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Radio Terminology Bibliography

Frequency Modulation (FM) Tutorial

Frequency modulation was devised to overcome the problem that A.M. reception was susceptible to noise interference.

With F.M.  instead of the carrier having its amplitude modulated the signal frequency is varied or controlled by the modulating (audio) frequency.

In the receiver the signal undergoes a great deal of amplification where the tops and bottoms are chopped of the signal - this is called 'limiting'. By limiting the amplitude of the signal all a.m. components (including noise) are thereby removed. This is why F.M. is preferred for quality music transmission. On the downside it tends to occupy greater bandwidth although narrow band F.M. does exist for two-way communication.

Commercial F.M. broadcasts occupy 200 Khz channels throughout the 88 - 108 Mhz band. This compares with the 10 Khz (or 9 Khz) channel spacing in the a.m. radio band or short wave broadcasting.

A process of translating baseband information to a higher frequency; carrier frequency, so more information can fit into a transmitted or recorded space. The video and two analog audio channels are FM modulated on to their own carriers so that they will all fit on the disc. The process: The two input signals to an FM modulator are the baseband signal (video or one of two audio channels) and the carrier frequency (a constant amplitude and constant frequency signal).

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