephone circuits and other outside influences. They were also equipped, every 40 to 50 miles along their run, with special high-quality amplifiers and 'distortion-compensating' networks to maintain good strength and quality of the programme currents along the route.
Underground cable circuits, 1931–3
In an effort to improve the quality of music transmission in particular, in the early 1930s the BBC switched over from the old system of overhead cables to the GPO's new network of underground telephone circuits (or 'music circuits' as they were sometimes referred to within the BBC).
The most important features which differentiated the underground and overhead systems were the 'loading coils' which had to be inserted into the underground circuits every mile or so along the route, and the repeaters or speech amplifiers and associated equipment, which had to be provided every 40 or 50 miles. Various adaptations had to be made, however, to meet the more demanding requirements of broadcasting:
The band of frequencies which must be efficiently transmitted for intelligible speech required in commercial telephony is approximately from 200 to 3000 p.p.s. It has been found possible, by careful selection of circuits, by redesigning and replacing all loading coils along the route and by developing new repeaters to replace those originally installed for telephone purposes, to modify circuits in the latest types of telephone cables so as to secure the even transmission of all frequencies from 50 to 6000 p.p.s. This latter frequency band corresponds to a quality of reproduction which, with the latest type of loudspeaker and amplifier equipment, is only just distinguishable from the original on a quick changeover test.
The first new broadcast link, between London and Leeds, came into service in July 1931. Within the following few weeks the new circuits had been extended to Manchester, and the route from London to Birmingham via Daventry also put into commission.
Early in 1932, a further extension from Leeds, northwards, via Newcastle to Edinburgh, was completed. This co-incided with the transfer of the BBC's main Scottish control room from Blythswood Square, Glasgow to the regional headquarters at Scottish Broadcasting House, Edinburgh. By the end of March, the line was linked-up with the section already in use between Dundee and Aberdeen, thereby completing the replacement programme.
In June 1933, the last links interconnecting nearly all the BBC stations were put into commission between Birmingham, Bristol, Cardiff and the new West Regional Station at Washford Cross.
By 1938 the BBC's transmitters and studios were linked by over 6,000 miles of special Post Office telephone lines.
- BBC Handbook 1928, 300.
- BBC Year Book 1933, 402.
- BBC Year Book 1933, 401.
- In fact, Leeds Broadcasting House was the amplifying and relaying centre for the trunk lines passing through. BBC Year Book 1934, 316.
- 'Better Scots broadcasting', Scottish Daily Express, 8 January 1932; BBC Year Book 1932, 299.
- BBC Year Book 1934, 329.
- BBC Handbook 1939, 86.