TELEPHONE

T E L E P H O N E




DEFINITION:

 "An apparatus for reproducing  sound, especially that of human voice(speech) at a great distance, by means of electricity; by conducting of transmitting and receiving instruments connected by a line or wire which conveys the electric current."

HISTORY:

We all heard that great inventions are made from some kinds of accidents, similarly the telephone also discovered by an accident. The telephone arrived as a practical instrument on March 10, 1876 as an outgrowth of experiments on a device to send multiple telegraph signals over a single wire. While conducting experiments Alexander Graham Bell spilled acids on his trousers. He reacted by shouting "Mr. Watson, come here, I want you." Thomas A Watson heard these words clearly carried by electricity, on a receiving set in another room. As a result on this accidental discovery rapid improvement was made on that simple instrument, being tested on that day in 1876. The concept has grown into an industry that provides over 100 millions telephone sets, profits of several billion dollars a year and employment for more than a million people.

FUNCTIONAL BLOCK DIAGRAM OF A STANDARD TELEPHONE SET:

a standard telephone set consists of a transmitter, a receiver, an electrical network for equalisation, associated circuitry to control side tone levels and to regulate signal power and necessary signaling circuit.In short a telephone is an apparatus that creates an exact likeness of sound waves with an electric current.

FUNCTIONAL BLOCK DIAGRAM OF A STANDARD TELEPHONE SET

The essential components are:-

RINGER: The purpose of the ringer is to alert the destination party about the incoming calls.Ringer circuit is placed directly across the the tip and ring of the local loop.

ON/OFF HOOK CIRCUIT: The on/off hook circuit switch placed across the tip and ring. The switch is mechanically connected to the telephone handset so that when the telephone is idle(on hook), the switch is open. When the telephone is in use (off hook), the switch is closed completing an electrical path through the microphone between tip and ring of the local loop.

EQUALIZER CIRCUIT: They are basically audio filters. They are used to regulate the amplitude and frequency response of the voice signals. This circuit consists of passive components like resistors , capacitors, and so on.

SPEAKER: The speaker is the receiver for the telephone. The speaker converts electrical signals received from the local loop to  acoustical signals(sound waves). It receives the electrical signals from local loop. Speaker is typically enclosed in the handset of  the telephone along with microphone.

MICROPHONE: It is the transmitter for the telephone. The microphone converts sound waves into electrical signals that are transmitted into the telephone network through the local subscriber loop.

HYBRID NETWORK: The hybrid network in a telephone set is a special balanced transformer usded to convert a two wire circuit into a four wire circuit and vice versa, thus enabling full duplex operation over a two wire circuit.

DIALLING  CIRCUIT: This circuit enables the subscriber to output the signals representing digits and this enables the caller to enter the destination telephone number.



BASIC TELEPHONE CALL PROCEDURES:

Simplified diagram illustration how two two telephone sets are interconnected through a central office dial switch

STEPS:-

1. Calling station  goes off hook.
2. After detecting a dc current flow on the loop, the switching machine returns an audible dial tone to the calling station , acknowledging that the caller has the asses to the switching machine.
3. The caller dials the destination telephone number using one of the two methods.
4. When the switching machine detects the first dialed number, it removes the dial tone from the loop.
5. Switch interprets the telephone number and then locates the local loop for the destination telephone number.
6. Before ringing the destination telephone, the switching machine tests the destination loop dc current to see if it is idle or in use. At the same time the switching machine locates a signal path through the switch between the local loops.
7. If the destination is in use then a busy signal is send back to calling station. If destination telephone is not in use then a calling signal is send to calling station.
8. When the destination answers the telephone. it completes the loop, causing dc current to flop.
9. The switch recognise the dc current as the station answering the telephone , at the same time the switch removes the ringing and ring back signals and completes the path through the switch, allowing the calling and called parties to begin heir conversation.
10. When either end goes on hook, the switching machine detects an open circuit on that loop and then drops the connections through the switch.

BASIC FUNCTIONS OF TELEPHONE SET:

1. Notify the subscriber when there is an incoming call with a audible signal and sometimes with a flashing light. These signals are enough to make people want to answer the telephone as soon as possible.
2. Provide a signal to the telephone network verifying when the incoming call has been acknowledged and answered.
3. Convert speech energy into electrical energy in the transmitter and vice versa in the receiver.
4. Incorporate some method of inputting and sending destination  phone numbers from the telephone set to central office switch over the local loop. This is accomplished using either rotary dialers or touch tone pads. 
5. Regulates the amplitude of the speech signal to prevent interference(crosstalk).
6. Incorporate some means of notifying the telephone office when a subscriber wish  to place an outgoing call. Subscribers cannot dial out until they receive  a dial tone from the switching machine.
7. Ensure that a small amount of the transmit signal is fed back to the speaker, enabling talkers to hear themselves speaking. This feedback signal is sometimes called side tone or talkback.
8. Provide an open circuit to local loop when the telephone is not in use and a closed circuit to local loop when the telephone is in use.
9. Provide a means of transmitting and receiving cell progress signals between the central office switch and the subscriber, such as on and off hook, busy, ringing, dial pulses, touch tone signals and dial tone.

WHY DC VOLTAGE IS USED RATHER THAT AC VOLTAGE ?

There are mainly three reasons they are:-

1. To prevent power supply hum.
2. To allow service to continue in the event of power outage.
3. The people were affraid of ac. The -48volts was selected to minimise the electrolytic corrosion on the loop wire