Radio: AM, FM, Analog, Digital. History

The term ‘‘radio’’ includes many different modes of wireless transmission. In the late nineteenth and early twentieth century, wireless telegraphy used spark-gap technology and intermittent waves to transmit Morse signals. Only with the development of continuous wave transmission in the early twentieth century did wireless telephony become possible, allowing effective transmission of the human voice and music.

All wireless or radio signals consist of a carrier wave (a continuous wave, altered by amplitude or frequency, to which is attached the intended voice or music intelligence being transmitted) and one or more sidebands (the band of frequencies produced by modulation). The intended information or content of a given signal is carried on one of the modulated sidebands. While only analog amplitude modulation (AM) was used until about 1940, frequency modulation (FM) became increasingly important after that date. Digital transmission developed beginning in the 1980s.

AM. Amplitude modulation indicates that the strength of the sideband signal is modulated (several thousand times per second) in accordance with the amplitude or strength of the carrier wave. Broadcast radio, which was developed in the 1920s, was assigned to medium-wave frequencies in most nations. In the U.S., AM radio stations are assigned to 10-kilohertz channels; in much of the rest of the world, AM or medium-wave radio uses 9-kilohertz channels.

While AM stations use narrower channels (and thus less frequency space) than FM stations, AM is prone to natural and most man-made electrical interference, or noise, which cannot be separated from the desired signal. Attempts to overcome static using more transmitter power failed. In part due to their narrow bandwidth and crowding on the AM band, signal response or ‘‘sound quality’’ is much poorer for AM (up to about 5 cycles per second) than FM, which regularly provides a signal response of up to 15 cycles per second due to its greater bandwidth.

Because they are located on medium-wave frequencies in most nations (540 to 1705 kilohertz in the U.S.), AM broadcast stations utilize ground wave propagation during daylight hours and sky wave propagation at night. Sky wave propagation, in which signals are bounced back to earth from the ionosphere, can carry a signal hundreds of kilometers, especially on cold, clear nights, though not in a predictable fashion. Station coverage or ‘‘reach’’ therefore varies by time of day and season.

FM. Frequency modulation signals vary by a swing of frequency rather than power output within the assigned channel. Edwin Howard Armstrong found that using a channel 20 times wider than an AM channel (200 kilohertz) would allow an analog signal with excellent frequency response (up to 15,000 cycles per second) that could avoid atmospheric interference (e.g., static from electrical storms) and much man-made interference as well.

An FM signal needs to be only twice as strong as a more distant competing transmitter to suppress the interfering signal. Utilizing very high-frequency (VHF, or VHF radio outside of the U.S.), FM signals are propagated by direct line-of-sight means day or night, limiting transmitter coverage to a radius of no more than 95 to 110 kilometers depending on local terrain, but eliminating multipath AM interference.

FM radio was developed from 1928 to 1933 by Edwin Armstrong, a professor at Columbia University in New York and prolific radio inventor. Armstrong fought many patent battles, as did most early American radio inventors. Two were very important and lasted for years: the fight with inventor Lee de Forest from 1914 to a Supreme Court decision two decades later over the rights to the regenerative circuit, which he lost; and the battle over his basic FM patents, fought against RCA and only settled after his death in 1954.

After considerable experimentation by Armstrong and others, FM was introduced as a broadcasting service in the U.S. in 1941, on 42-50 megahertz. About 50 stations were on the air before a wartime freeze was imposed on new construction in 1942. Television standards approved in 1941 required FM for the sound portion of the signal. After extensive research and hearings, the Federal Communications Commission (FCC) in early 1945 reallocated FM to its present 88-108 megahertz range, thus providing more channels but at a cost to the medium of ‘‘starting over’’ in the face of television and revived AM competition.

In 1955 the FCC allowed FM stations to multiplex an additional nonbroadcast signal as a means of generating revenue. The service usually provided background music to offices and stores. Six years later, stereo multiplex FM transmission standards were approved for commercial operation in the U.S. The National Stereophonic Radio Committee has been formed by the radio manufacturing industry in 1959 to test various proposals.

The system developed by General Electric was recommended to and approved by the FCC in early 1961. Stereo transmission is downward compatible, meaning it allows monaural reception in radios not equipped with stereo reception.

Digital. AM and FM transmissions are analog signals, subject to an inherent background electrical noise (or ‘‘hiss’’), although FM suffers less than AM. Most consumers were introduced to digital sound and its superior signal-to-noise ratio with the success of the digital compact disc in the 1980s, and began to seek similar quality over the air.

By the late 1990s, most American and many international radio stations used the Internet to stream their signals, making worldwide reception possible. Audio streaming quality of sound was most often limited by the computer speakers used.

Based on research that began in the early 1980s, digital audio broadcasting (DAB) became operational in Europe and Canada, which in the 1990s agreed to use the ‘‘Eureka 147’’ technical standard, allowing transmission of digital-quality sound as well as information and data. Depending on the country, stations operated on Band III (around 221 megahertz) or L-band (1452 to 1492 megahertz) frequencies, well above current FM/VHF frequencies.

L-band was allocated for digital radio at the World Administrative Radio Conference in 1992. The first commercial DAB receivers became available in 1998; and terrestrial DAB service was available by late 2001 to nearly 300 million people from more than 400 digital radio stations, with plans for satellite delivery in the future.

Declining to adopt Eureka 147, U.S. manufacturing companies formed the National Radio Systems Committee (NRSC) to determine which of a half-dozen schemes to recommend for approval by the FCC. To ease the eventual transition to digital radio from existing analog stations, the industry and FCC sought an in-band, on-channel (IBOC) scheme whereby the new service would operate during a transition period side-by-side the analog stations that it would eventually replace.

However, agreement on a workable technical standard was continually delayed. In the meantime, digital audio radio service (DARS), transmitted from domestic satellites, was authorized by the FCC in the late 1990s. XM Satellite Radio began to offer its subscription- based 100-channel service in late 2001 while competitor Sirius planned to begin advertising- free transmissions by early 2002. Both required the purchase of special digital receivers.