A typical telephone instrument (see Figure 1) includes a number of main blocks (a switchhook, a ringer, a dial, a receiver, a transmitter, and a balancing network), which make it capable of numerous operations. 1) It requests the use of the telephone system when the handset is lifted, and signals the system that a call is finished when a caller returns the handset to the cradle. 2) It announces the incoming call by sending audible signals. 3) It sends to the system the telephone number which is specified by the caller, who rotates a dial or presses number keys. 4) It converts the speech of the caller to electrical signals for transmission through the system and changes received electrical signals to speech. 5) It controls the level of unwanted sounds.
The switchhook interrupts the connection with the central office when the telephone set is idle (the handset is on the cradle) and completes the circuit whenever the handset is lifted off. The ringer is initiated by the system each time when an incoming call is waiting. Alternating current passing through a coil drives a permanent magnet, which makes the attached clapper periodically strike the bell.
The rotary dial is an electric contact that interrupts the circuit periodically sending to the central office a series of current pulses, corresponding to the identification number of the telephone called. When the tone dialing is used, as in touch-tone service, pressing of a number key results in sending a signal of two frequencies to the central office circuit, which identifies it and accordingly sets up the switching for the communication path. The use of tones speeds up the dialing operation and allows sending commands and control information directly to the called location.
The telephone transmitter responds to acoustic frequencies from 250 to 1,000 Hz. It consists of an electrical capacitor adjacent to a diaphragm, so that air-pressure vibrations caused by sound make the membrane produce changes in the value of capacitance of the circuit and, therefore, the variable electric voltage. As a result, a pulsating direct current occurs.
The typical receiver consists of a lightweight diaphragm attached to a permanent magnet associated with a coil of wire. When the alternating current of the telephone signal passes through the winding of the coil, changes are produced in the magnetic forces acting on a permanent magnet. The diaphragm moves in response to these variations, creating an acoustic pressure, more or less exactly reproducing the original sound wave from the distant telephone transmitter.
Sidetone is the sound of the speaker's voice heard in his own receiver. To keep the level of this unwanted sound in acceptable range, the balancing network is used.
Special signals informing users on the status of their calls originate from the central office. When a caller lifts off the handset from the cradle the direct current flows through a complete circuit to the central office, which immediately registers the line initiating a call and places a dial tone on it. This tone signals to the subscriber that he may proceed with dialing the number of the called party. If the connection cannot be established, the tone generator in the central office produces one of three possible busy signals. A signal interrupted 60 times per minute means that the called party is busy. A signal interrupted only 30 times per minute indicates that the toll line between the central offices is busy, while a signal with 120 interruptions per minute means that all the intra-office paths are busy.
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