AM stereo

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AM stereo is a term given to a series of mutually incompatible techniques for radio broadcasting stereo audio in the AM band in a manner that is compatible with standard AM receivers. There are two main classes of systems: independent sideband (ISB) systems, promoted principally by American broadcast engineer Leonard R. Kahn; and quadrature amplitude modulation (QAM) multiplexing systems (conceptually closer to FM stereo).

Radio broadcasting distribution of audio content to a dispersed audience via any audio mass communications medium

Radio broadcasting is transmission by radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both. The signal types can be either analog audio or digital audio.

An audio frequency or audible frequency is a periodic vibration whose frequency is in the band audible to the average human. The SI unit of audio frequency is the hertz (Hz). It is the property of sound that most determines pitch.

Amplitude modulation in amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to the waveform being transmitted

Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the carrier wave is varied in proportion to that of the message signal being transmitted. The message signal is, for example, a function of the sound to be reproduced by a loudspeaker, or the light intensity of pixels of a television screen. This technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied.


Initially adopted by many commercial AM broadcasters in the mid to late 1980s, AM stereo broadcasting soon began to decline due to a lack of receivers (most "AM/FM stereo" radios only receive in stereo on FM), a growing exodus of music broadcasters to FM, concentration of ownership of the few remaining stations in the hands of large corporations and the removal of music from AM stations in favour of news/talk or sports broadcasting. By 2001, most of the former AM stereo broadcasters were no longer stereo or had left the AM band entirely.

AM stereo receiver. Tandy AM Stereo.jpg
AM stereo receiver.


Early experiments with stereo AM radio involved two separate stations (both AM or sometimes one AM and one FM) broadcasting the left and right audio channels. This system was not very practical, as it required the listener to use two separate receivers. Synchronization was problematic, often resulting in "ping-pong" effects between the two channels. Reception was also likely to be different between the two stations, and many listeners used mismatching models of receivers.

After the early experiments with two stations, a number of systems were invented to broadcast a stereo signal in a way which was compatible with standard AM receivers.

FM stereo was first implemented in 1961. In the United States, FM overtook AM as the dominant broadcast radio band in the late 1970s and early 1980s.

United States federal republic in North America

The United States of America (USA), commonly known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.


WDRC (AM) news/talk radio station in Hartford, Connecticut, United States

WDRC is a radio station owned by Red Wolf Brodcasting. It operates full-time with 5,000 watts of power with studios and transmitters located on Blue Hill Avenue in Bloomfield, Connecticut.

New Haven, Connecticut City in Connecticut, United States

New Haven is a coastal city in the U.S. state of Connecticut. It is located on New Haven Harbor on the northern shore of Long Island Sound in New Haven County, Connecticut, and is part of the New York metropolitan area. With a population of 129,779 as determined by the 2010 United States Census, it is the second-largest city in Connecticut after Bridgeport. New Haven is the principal municipality of Greater New Haven, which had a total population of 862,477 in 2010.

Tijuana City in Baja California, Mexico

Tijuana is the largest city in the Mexican state of Baja California and on the Baja California Peninsula. It is part of the San Diego–Tijuana international transborder agglomeration and the larger Southern California megalopolis. As one of the largest and fastest-growing cities of Mexico, Tijuana exerts a strong influence on local economics, education, culture, art, and politics. As the city has become a leading center in the country, so has the surrounding metropolitan area, a major industrial and paramount metropolis in northwestern Mexico. Currently one of the fastest-growing metropolitan areas in Mexico, Tijuana maintains global city status. As of 2015, the city of Tijuana had a population of 1,641,570.

Broadcasting systems

The Magnavox PMX, Harris Corporation V-CPM, and Motorola C-QUAM (Compatible—Quadrature Amplitude Modulation) were all based around modulating the phase and amplitude of the carrier, placing the stereo information in the phase modulated portion, while the standard mono (L+R) information is in the amplitude modulation. The systems all did this in similar (but not completely compatible) ways. The original Harris Corporation system was later changed to match the Motorola C-QUAM pilot tone for indicating the station was in stereo, thus making it compatible with all C-QUAM receivers.

Harris System

This system, known as V-CPM for Variable Angle Compatible Phase Multiplex, was developed by Harris Corporation, a major manufacturer of radio/TV transmitters. It incorporated a left minus right component which was frequency modulated by about 1 kHz. Harris is the successor to the pioneer Gates radio line, which has changed its name in 2014 to Gates-Air. The Harris system eventually changed their pilot tone to be compatible with C-QUAM, after C-QUAM became the more popular and eventually, the FCC approved standard. CKLW in Windsor, Ontario, Canada (also serving nearby Detroit, Michigan) was among the first stations to broadcast in Harris AM stereo. The Harris system is currently no longer used in its original form.

Harris Corporation is an American technology company, defense contractor and information technology services provider that produces wireless equipment, tactical radios, electronic systems, night vision equipment and both terrestrial and spaceborne antennas for use in the government, defense and commercial sectors. They specialize in surveillance solutions, microwave weaponry, and electronic warfare.

Windsor, Ontario City in Ontario, Canada

Windsor is a city in Ontario and the southernmost city in Canada. It is on the southern shore of the Detroit River, due south and directly across the river from Detroit, Michigan. Windsor is a major contributor to Canada's automotive industry and has a storied history and a diverse culture. Known as the "Automotive Capital of Canada", Windsor's industrial and manufacturing heritage is responsible for how the city has developed through the years.

