Telecommunications by R, Bayfield.
At the Durrington Community Centre, 14th February, 1990.
This talk dealt mainly with line transmission, taking the methods in chronological order of introduction.
1. Audio frequency. Initially this was simply amplified speech, needing a pair of wires in each direction to permit conversation. Mr. Bayfield described the terminal equipment enabling the house connection to be made with only one pair. In addition d.c. signals are needed (e.g. to obtain dialling facility), and a circuit to achieve this, using capacitors, was shown. This last was known as the phantom circuit as it enabled three functions (send, receive, signal) to be made over two pairs of wires. Valve amplifiers were located at intervals in both directions to boost the signals.
2. CarrierTransmission used radio principles – put a carrier frequency
on each one of a number of conversations, “tuning in” to the one required at the receiving end. Normal speech requires a bandwidth of only 4 KHz to be intelligible, so a group of 12 conversations can be accommodated between 12 and 60 KHz. If repeaters are close enough the number can be increased to 24 (12 to 108 KHz) and transmitted over one pair of wires, duplicated for the reverse direction. Repeaters spaced at 12 miles, in positions formerly occupied by the Heaviside loading coils of the audio frequency system. To prevent over loading the amplifiers, the carrier and one sideband are suppressed before transmission, the carrier being reinstated at the receiving end. Synchronisation of the carrier is achieved by a 4 KHz pilot signal sent out country-wide.
3. Coaxial cable. Similar in principle to the carrier but allows a much wider bandwidth. Thus a “super-group” of 60 conversations can be accommodated between 312 and 552 KHz, modulation being used to create other bandwidths. Repeater stations are required at 6 mile intervals. The Brighton – London link was a 10 super group system (600 conversations) with the terminal at Withdean and repeater stations at Clayton, Balcombe. etc. The carrier excludes d.c. for signalling, so the earlier systems used audio frequencies of 600 and 750 Hz.
4. Submarine cables. These are sophisticated co-axial cables. The first transatlantic crossing (TAC1, Oban to Newfoundland) had 36 channels, but still needed 52 repeaters at 37 mile spacing. Repeater power was d.c., + 2 KV at one end, – 2 KV at the other, the cable withstand voltage being 2.5 KV; a margin for variation in earth potential had to be allowed. Mr. Bayfield then described cable laying operations and the purpose- built ships. Alert, Iris and Monarch.
5. Pulse Code Modulation. TACl’s 36 channels can be doubled by time assignment speech interpolation (TASS1). The TASS! circuit recognises the direction of speech in a conversation and uses the return (unoccupied) channel for a second speaker. A user can be given a fraction of the bandwidth all the time or the whole bandwidth for a fraction of the time. The version of the latter now used, looks at the voltage level of each conversation each 125 micro seconds, equates it to one of 128 voltage levels, then transmits, employing a pulse code from 7 binary digits. The system can accommodate 24 conversations, but needs a bandwidth of 1.5 MHz, and, therefore, repeaters every 2,000 yards. Since only pulses are used, noise is excluded.
In discussion, problems of reliability of cordless telephones were raised, together with some interesting possibilities for cross-charging: we learned that a cordless phone will not work in conjunction with a shared line. P.C.M. can be adopted to any form of transmission, e.g. by fibre optics. It is not susceptible to interference from, for example, traction systems. The Compact Disc uses the P.C.M. principle, which also lends itself to transmission via satellite, where a poor signal/noise ratio exists. In a submarine cable, each repeater (normal volt drop 52) can be separately tested. Replacement necessitates an additional piece of cable, i.e. the total length of the cable is increased. So a simulated length is added to the cable when new, and a piece removed after each repair.