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As regards electrical properties, the trend is towards higher frequencies andhigher powers. This trend may be expected to continue. Electrical propertiescan clearly be built into semiconductors almost at will. But special types canhardly be expected to be evolved for telephony purposes apart from exceptionalapplications - the market would be too small.Within the component field the ferrites have attained very wide usageduring the fifties for inductance elements. Their permeability has been successivelyraised to a very high level without increasing their losses. A continuationof this trend may be expected, although hardly to the same extent asduring the past decade.For electronic exchanges the memory technique has come into the foreground.Several new memory elements have been evolved. New memory tubes,magnetic memories of different kinds and dielectric memories have beensuggested. During the sixties it is likely that technology will take a definitivedirection, with only a few types of memory as realistic alternatives. For certainrequirements, especially for semipermanent memories, there is at present nosaticfactory solution, and we look to new inventions in this sphere.TelephonesThe main evolution in telephone sets has been the successive improvementsin reference equivalents for sending and receiving. This has permitted higherattenuation in local networks, so longer local lines and smaller diameters ofwire. As regards the transmitter, the type that was patented in 1890 still stands,though, of course, improved in all details. The carbon granule transmitter hasthe advantage that it is itself an amplifier-about 25 db-and that it is cheap.This must be paid for in terms of high harmonic distortion, which gives thetelephone voice another timbre. The output of the carbon granule transmitterhas been increased in recent years by about 5 db, and a few additionaldecibels are perhaps within reach in the near future.The electromagnetic receiver principle originally evolved by Bell is stillused to a large extent. By the use of improved materials the efficiency ofreceivers has been increased by about 8 db within the past few years. Theelectromagnetic receivers of balanced type add a further 2 to 3 db. Noparticular improvement over these figures is likely in the near future.Thus, all in all, the efficiency of telephones has been increased by some15 db, but differently distributed between sending and receiving, so that aredistribution of the attenuation must be made in the differential coupling ofthe telephone from sending to receiving.But the greater efficiency in sending and receiving has had certain drawbacks.On short lines the volume has become unpleasantly high, severerrequirements must be placed on sidetone attenuation, and the requirements oncrosstalk attenuation in exchange and cables have also had to be accentuated.With modern telephone sets the system reference equivalent may lie withinthe range - 10 to + 40 db. The most agreeable range as regards volume isbetween + 10 and +25 db. For system reference equivalents below 0 thevolume is definitely annoying. Measures should therefore be taken to dampthe signal. In the U.S.A. (Bell System) and in the U.K., non-linear elements,whose attenuation of the signal is a function of the line current, have beenintroduced into the transmission circuit of telephone sets. Bell System employs
varistors and the British Post Office Counter-coupled rectifiers, the bias and,therefore, impedance of which is varied by a resistance which varies greatlywith the line current.The advent of transistors has made it possible to introduce amplificationinto telephone sets, the amplifier being powered from the exchange. Thiswould make possible the use of a better quality transmitter. The attenuation inlocal networks could be increased still further, though this would lower thecost of line plant only to a limited extent.The signalling element in the telephone set has presumably always been alow-frequency a.C. bell, which is now made with mechanical volume regulation.There is every reason to believe that the same form of bell will be used infuture. An alternative exists, however, which has certain advantages. Jt is atransistor converter of the ringing current to a v.f. current which operates asignal generator, possibly the receiver itself. The ringing signal has a morepleasant sound than the present one and is equally audible. But the pricewould perhaps often appear prohibitive.Fig. 1Suggested diagram of telephone set forelectronic systemRGTr4Tr 1MTr2 1Tr3 JT1.T2f,. UT4HgSignal generatorTransistor tor amplifier for RG» •• stabilizationTransmitterTransistors for transmitter amplifier andfor digit sending• Keying signal circuitTelephone transformerReceiverSwitchhookIt may be definitely predicted that the sixties will see dials replaced bykeysets on a broad front. Actually it has taken unexpectedly long for thisinvention to break through. The Bell group in U.S.A. has now made a proposalfor a keyset signalling system. It is a v.f. system, each digit consisting of twofrequencies, each from two groups of four frequencies. This system appearsto be the best devised hitherto, being insusceptible to interference by speechand not requiring reversal of polarity in keying position. In the U.S.A. considerationis being given to the question of a higher charge for keyset telephones.There is little doubt that many subscribers would be prepared to paythe moderate additional fee which would make keyset systems an economicproposition for administrations. Keying is more convenient than dialling andsaves time.105
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