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These new features - speech amplification, call signalling device and keysetimpulsing - are characteristic of the telephone sets for electronic systems whichare now at an advanced stage of study. Amplifying and active units must beprovided for calling and impulsing, the former because an electronic exchangecannot send ordinary ringing current but only v.f. current. It was therefore anatural step to introduce transmitter amplification as well. As is seen fromfig. 1, the requirement of material is fairly large.Loudspeaking telephones have every prospect of a bright future. Theirdesign is perhaps not yet ideal, but when they have been perfected in thatrespect at a reasonable cost, they should find their way to many homes.As regards the external design of telephone sets, the fifties have been arevolutionary period. Coloured sets have become common - most being ofthermoplastic. In U.S.A. there are two-colour sets, the case and handset beingof different colours, so avoiding the differing discolouration of these twoparts during the life of the set. The fifties saw the birth of the Ericofon,which has been very well received by subscribers. During the sixties peoplemay well come to think of the telephone as an integral part of the furnishingof their home. Administrations and manufacturers must be prepared to providethe public with elegantly designed sets in many different colours, and to adapttheir colour range to the dictates of fashion. We may have to produce newtypes of set fairly often. There is even a risk that less scrupulous manufacturerswill sell telephones direct to subscribers for plugging into their wall sockets.Line PlantThe line plant is the most costly part of a telephone network. Creditableefforts have been made to reduce this cost. The principal items are cable andinstallation work. The main striving has been to lower the cost of cable andwire through use of the new materials that have become available. Secondly,methods of installation have been rationalized. And only in the third place havetechnical developments been drawn upon for the modernization of linematerial such as cable fittings, protective equipment etc.During the fifties plastic cables took a big stride forward. This tendencymay be expected to continue during the sixties, line materials being successivelyadapted to suit plastic cables.For local line plant, even in the forties, 0.4-mm wire began to oust largergauges of wire wherever the attenuation conditions permitted. A furtherdevelopment in this direction-to 0.3-mm wire-is proceeding in some places.The problems of manufacture and jointing are troublesome, since 0.3-mmwire is rather fragile. The line equipment must also be reconstructed to fitthe new wire.The replacement of copper by aluminium in local cables does not appearso attractive. From the attenuation point of view this would often be possible,but aluminium is difficult to joint. It also involves problems of manufacturebecause of its lower strength and ductility. Considerable changes in cablemakingmachinery would probably be necessary. Aluminium cables arethicker than copper cables, moreover, which is undesirable in local plant.Any general change from copper to aluminium will depend on the price factor.
Gas pressure is being increasingly employed to maintain the insulation ofcables. An overpressure is applied in the cable which prevents the entry ofmoisture in the event of a leak in the sheath. The method has been used forthirty years in paper-insulated lead-covered cables, at the outset mainly onlong distance cables, and latterly also on local cables. The use of gas controlon plastic cables is under development. Some advantages would be gained, forexample in jointing.TransmissionAs regards the carrier systems the trend has been towards the use of largerbandwidths in the coaxial cables. Systems now exist for up to 12 Mc/s. Whenthis figure will be exceeded is uncertain, but within a few years there shouldbe technological and economic possibilities of perhaps 36 Mc/s transmittedbandwidth.From the equipment point of view the carrier systems are being progressivelytransistorized, and this will undoubtedly continue. Amplifiers with transistorsfor coaxial systems for 12 Mc/s are within reach. The only doubt is concerningthe shock resistance of high frequency transistors. Transistor amplifiers havemany advantages, especially in the matter of power requirements. Transistorizedsystems also appear to be cheaper than tube systems.Fig. 2X7797TASI system (Bell Laboratories Record, March1959)The system concentrates the traffic on 72 lines to36 channels. To each line on the transmitting sideis connected a speech detector, which indicates whetherthere is speech on the line or not. If there is, the lineis connected by the switch to a free channel; at thesame time the receiving end is informed over a controlcircuit of the connection that has been established andthe receiving switch is operated accordingly. Whenthere is no speech on a line, the line is disconnectedfrom all channels.Even if bandwidth has on the whole become cheaper, there is a tendency tocut down the bandwidth on expensive circuits. On the Atlantic cables thecompromise of 3 kc/s instead of 4 kc/s spacing on the telephone channels isto be accepted, which may seem a step backwards. In the TASI system - TimeAssignment Speech Interpolation - (fig. 2), which is on the verge of switchingtechnique, the number of channels has been doubled. In this system a channelis seized only if there is speech on the line. Energetic work is being done onvarious Vocoder systems, which would permit a radical reduction in bandwidth.But in view of their cost, it is doubtful whether such systems will beusable in commercial telephony.There has been much talk during the fifties of short-haul carrier systems,which would supersede the usual method of loaded cables. Here, it must besaid, the results have been disappointing. It seems impossible to cut the costbelow a certain level, and there would be no gain in economy at distances107
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