AM vs FM

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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.

Comparison chart

AM versus FM comparison chart
	AM	FM
Stands for	AM stands for Amplitude Modulation	FM stands for Frequency Modulation
Origin	AM method of audio transmission was first successfully carried out in the mid 1870s.	FM radio was developed in the United states in the 1930s, mainly by Edwin Armstrong.
Modulating differences	In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. The frequency and phase remain the same.	In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. The amplitude and phase remain the same.
Pros and cons	AM has poorer sound quality compared with FM, but is cheaper and can be transmitted over long distances. It has a lower bandwidth so it can have more stations available in any frequency range.	FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has better sound quality due to higher bandwidth.
Frequency Range	AM radio ranges from 535 to 1705 KHz (OR) Up to 1200 bits per second.	FM radio ranges in a higher spectrum from 88 to 108 MHz. (OR) 1200 to 2400 bits per second.
Bandwidth Requirements	Twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz.	Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz.
Zero crossing in modulated signal	Equidistant	Not equidistant
Complexity	Transmitter and receiver are simple but syncronization is needed in case of SSBSC AM carrier.	Tranmitter and reciver are more complex as variation of modulating signal has to beconverted and detected from corresponding variation in frequencies.(i.e. voltage to frequency and frequency to voltage conversion has to be done).
Noise	AM is more susceptible to noise because noise affects amplitude, which is where information is "stored" in an AM signal.	FM is less susceptible to noise because information in an FM signal is transmitted through varying the frequency, and not the amplitude.

History

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.

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.

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.

Popularity

FM radio became popular in the 1970s and early 80s. 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.

Technical Details

A signal may be carried by an AM or FM radio wave.

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.