How does a Video Telephone Work?

            A video telephone is a communication device used foe simultaneous exchange of visual images and the associated speech. In this system one can see the face of the person talking at the other end and also listen to his voice. Do you know how this is possible?

Video Telephone

            A complete video telephone system consists of the following parts:

1.                   Terminal equipment which has a camera display screen microphone and a speaker phone. These instruments transform both voice and visual inputs into electrical signals and vice versa.

2.                  Transmission facilities which carry the electrical signals up to long distances.

3.                  A switching system to allow a choice of terminals to be interconnected. At the transmission point, voice and picture both are converted into electrical signals. Which are carried to a long distance through transmission arrangement? At the receiver end these signals are converted into voice and picture which can be heard and seen by the person at the receiver end.

            Video telephone system was developed in 1927 by H.E. lves who transmitted one way images by wire from Washington D.C. also transmitted from blew jersey to New York by radio. These experiments were aimed at transmitting and reproducing a recognizable human face.

            In 1930, lves demonstrated a two-way video telephone over a wire path in New York City. From 1936 to 1940, a public video telephone service was provided on a local and intercity basis by the German post offices. Call could be set up by appointment between any two subscribers in Berlin, Leipzig, Nuremberg and Munich. A similar system was inaugurated by the soviets in 1961 for subscribers in 8 cities, including Moscow, Kiew and Leningrad.

            Since the 1960s, most major communication agencies throughout the world have been exploring the feasibility of a commercial video telephone service. The first limited commercial video telephone service was offered in the early 1970s by bell system.

            In this system, ordinary telephone wires utilizing specially designed repeaters were used to transmit the video signals. Video requires two pairs of wires, one for each direction of transmission. Early service between Chicago and Pittsburgh provided valuable insight into customer needs, but customer acceptance did not meet the early expectation.

            Nowadays, researchers have developed video telephone systems, using communication satellite links. In many developed countries, this system has now become popular after vase improvement of its mechanism in terms of perfection at the level of customer’s expectation.

What is FM Transmission?

            Over the year frequency modulated FM transmission has attained wide popularity due to its clarity and high fidelity in reproducing the broadcast speech or music. It can also provide a stereophonic effect to the sound produced at the receiving end. Before television came into our homes, radio was a popular entertainment medium. With FM transmission capturing the imagination of people, radio is likely to become popular once again.

FM Transmission Provider

            The earlier transmission services were basically amplitude modulated AM, which could not filter the noise and hence lacked the clarity that FM can provide. But what is the fundamental difference between AM and FM transmission?

            In both AM and FM transmission, radio waves are modulated by mounting it on the carrier frequency, which is essential for broadcasting and propagation. In AM transmission, the waves are constant in frequency but the amplitude varies, whereas in FM transmission, the amplitude is constant and the frequency varies. In simple terms, in AM amplitude is modulated and in FM, the frequency is modulated.

            Modulation is the process of coding the sound waves for transmission, and is done either by altering the amplitude of the wave or its frequency.

            We now need to understand three fundamental concepts- modulating wave, carrier wave and modulated wave. The modulating wave is the information bearing signal such as human voice or music, audible to the human ear, and intended to be communicated to a receiver. The carrier wave can be transmitted over long distances and is not audible to the human ear. The modulated wave is the wave derived by impressing the information bearing signal on the carrier wave. This wave is transmitted through space to the receiver.

            How is it that the FM transmission provides excellent reception?

            Speech itself is represented by an irregular wave pattern. An FM receiver has to detect frequency changes only; it does not reproduce any amplitude changes caused by electrical interferences. As a result, almost all background noise can be eliminated using filters, producing excellent reception.

            FM transmission is used in radio broadcasting services, multi-channel carrier telephones, communication satellite links, telegraphy, mobile communications, navigational and meteorological equipment as well as for medical diagnostic instrumentation. FM also exhibits channel grabbing that is, if two FM signals are available, the stronger of the two is received and the other is nearly excluded. This permits low-power radio stations operating in the same frequency to operate close to each other. AM transmission does not possess this ability.

What is an Electronic Mail?

            Electronic mail is a modern method of instant delivery of mail without the involvement of a postman or post office. In this system of mailing messages are sent with the help of a computer.

The Network by which E-Mails are sent

            In electronic mail system, a letter or a document is typed and edited on a computer. It is then set to the addressee through a computer network in the form of electrical data. The receiver could be anywhere, but he should also have a computer and an electronic mail system, the contents of the transmitted letters or documents get displayed on the computer screen of the addressee.

            If the addressee is not available at his computer, the message is automatically stored in the computer’s memory. When he returns, the computer gives an indication that a mail has arrived. If he desires, he can get a printout of the message.

            With the help of an electronic mail system, both typed and handwritten messages can be transmitted. Electronic mail is swift and saves time and paper work. It needs a telephone or a satellite, or a cable links for visual, voice and data transmission. This system is prevalent in developed countries but spreading fast throughout the world. E- Mail is widely sued in internet services. It is also connected to mobiles, laptops and palmtops.