 the speech. Above 4kHz the sensitivity should fall off Telephone transmission performance rapidly, to prevent unwanted out of band energy to be transmitted to the network, which might interfere with PCM circuits. At low To assess the transmission performance of an individual telephone, frequencies the sensitivity should roll off to prevent mains hum or telephone network, a number of parameters have been estab- pickup. The gain introduced into the sending path is chosen to lished. The voice operation of a telephone can be broken down into ensure that the telephone send sensitivity is in accordance with the the following elements: transmission plan for the telephone network. A typical send sensi- tivity response is shown in Figure 58.7. 1. The path from talker's mouth to the transmitter. 2. The path from the telephone receiver to the listener's ear. 3. The near end path between the transmitter and the receiver. 3. The telephone connection from transmitter and receiver to the distant end. Different methods exist for comparing the performance of various telephones and for defining the standard to which a particular telephone or system should perform. The four key parameters in determining telephone performance are: 1. Sending sensitivity. 2. Receiving sensitivity 3. Impedance the telephone presents to the line. 4. The line impedance for minimum sidetone. Frequency (Hz) Each of these parameters can be a function of the frequency range of the telephone and the available line current. Algorithms exist for converting the sensitivity of the transmission path or receive path Figure 58.7 Typical send sensitivity response Telephone transmission performance 58/7 58.6.2 Receive sensitivity One essential function of any speech circuit of a telephone is to combine the four wire path from the handset (transmitter connec- The receive sensitivity is defined as the ratio of the sound pressure tions and receiver connections) onto the two wire path to the level in the artificial ear, to the voltage applied at the terminating exchange