al. The dial then returns to its original position by a spring, which are geometrically spaced to ensure that any two frequencies of a was wound up when the dial was turned. On its return the dial speed valid combination are not harmonically related. Table 58.1 details is held constant by a mechanical governor and a cam with notches the frequencies and valid combinations. Interdigit pause Pulsing contacts closing (Make) Digit train Digit 4 Next digit Pulsing contacts (Break) Break period Make period Pulse duration Figure 58.12 Dial pulse train 58/10 Telephones and headsets Table 58.1 DTMF tone assignments The group listening telephone normally has call progress monitor, but in addition has a circuit to enable the user to press a button on the telephone and broadcast the incoming speech signal to others listening in the room. The circuit may also drive the receiver in the handset simultaneously with the loudspeaker in the base of the 1209Hz 1336Hz 1447Hz 1633Hz telephone. Outgoing speech is only possible through the handset 697Hz transmitter. 770Hz The full handsfree telephone is equipped with both a handset and 852Hz a separate microphone and loudspeaker for handsfree operation. The microphone and speaker are now all provided as part of the 941Hz telephone, although some older designs consisted of separate units connected in parallel with the standard telephone. The design of the handsfree telephone creates particular problems. The ICs are designed to ensure that frequencies do not deviate The level of sound being emitted from the loudspeaker is required more than 1.5% from the standard and that no unwanted signals to be large enough to fill the volume of air in the room where it is are sent to line. In addition the speech transmission is normally being used, SO that users can be anywhere in the room and still hear muted during signalling, but the receiver normally receives some of the incoming speech. The microphone in the telephone will also the tone to give the user confidence that the tone is being sent to li