Frequency or frequency modulation is a form of modulation that transmits information by varying the frequency of a carrier wave; The amplitude modulation AM greater or varies the amplitude of the carrier, with its frequency remains constant. With FM the frequency deviation of the assigned carrier frequency at any instant is directly proportional to the amplitude of the input signal and determines the instantaneous frequency of the transmitted signal. As the transmitted FM signals use more bandwidth of AM-band signals, this form of modulation is generally used with the higher (VHF or UHF) frequencies used by television, FM broadcast band systems, and telephony Land mobile.
Armstrong’s first FM transmitter, located in the Empire State Building, New York City, which allows testing of the secrets of its system between 1934 and 1935. licensed as a W2XDG experiment station, is broadcast 41 MHz at a power of 2 KW
A random noise having a triangular spectral distribution of an FM system, with the effect that the noise occurs primarily at the highest frequencies in the audio band. This can be compensated to a limited extent by increasing the high frequencies prior to transmission and further reducing in the receiver. Reducing high audio frequencies in the receiver also reduces high-frequency noise. These amplification methods and thus reduce certain frequencies are known respectively as pre-emphasis and de-emphasis ,.
The amount of pre-emphasis and de-emphasis used is defined by the time constant of a single RC filter circuit. In most countries, a time constant of 50 ms is used. The Americas and South Korea, 75 ms is used. This is worth both mono and stereo transmissions. Pre-emphasis is applied to the left and right channels before multiplexing, stereo.The amount of pre-emphasis that can be applied is limited by the fact that many forms of contemporary music contain higher frequency energy than the musical styles in effect at the birth of FM broadcasting. It can not be pre-emphasized as it would cause excessive deviation of the FM carrier. More modern FM broadcasting systems tend to use the program according to variable pre-emphasis; For example, dbx in the sound system of the BTSC TV, or not at all.