AM (or Amplitude Modulation) and FM (or Frequency Modulation) are ways of broadcasting radio signals. Both transmit the information in the form of electromagnetic waves. AM works by modulating (varying) the amplitude of the signal or carrier transmitted according to the information being sent, while the frequency remains constant. This differs from FM technology in which information (sound) is encoded by varying the frequency of the wave and the amplitude is kept constant.
AM method of audio transmission was first successfully carried out in the mid 1870s to produce quality radio over telephone lines and the original method used for audio radio transmissions. FM radio was developed in the United states mainly by Edwin Armstrong in the 1930s.
edit Differences in Spectrum Range
AM radio ranges from 535 to 1705 kilohertz, whereas FM radio ranges in a higher spectrum from 88 to 108 megahertz. For AM radio, stations are possible every 10 kHz and FM stations are possible every 200 kHz.
edit Pros and Cons of AM vs. FM
The advantages of AM radio are that it is relatively easy to detect with simple equipment, even if the signal is not very strong. The other advantage is that it has a narrower bandwidth than FM, and wider coverage compared with FM radio. The major disadvantage of AM is that the signal is affected by electrical storms and other radio frequency interference. Also, although the radio transmitters can transmit sound waves of frequency up to 15 kHz, most receivers are able to reproduce frequencies only up to 5kHz or less. Wideband FM was invented to specifically overcome the interference disadvantage of AM radio.
A distinct advantage that FM has over AM is that FM radio has better sound quality than AM radio. The disadvantage of FM signal is that it is more local and cannot be transmitted over long distance. Thus, it may take more FM radio stations to cover a large area. Moreover, the presence of tall buildings or land masses may limit the coverage and quality of FM. Thirdly, FM requires a fairly more complicated receiver and transmitter than an AM signal does.
FM radio became popular in the 1980s and by the 1990s most music stations switched from AM and adopted FM due to better sound quality. This trend was seen in America and most of the countries in Europe, and slowly FM channels exceeded AM channels. Today, speech broadcasting (such as talk and news channels) still prefers to use AM, while music channels are solely FM.
edit Technical Details
AM was initially developed for telephone communication. For radio communication, a continuous wave radio signal called double sideband amplitude modulation (DSB-AM) was produced. A sideband is a band of frequencies higher (called upper sideband) or lower (called lower sideband) than the carrier frequencies which is a result of modulation. All forms of modulations produce sidebands. In DSB-AM the carrier and both USB and LSB are present. The power usage in this system proved inefficient and led to the double-sideband suppressed-carrier (DSBSC) signal in which the carrier is removed. For greater efficiency, single-sideband modulation was developed and used in which only a single sideband remained. For digital communication, a simple form of AM called continuous wave (CW) operation is used in which the presence or absence of carrier wave represents binary data. The International Telecommunication Union (ITU) designated different types of amplitude modulation in 1982 which include A3E, double sideband full–carrier; R3E, single-sideband reduced-carrier; H3E, single-sideband full-carrier; J3E, single-sideband suppressed-carrier; B8E, independent-sideband emission; C3F, vestigial-sideband and Lincompex, linked compressor and expander.
FM radio characteristics and services include pre-emphasis and de-emphasis, stereophonic FM sound, Quadraphonic sound, Dolby FM and other subcarrier services. Pre-emphasis and de-emphasis are processes that require boosting and reducing certain frequencies. This is done to reduce noise at high frequencies. Stereophonic FM radio was developed and formally approved in 1961 in the USA. This uses two or more audio channels independently to produce sound heard from various directions. Quadraphonic is four-channel FM broadcasting. Dolby FM is a noise reduction system used with FM radio, which has not been very successful, commercially.
Below is an old training video from the U.S. Army that talks about the technical workings of AM and FM radio.