24  Distribution of Audio Signals

Electromagnetic Telephones

Basic characteristics and functions:

1.  Hybrid. The telephone is a four-wire device but needs to be converted to two-wires for connection to the exchange.

2.  Side-tone. Some of the transmitted signal is fed into the receiver.

3.  Power feed. The transmitter has to be fed with d.c. but the receiver needs to be isolated from this.

4.  Impedance matching. The a.c. impedance of the telephone needs to be matched with the impedance of the line. The telephone must be able to cope with a wide variation in line impedances.

5.  Protection – from extraneous voltages, power line surges, lightning.

The speech circuit. Figure 130 shows the use of a hybrid transformer for use with electromagnetic ...

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