The telephone consists of a microphone and a receiver which is part of the speech circuit, a
    bell, cradle switch and a keypad for the push-button type or a dial for the type.

   The function of the microphone is to convert the sound into electrical signal whereas the
       receiver converts the electrical signal back to sound. When one speaks, this causes an
       alternate ' high and low ' pressures in front of the microphone. This pressure variations
       however are converted to an electrical signal by the microphone. The electrical voltage
       variations are then transported to the exchange via the subscriber line.

   At the other end, these voltage variations then have to be converted back to audible sound.
      This is done by the receiver in the handset. The electro-magnet in the receiver causes the
      membrane to vibrate in phase with the voltage variations.

   Apart from the microphone and the receiver, the speech circuit also consists of two
      amplification units, one to amplify the microphone current and the other for feeding the
      receiver. The degree of amplification is being regulated by a control circuit.
 

       Figure shows schematic diagram of a telephone set.
 

 

   The functions of the telephone can be summarised in the following points ......

   By lifting the handset from its resting position, the cradle function activates the exchange.

   Send the required subscriber number either by two-tone signals or pulses.

   Convert speech to electrical voltage.

   Convert electrical current to audible sound.
 
   Alarm the called subscriber with a built-in bell.

   Inform the exchange when the call is over.

   A special variant of the telephone is the coin-box phone or more accurately the pay phone.
   There are several different types, of which the following two are those in most common use.

   The simpler (and older) variant sends information to the exchange on the amount of money
       inserted. The exchange registers the amount and then controls the charging. When the
       amount is used up, the exchange sends a tone signal to the coin-box phone. The tone signal
       is a warning to put more money in if the call is not to be cut off.

   The more intelligent types of pay phone register the amount inserted themselves, and control
       the chaging with the aid of charging pulses sent the exchange.
 

   There is an equipment that is often use together with a telephone which is known as telephone
   answering machine. The telephone answering machine has a built-in tape recorder, where
   messages can be left for those calling in, and it usually also has a tape for recording messages.
   The machine has its own 'cradle switch' function which can be activated after a couple of
   ringing  signals. after the message has been sent, and perhaps received, the cradle switch
   function automatically indicates 'hook on' to the exchange. Certain telephone answering
   machines can also be controlled remotely from another telephone. By phoning the answering
   machine, incoming messages can be listened to and then erased. It is also often possible to
   speak in a new message. The remote control is by means of tone signals from a small
   loudspeaker which is pressed againts the microphone.

   The telephone network can be used for data communication. However, since the network is
   designed for transmitting speech, it cannot handle the computers' digital signalsdirectly. These
   must first be converted to a form of signal which is suited to the telephone network, and this is
   done by the modem. The word modem comes from the concepts of MOdulator and
   DEModulator. The modulator converts the computer's digital voltage pulses to tone-like signals
   (using phase shift or frequency shift), and the demodulator converts this tone-like signal  back
   to digital pulses. Figure shows the connection between a computer and the telephone line.
 

 

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