elay coils, and speech is coupled from one line to This is a hybrid transformer which performs the dual function of another through the exchange switching apparatus by means of matching the receiver and transmitter to the line and controlling capacitors as shown in Figure 31.5 or through a transformer. The sidetone. Structure of telephone systems 31/5 First Seize Interdigital Second digit pause digit Clear (>400 ms) Off-hook On-hooki(0) 66 2/3 ms break 331/3 ms make Ratio (% break) Exchange equipment performance limits Dial limits, Dial spec Effects of line etc distortion 'Make' (ms) Figure 31.6 Telephone subscriber's line signalling: (a) waveform; (b) performance curves Sidetone is the reproduction at the receiver of sound picked up by are often used to describe signalling conditions corresponding to the the transmitter of the same instrument. It occurs because the trans- contact closed or open respectively. mitter and receiver are both coupled to the same two-wire line. The most comfortable conditions for the user are found to be when he hears his own voice in the receiver at about the same loudness as he 31.2.1.5 would hear it through the air in normal conversation. Too much sidetone causes the talker to lower his voice, reduces his subsequent This is a spring-operated mechanical device with a centrifugal listening ability and increases the interfering effect of local room governor to control the speed of return. Pulses are generated on the noise at the receiving end. The complete absence of sidetone, return motion only, the number of pulses up to ten depending upon however, makes the telephone seem to be 'dead'. how far round the finger plate is pulled. The use of a hybrid transformer divides the transmitter output The pulses consist of interruptions (breaks) in the line current between the line and a corresponding balance impedance. If this produced by cam-operated pulsing contacts. A set of off-normal impedance exactly balanced that of the line there would be no contacts operate when the finger plate is mov