FM broadcasting is a method of radio broadcasting using frequency modulation technology. Invented in 1933 by American engineer Edwin Armstrong, it is used worldwide to provide high-fidelity sound over broadcast radio, FM broadcasting is capable of better sound quality than AM broadcasting, the chief competing radio broadcasting technology, so it is used for most music broadcasts. FM radio stations use the VHF frequencies, the term FM band describes the frequency band in a given country which is dedicated to FM broadcasting. Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 to 108.0 MHz is used, or some portion thereof, with few exceptions, In the former Soviet republics, and some former Eastern Bloc countries, assigned frequencies are at intervals of 30 kHz. This band, sometimes referred to as the OIRT band, is slowly being phased out in many countries, in those countries the 87. 5–108.0 MHz band is referred to as the CCIR band. In Japan, the band 76–95 MHz is used, the frequency of an FM broadcast station is usually an exact multiple of 100 kHz. In most of South Korea, the Americas, the Philippines, in some parts of Europe, Greenland and Africa, only even multiples are used. In the UK odd or even are used, in Italy, multiples of 50 kHz are used. There are other unusual and obsolete FM broadcasting standards in countries, including 1,10,30,74,500. Random noise has a triangular spectral distribution in an FM system and this can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis, the amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used, in the Americas and South Korea,75 µs is used. This applies to both mono and stereo transmissions, for stereo, pre-emphasis is applied to the left and right channels before multiplexing. They cannot be pre-emphasized as much because it would cause excessive deviation of the FM carrier, systems more modern than FM broadcasting tend to use either programme-dependent variable pre-emphasis, e. g. dbx in the BTSC TV sound system, or none at all. Long before FM stereo transmission was considered, FM multiplexing of other types of audio level information was experimented with. Edwin Armstrong who invented FM was the first to experiment with multiplexing and these original FM multiplex subcarriers were amplitude modulated
FM Radio Broadcasting : Big Picture in Full Electromagnetic Spectrum
A commercial 35 kW FM radio transmitter built in the late 1980s. It belongs to FM radio station KWNR in Las Vegas, Nevada, and broadcasts at a frequency of 95.5 MHz.
FM has better rejection of static (RFI) than AM. This was shown in a dramatic demonstration by General Electric at its New York lab in 1940. The radio had both AM and FM receivers. With a million-volt arc as a source of interference behind it, the AM receiver produced only a roar of static, while the FM receiver clearly reproduced a music program from Armstrong's experimental FM transmitter in New Jersey.
Crossed dipole antenna of station KENZ's 94.9 MHz, 48 kW transmitter on Lake Mountain, Utah. It radiates circularly polarized radio waves.