Abstract :
The aim of the telephone designer has generally been the attainment of greater sending and receiving sensitivity, so that conversations may be carried on over connections of greater loss. With the telephone sets hitherto available, the sensitivity has not been so high as to give excessively loud reception on short connections. However, recent developments, particularly in the electromagnetic design of telephone receivers, have made available substantial increases in sensitivity, and the new British Post Office subscriber´s telephone set (the 700 type) is more sensitive on long lines than its predecessor (the 300 type) by some 4 dB in both sending and receiving directions. This enhanced sensitivity, which is of great advantage in permitting the use of longer or smaller-gauge subscribers´ local lines without degradation of performance, is liable if uncontrolled to be an embarrassment on short lines, and, in fact, subjective tests carried out in the field and in the laboratory have established that, on short connections, most subscribers would find this sensitivity too high. The problem is discussed, and it is shown that the best solution is the incorporation in the telephone set of an automatic regulating device which reduces the sending and receiving efficiencies and the sidetone level when the line current is high, while leaving the full sensitivity unimpaired on longer lines. The regulator makes use of the non-linear voltage/current characteristics of selenium-rectifier elements, biased in the forward direction by a control voltage derived from the voltage drop across a metal-filament lamp in series with the line. These elements are used to provide shunt losses on short lines of up to 6 dB on sending and 4 dB on receiving, and also to reduce sidetone. Details are given of the performance of the new Post Office Telephone No. 706 which embodies such a sensitivity control as an integral part of the telephone set. It has as good a performance on a 1-kilohm local line- as the 300-type set had on a 660-ohm line; at the other extreme, on very short lines, the sensitivity of the new set is, owing to the action of the regulator, no greater than that of the old set.
