Telephony, as you know, is the technology related to voice transmission between two parties placed in distant geographical locations. Today, in addition to analog telephone systems, there are telephone systems, which operate through digital-transmission of voice. There are also systems, which allow radio to transmit telephone signals.

What is an Electro-magnetic wave?

When you dial your satellite phone, it beams the signal to the satellite up above in the form of radio waves. Radio waves along with 'Gamma' rays, X-rays, ultra-violet rays, infra-red rays, visible light, radar waves, micro-waves etc. are known as electro-magnetic waves as they are made up of 'electric' as well as 'magnetic fields'. The total range is known as 'Electro-magnetic Spectrum'. They all travel at the speed of light (299, 274 kilometers per second). They

have different wavelengths and can carry different amounts of energy and information. Shorter wavelengths have more energy while longer ones have less energy.

How voice transmission is effected?

When you talk to your satellite phone, there is a special accessory, usually a modem, which has a basic electric signal that oscillates back and forth regularly at a constant rate, known as "Carrier". This 'Carrier' signal is blended with the voice, which of course has different pitches. The resulting signal is slightly different in pitch compared to the "Carrier". This modulated and amplified signal is sent to the satellite, which sends it back to the LES, which returns it to the satellite after encoding. The satellite then sends the signal to the recipient's satellite phone where the special accessory, which knows the carrier modulation, separates it thereby redeeming the voice signal.

What is Telephony?

It is voice transmission to a recipient at a distant location through wire, cable or sound waves. Many innovations have taken place in the last 130 years and, today; Alexander Graham Bell's simple invention is the main catalyst in ushering a veritable revolution in global communication. The voice of the caller is sent, (1) through analog format via wires, (2) through digital format via wires/waves, (3) and through sound waves.

(1) Analog format

Originally all voice messages were sent through analog format only. What is analog format? Any electronic device is said to be "analog" if the amount of electrical current it produces is proportional to its input. In earlier telephones, the amount of electrical current sent between two telephones was proportional to the volume of sound. Disadvantages (a) Distortion possible (b) Signal becoming weak (signal loss) over a long distance needing an amplifier. (c) Possible to get a lot of interference (d) Noise pollution.

The system needs special filters to block distortions and noise. This sometimes, weakens the primary sound, i.e., voice. Despite all these drawbacks, over the years, the analog model was found to be easy and understandable.

The invention of IC - Integrated circuits:

The scientists at Bell Laboratories, U.S.A. developed a solid state switch called a "transistor". Transistors have three layers - collector, base and emitter. They are used to amplify or switch 'on' and 'off' current and vary in the frequency range of signals they handle. This single invention had put a new fillip to the electronic industry. A little later, a most breath-taking invention revolutionized the electronics, computers and telecommunication industries. It was the invention of "Chip", or, "silicon chip", or, "integrated circuits". Now what is an IC? It is a tiny silicon chip containing in itself a miniature circuit with thousands of components, like, diodes, transistors, capacitors, resistors, etc. Complex devices need many chips mounted on a printed circuit board, which have all the connections between the chips and other components "printed" on copper. Continuous innovations have made IC's most sophisticated and today, it is possible to build an IC with millions of transistors and other components.

(2) The digital format.

The digital triumph started with the inventions of transistors and integrated circuits. Computers, today, have become very compact only because of silicon chips. Information stored in a computer is in the form of numbers (digits). Any electronic technology, which produces, processes or stores information in 'binary digits' is said to be a digital technology. 'Binary digit', or 'bit' is either '1' or '0'. Information in voice format can be encoded in digital form. This ensures, communication to be smooth, with no noise pollution to affect, no distortions enroute and no weakening of signal. However the encoded digital voice has to be sent in analog format only if you use the PSTN. Voice transfer is possible with digital encoding. At the sending-end the system converts an analog voice signal into a series of numbers and the connected computer sends it across to the telephone network where they are converted back to analog signal. An integrated circuit performs the conversion i.e.; from 'Analog to Digital' and 'Digital to Analog'.

Graphically:

                    

Integrated Circuit that performs analog-to-digital conversion

Then, you require another chip, which converts the digital back to analog.

Graphically:


                      
                                 Integrated Circuit that performs digital-to-analog conversion


In the digital system, the dialing process of telephone companies used to have two sets of wires, one to carry digital signals used for dialing and the other for carrying analog (electrical) signals used for voice transfer. Further inventions brought in merging of voice and dialing process, and, today, telephone systems use digital technology entirely.

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