Rocking armature transmitter In speech communications we are primarily concerned with intel- The rocking armature transmitter is shown in Figure 58.3. Sound ligibility i.e. the percentage of voice signals transmitted from one pressure impinges on the diaphragm causing the armature to pivot, Transmitter Speech transmission circuit Receiver Power Ringing extraction Dialling circuit circuit circuit Figure 58.1 Functional diagram of a telephone 58/4 Telephones and headsets Carbon granules Perforated guard Supply voltage Diaphragm Carbon electrodes Figure 58.2 Carbon granule transmitter altering the magnetic field and inducing an alternating current in the apertures at the front of the transmitter have variable hole patterns coils. This alternating current represents the speech signal. which are selected to improve the frequency response of the unit. 58.3.3 Piezo-electric transmitter Telephone receivers Piezo-electric transmitters work on the principle that certain types Like telephone transmitters, telephone receivers have been de- of quartz crystals become electrically polarised when subjected to signed using numerous principles, but the earliest type tended to be pressure from sound waves. Sound impinges on the disc of piezo electromagnetic as shown in Figure 58.5. material, causing the disc to be stressed, varying the charge on the An electromagnet winding terminates the telephone line and picks capacitor within the disc, which in turn varies the gate voltage of the up the a.c. signals from the line, varying the magnetic field. The thin FET impedance conversion amplifier, shown in Figure 58.4. The iron diaphragm, held in close proximity to the ends of the magnetic disc and the FET are all housed in a pressed aluminium casing. The poles, is moved to and from the poles of the electromagnet. The Perforated cover Diaphragm Pivot Armature Linking pin Coils Magnet Figure 58.3 Rocking armature transmitter Telephone handset design 58/5 FET impedance converter Cover Diaphragm Connections Output Holes Gauze Piezo electric d