emale voice from 1875-77, was an apparatus named the 'Electrical Speaking Tele- 400Hz to 2000Hz. CCITT recommend a bandwidth of 300Hz to phone'. It was a means of transmitting sound (especially voice) over 3400Hz as being adequate for telephony and provide an acceptable a distance, by converting sound vibrations into electrical signals level of intelligibility on a speech connection. which passed through wires as electrical signals, and were then reconverted to sound at the distant end. This will provide one direction of communication. In a practical telephone bothway com- Telephone transmitters munication is necessary, SO in a simple telephone system each end is provided with means for transmitting and receiving sound. Telephone transmitters have been designed using numerous tech- In addition some form of mechanism is required to signal to the niques to convert sound impinging onto the transmitter into electri- distant end to attract the distant party's attention to the fact that the cal energy. caller wishes to talk to the distant end. Numerous forms of signall- ing schemes have been devised and they vary depending on the type 58.3.1 Carbon granule transmitter of telephone system to which the telephone is connected. Some form of power supply is required to generate the electrical signals Carbon granule transmitters were very common from the earliest and many forms of circuits exist to provide power to the telephone. days of telephony until comparatively recently. The principle of These consist of local batteries to power only an individual tele- operation is shown in Figure 58.2. phone or central batteries in the telephone exchange (Central Office Sound pressure impinges on the diaphragm causing it to vibrate. or CO in the USA) where the power is sent down the individual The centre of the diaphragm is attached to a carbon electrode which telephone line to the telephone, where circuits exist to extract the moves, compressing and decompressing the fine carbon granules power from the line and feed the telephone ci