How does your Telephone Work?

Have you ever thought of it? How does your telephone work? We use phones of varying sorts every day to call friends, business colleagues and relatives. We call locally and long distance, yes even to the other side of the world. It is truly amazing how far communications have developed.

Back in the 1800's there was just a telegraph where messages were sent as a simple code. At that time reproducing ‘voice' was unheard of. However, by 1871 Antonio Meucci was on the track of reproducing ‘voice' and filed a patent caveat in the US patent office describing it as ‘communication of voice between two people by wire'. However, he is not a man of money and in the end he lets the patent caveat lapse. In the several years following there is a rush to invent a good system of being able to transmit the human voice over wire. Bell, Gray, and finally Edison were in a ‘fight to the finish' to patent a method that was both practical and reliable. It was one thing to produce a click or buzz down a wire but the human voice is a huge step forward back then. Eventually it was Bell and Edison that won out, but not after many lawsuits had been fought.

So just what was it they had to do? They had to invent a way to transmit from one transmitter to a receiver a sound that was the same as the human voice. Just how was it done? How was a human voice to be transmitted with all its varying qualities such as loudness, tone, pitch etc.

To get right down to understanding the real difficulties let's look at the sensation of sound. The sound sensation we hear is produced because of fluctuations in the pressure of the atmosphere that enter our ear. The delicate inner ear mechanisms vibrate and produces the sound sensation that we call hearing. Now if the fluctuations are very irregular and none periodic we would probably be putting our hands over our ears and considering it a noise. If the pressure fluctuations are regular and say – cyclic we would be hearing what we term as music. Music is complex being made up of pitch, quality and loudness. Regarding pitch, this depends on the number of cycles passed through by the fluctuations of the pressure in a given time. Loudness would depend on the amount or amplitude of the fluctuation in each cycle. Quality depends on the nature or form of the fluctuation in each cycle. Does this sound daunting? Well it did to the men who were trying so hard to invent a way to reproduce successfully all this by an electrical current traveling down a wire and then convert back again! So any telephone system has to do all this, reproducing all these characteristics.

So we come back to our original question. How does your telephone work? So let us take a simple land line telephone as an example to use.

Your telephone basically has to do two things using 2 wires, transmit signals and your voice. The signal is the ring tone you get. It used to be a bell that alerts you to an incoming call. Then there is a dial (now a digital push button) that is used to enter the phone number of the person you wish to speak to. When you lift the handset you activate a switch which puts the phone in what is termed 'active mode'. This is done by creating a resistance short across the wires so that a current can flow along the telephone wires. Once this is done the telephone exchange detects the DC current, connects a digit receiver and sends to you a dial tone which indicates it is ready for you to dial or push the number buttons. These buttons are connected to a tone generator in your phone and generates DTMF tones. Now the exchange will know who you desire to be connected to. Whilst your phone is inactive with the handset replaced, its bell or other alerting device is connected across the line through a capacitor. Because this does not short the line the telephone exchange knows that your telephone is not being used and 'on hook'. This also means that only the bell is connected electrically, so when someone calls your number the exchange sends a high voltage pulsating signal that causes the phone to ring. When you pick up the handset the switch disconnects the bell then connects the voice parts of your phone putting the resistance short on the line as described previously. This now tells the exchange that you have answered your phone. Now that both phones have their handsets of the cradle this is the signaling job done. You can now talk to your friend using the voice parts of your phone. The voice parts are in the handset and consist of a transmitter (microphone) and a receiver. The transmitter being powered by the line puts out an electric current which varies in response to acoustic pressure waves caused by your voice. This causes variations of the electric current transmitted along the telephone line to the other phone. Once these hit the other phone they cause the coil in the receiver to move back and forth, this reproducing the same acoustic pressure waves as the transmitter received from your voice. So your friend now hears what you said like you were stood next to them. This applies vice versa to you, so you have your conversation. Once you have finished the conversation, when you replace the handset the DC current ceases to flow and the exchange detects this so disconnects the callers.

The greatest difficulty to those early pioneers of telephone was the development of the transmitter and the receiver. It is so easy to take these for granted.

Well if the above has had your head spinning then I ask you to think about those early pioneers of the telephone. They had to work out how to make the transmitter and receiver to vary the electric current in a perfect way so as to transmit voice. They were the ‘brains' of their day and we owe them a lot.

Nowadays, we have a huge raft of new technology allowing conversation worldwide effortlessly (so it may seem) but always think of those guys out there somewhere who are making it just that – effortlessly easy!