Chapter 10 Principle of Operation
10.1 Telephone System
10-1
10.1 Telephone System
10.1.1 Converting and Sending Voice in the Form of Electrical Signals
The human voice is a sound wave; in other words, air vibrations. Conversation between two people results
when such vibrations travel through the air and reach each other's ears. A string telephone transmits the air
vibrations generated between two people along a string stretched tight, thus allowing conversation over a dis-
tance. In this system, a paper cup at one end of the string receives the air vibrations, which are then transmit-
ted along the string. A paper cup at the other end of the string transmits them back to the air, so that they
again become again audible sound waves.
A telephone is a device that replaces the vibrations transmitted by string with electrical signals. The two paper
cups correspond to microphone and speaker and the string to the telephone line. Because electrical signals
travel over the telephone line at a high speed with minimal attenuation, the telephone enables conversation
over great distances.
Voice is changed into electrical signals using electromagnetic induction, a process by which electrical signals
are generated by vibrating a coil in a magnetic field. Both the microphone and speaker exploit this process.
The microphone transduces sound into electrical signals using electromagnetic induction that occurs at a
moving coil coupled to a diaphragm picking up air vibrations.
On the other hand, the speaker functions in the reverse manner, transducing electrical signals back to air
vibrations. Therefore, its construction is basically the same as that of a microphone. Electrical signals passing
through a coil in the magnetic field vibrate the coil, which in turn vibrate the air to reproduce the voice.