 line, and to do this with minimum coupling from the impedance of the exchange feed bridge. This ratio is also measured transmitter into the receiver (sidetone). Sidetone plays an important over the full frequency range and at different line lengths. The part in the subjective performance of any telephone but it is desir- receiving sensitivity is nominally flat within the the 300Hz to able to be able to control the amount of sidetone that can occur over 3400Hz speech band and tends to roll off at low frequencies. To the range of exchange line connections, to ensure that transmission reduce the effects of picking up mains hum in the telephone, the cut difficulties do not arise. off at low frequencies cannot be too sharp, since the absence of low One way to assess the efficiency of the 2/4 wire converter in the frequencies increases the difficulty of hearing on quieter calls. telephone circuit is to check for minimum sidetone, or zero sidetone Above 3400Hz the sensitivity drops off sharply to exclude the when the telephone is connected into an impedance which com- unwanted high frequency by-products of pulse code modulation pletely suppresses the sidetone. Many telephone speech integrated systems. This is because PCM systems sample at 8kHz and all circuits perform the 2/4 wire conversion by means of a Wheatstone codecs put some part of the sampling frequency onto the line, albeit bridge principle, as in Figure 58.9, to obtain proper decoupling at a very low level. The aim is to reduce the level as much as between the send and receive signals. For balance of the bridge possible, to prevent it being an annoyance to the user, even if it is Equation 58.1 must be satisfied. heard. A typical receive sensitivity response is shown in Figure 58.8. Receiver Receive amplifier Frequency (Hz) Figure 58.8 Receive sensitivity 58.6.3 Impedance balance The impedance that the telephone should present to the line is chosen to suit the transmission plan for the network. As more digital Figure 58.9 Telephone bridge local exchanges are 