58.6.2 Receive sensitivity

The receive sensitivity is defined as the ratio of the sound pressure level in the artificial ear, to the voltage applied at the terminating impedance of the exchange feed bridge. This ratio is also measured over the full frequency range and at different line lengths. The receiving sensitivity is nominally flat within the the 300Hz to 34OOHz speech band and tends to roll off at low frequencies. To reduce the effects of picking up mains hum in the telephone, the cut off at low frequencies cannot be too sharp, since the absence of low frequencies increases the difficulty of hearing on quieter calls.

Above 3400Hz the sensitivity drops off sharply to exclude the unwanted high frequency by-products of pulse code modulation systems. This is because PCM systems sample at 8kHz and all codecs put some part of the sampling frequency onto the line, albeit at a very low level. The aim is to reduce the level as much as possible, to prevent it being an annoyance to the user, even if it is heard. Atypical receive sensitivity response is shown in Figure 58.8.

58.6.3 Impedance

The impedance that the telephone should present to the line is chosen to suit the transmission plan for the network. As more digital local exchanges are installed in networks, control of the impedance presented to the network becomes more important. Digital ex­changes use four wire transmission paths - two for transmitting and two for receiving. However, the cable between the exchange and the telephone is only two wire, therefore a mechanism is necessary to terminate the bi-directional two wire path onto the unidirectional send and receive paths of the four wire circuit. The four wire circuits are balanced to prevent singing or echo and it is also necessary to include the impedance of the line and the telephone to ensure correct balance. Echo is where the speaker hears his own voice repeated back. Part of the speech signal energy transmitted in one direction is returned in the other direction due to imperfect balance at the 2/4 wire conversion. The length and type of cable connecting the telephone to the exchange will vary and hence its impedance, therefore trying to match the impedance of the telephone line to the balance network would involve individual measurements of each line. In practice a nominal impedance of 600 ohms has been chosen to terminate the line and most telephones have been designed around this value. More recently complex impedances have been specified by the network provider as this is thought to provide a more ideal balance network.

One essential function of any speech circuit of a telephone is to combine the four wire path from the handset (transmitter connec­tions and receiver connections) onto the two wire path to the exchange line, and to do this with minimum coupling from the transmitter into the receiver (sidetone). Sidetone plays an important part in the subjective performance of any telephone but it is desir­able to be able to control the amount of sidetone that can occur over the range of exchange line connections, to ensure that transmission difficulties do not arise.

One way to assess the efficiency of the 2/4 wire converter in the telephone circuit is to check for minimum sidetone, or zero sidetone when the telephone is connected into an impedance which com­pletely suppresses the sidetone. Many telephone speech integrated circuits perform the 2/4 wire conversion by means of a Wheatstone bridge principle, as in Figure 58.9, to obtain proper decoupling between the send and receive signals. For balance of the bridge Equation 58.1 must be satisfied.

T he a.c. signal from the microphone is fed into one diagonal of the bridge and a small amount is fed into the balance impedance, but the majority is fed to line via R_y The signal coming from the line is detected on the other diagonal of the bridge then amplified and applied to the receiver.