The History of Amplitude Modulation

The History of Amplitude Modulation
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Introduction - What is AM or Amplitude Modulation

A radio signal is usually very weak and cannot travel long distances on its own. A method to deal with this problem is to use a technique called Amplitude Modulation or AM. The technique imposes a second signal on the already existing carrier wave that transmits the information. The amplitude of the second signal varies (increases or decreases) correspondingly to the amplitude of the first wave. These variations in amplitude or strength are used by the receiver in order to reproduce audio and visual information (different sounds and images).

The range of frequencies available for AM transmission is from 535 to 1705 kilohertz. There is also the Frequency Modulation or FM technique, in which the frequency - and not the amplitude - of the second signal is regulated according to the carrier wave. For a better understanding check the comparison image.

An Audio Radiosignal Modulated by Amplitude (AM) and Frequency (FM)

Early History

The discovery that electromagnetic waves are capable of transmitting information sparked the invention of radio. The technology was based on the already existing inventions of the telegraph and the telephone. According to some researchers, the amplitude modulation technique originated from the experimental and theoretical work of Leblanc, back in 1886, Mayer (1875) and Rayleigh (1894). For example, the first successful attempt to transmit audio signals over telephone lines took place in the mid-1870s and this has been acknowledged as one of the first transmissions taking place with the help of some form of amplitude modulation.

The inventors and engineers of the early 20th century were not aware of this work. They rediscovered the same principles and developed them into new and innovative devices.

The Advent of AM Radio

The development of the technique is attributed to Lee de Forest and Reginald Fessenden.

A number of inventions are attributed to Lee de Forest, including the triode amplifier, the space telegraphy and the Audion, another type of vacuum tube. The use of the term “radio” is also attributed to him. His work had to do with the amplification of very weak signals that needed to be picked up by the antenna before application to the receiver detector. This work is thought to have significantly contributed to the development of the amplitude modulation technique.

The first transmission however, took place in 1906 from a garage in Brant Rock, Massachusetts. After six years of hard work, Reginald Fessenden, a Canadian inventor transmitted the world’s first voice message by using an Alexanderson alternator and a rotary spark-gap transmitter. His message was heard by radio-equipped ships within a range of several hundred miles away from the transmission point

The use of AM broadcasting became more extensive in the years before World War I. Back in 1909, Charles David Herrold founded the first radio station in San Jose, California, known as KQW. Radio station KCBS from San Francisco, is believed to be the descendant of this first broadcast station. After World War I, the number of radio experimentalists increased dramatically and so did the use of AM radio.

The year 1920 was the year of the first “commercial” AM broadcasting by XWA in Montreal, Quebec, Canada. The station is now called CINW. Correspondingly, the first commercial broadcasting in the US took place in Pennsylvania by Frank Conrad, who was also responsible for the founding of the first licensed broadcast station in the world, KDKA. Conrad was an Assistant Chief Engineer in the Westinghouse Electric Corporation that created KDKA on November 2, 1920. The station is now owned by the CBS Radio and its transmitter is located in Allison Park.


Reginald Fessenden


In 1933, another modulation technique was invented by Edwin Howard Armstrong. The well-known frequency-modulated or FM radio managed to dominate the world of music and public broadcasting, especially during the late 1970s. The reason was that the new improved signal was less susceptible to noise, static, and electrical or atmospheric interference. For example, electrical motors, domestic appliances, light bulbs, lightning, and even tall buildings and metal structures could easily disrupt an AM signal, whereas an FM signal would stay intact.

Despite this fact, AM remains widely used even in the 21st century, especially in areas where FM frequencies are in short supply or in thinly populated or mountainous areas where the coverage is too poor. In the late 1960s, radio stations such as the WABC transmitted rock and roll music on this band with huge success. Although it is not very popular for music broadcasting nowadays, it is a good option for talk radio, news, sports, etc.

Still you can find certain music such as country and ethnic music transmitted on the AM band.