Over the years Frequency Modulated (FM) transmission has attained wide popularity due to its clarity and accuracy in reproducing the broadcast speech or music. It can also provide a stereophonic effect to the sound produced at the receiving end. FM transmission service has rekindled the hope of regaining the fading popularity of radio. The earlier transmission services were basically Amplitude Modulated (AM) which could not filter the noise and hence lacked the clarity of FM. But what is the fundamental difference between AM and FM transmission?
In both cases radio waves are altered for the transmission of broadcasting signals. In AM transmission the waves are constant in frequency but the amplitude of the transmitting wave varies in accordance with the signal being broadcast whereas in FM the amplitude is constant and the frequency varies in accordance with the signal being transmitted. In simple terms, in AM the amplitude is modulated and in FM, the frequency is modulated.
Modulation is the process of coding the radio wave and is done either by altering the amplitude of the wave or its frequency. Here comes the need of understanding three fundamental concepts: modulating wave, carrier and modulated wave. The modulating wave is the information-bearing signal such as human voice or music. It is the message intended to be communicated to a receiver. The carrier is the wave that is varied by the information bearing signal. The modulated wave is the wave developed by impressing the information-bearing signal on the carrier. This wave is transmitted to the receiver. Now the curiosity arises as to why does the FM transmission provided an excellent reception?
Speech itself is represented by an irregular wave pattern. As the receiver using FM had to detect frequency changes only, it did not reproduce any amplitude changes caused by electrical interference. As a result, almost all background noise can be eliminated using filters, giving excellent reception.
FM transmission is used in radio broadcasting services, multichannel carrier telephone, communication satellite links, telegraphy, mobile communications, navigational and meteorological aids 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 be located close to each other. This advantage is not possessed by the AM stations.