Canada Country in North America

Canada is a country in the northern part of North America. Its ten provinces and three territories extend from the Atlantic to the Pacific and northward into the Arctic Ocean, covering 9.98 million square kilometres, making it the world's second-largest country by total area. Canada's southern border with the United States is the world's longest bi-national land border. Its capital is Ottawa, and its three largest metropolitan areas are Toronto, Montreal, and Vancouver. As a whole, Canada is sparsely populated, the majority of its land area being dominated by forest and tundra. Consequently, its population is highly urbanized, with over 80 percent of its inhabitants concentrated in large and medium-sized cities, many near the southern border. Canada's climate varies widely across its vast area, ranging from arctic weather in the north, to hot summers in the southern regions, with four distinct seasons.

Magnavox System

This system was developed by electronics manufacturer, Magnavox. It is a phase modulation system. It was initially declared the AM stereo standard by the FCC in 1980, but the FCC later declared that stations were free to choose any system. As with the Harris system, it was popular in the 1980s, but most stations stopped broadcasting in stereo, or downgraded to the C-QUAM system as time went on. 1190 WOWO in Fort Wayne, Indiana was the (then) 50,000-watt clear channel Magnavox flagship station.

Magnavox American Electronics Company

Magnavox is an American electronics company founded in the United States. Since 1974 it is a subsidiary of Dutch electronics corporation Philips.

Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms of angle modulation, together with frequency modulation.

WOWO (AM) news/talk radio station in Fort Wayne, Indiana, United States

WOWO is a commercial AM radio station in Fort Wayne, Indiana. It is airs a news/talk radio format. The call letters are usually pronounced as "Wo-Wo," rhyming with "go-go." WOWO is licensed to Pathfinder Communications Corporation, a subsidiary of Federated Media, a company that owns about a dozen radio stations in Indiana, Ohio and Michigan. The studios and offices are located along Maples Road in Fort Wayne, while its transmitter is found along Hoosier-Heartland Highway in Roanoke, midway between Fort Wayne and Huntington.

Motorola C-QUAM

C-QUAM was developed and promoted primarily by Motorola, a longtime manufacturer of two-way radio equipment. It became the dominant system by the late 1980s, and was declared the official standard by the FCC in 1993. While many stations in the USA have since discontinued broadcasting in stereo, many still have the necessary equipment to do so. C-QUAM is still popular in other parts of the world, such as Canada, Japan, and Australia which it was declared the official standard.

C-QUAM is the method of AM stereo broadcasting used in Canada, the United States and most other countries. It was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and published in an IEEE journal.

Japan Constitutional monarchy in East Asia

Japan is an island country in East Asia. Located in the Pacific Ocean, it lies off the eastern coast of the Asian continent and stretches from the Sea of Okhotsk in the north to the East China Sea and the Philippine Sea in the south.

Australia Country in Oceania

Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania and numerous smaller islands. It is the largest country in Oceania and the world's sixth-largest country by total area. The neighbouring countries are Papua New Guinea, Indonesia and East Timor to the north; the Solomon Islands and Vanuatu to the north-east; and New Zealand to the south-east. The population of 25 million is highly urbanised and heavily concentrated on the eastern seaboard. Australia's capital is Canberra, and its largest city is Sydney. The country's other major metropolitan areas are Melbourne, Brisbane, Perth and Adelaide.

The C-QUAM exciter replaces the crystal stage in an AM transmitter. The C-QUAM signal, consists of a phase modulated portion which is made up of both the L+R and L-R audio information and a conventional L+R (mono) portion, which is amplitude modulated. C-QUAM is a modified form of quadrature modulation in that the phase modulated audio consists of both the L+R and L-R portions, modulated 90 degrees out of phase with each other.

Including the L+R audio in the phase modulated portion of the signal is the very reason the C-QUAM method of AM Stereo is, as the name implies, 100% compatible with mono AM radios. This technique resolves a distortion issue which arises when left only or right only audio is transmitted using a basic L-R quadrature modulation approach.

C-QUAM had been long criticized by the Kahn-Hazeltine system's creator, Leonard Kahn as being inferior to his system. First generation C-QUAM receivers suffered from "platform motion" effects when listening to stations received via skywave. Later improvements by Motorola minimized the platform motion effect and increased audio quality and stereo separation, especially on AMAX-certified receivers in the 1990s.


The Kahn-Hazeltine system also called ISB was developed by American engineer Leonard R. Kahn and the Hazeltine Corporation. This system used an entirely different principle—using independently modulated upper and lower sidebands. While a station using the system would sound best with proper decoding, it was also possible to use two standard AM radios (one tuned above and the other below the primary carrier) to achieve the stereophonic effect, although with poor stereo separation and fidelity compared to a proper Kahn system AM stereo receiver. One of the best known stations to use the Kahn system was 890/WLS, Chicago. WLS still transmits in AM stereo today but uses the Motorola C-QUAM system instead.

However, the Kahn system suffered from lower stereo separation above 5 kHz (reaching none at 7 kHz whereas FM stereo has 40 dB or more separation at 15 kHz) and the radio antenna array on directional AM (common on a lot of nighttime and some daytime stations) had to have a flat response across the entire 20 kHz AM channel. If the array had a higher reactance value (leading to a higher Standing wave ratio) on one side of the frequency vs the other, it would affect the audio response of that channel and thus the stereo signal would be affected. Also, Kahn refused to license any radio receivers manufacturers with his design, although multi-system receivers were manufactured by various companies such as Sony, Sansui, and Sanyo, which could receive any of the four AM stereo systems.

Nonetheless, this system remained competitive with C-QUAM into the late 1980s and Kahn was very vocal about its advantages over Motorola's system. Kahn filed a lawsuit claiming that the Motorola system did not meet FCC emission bandwidth specifications, but by that time, C-QUAM had already been declared as the single standard for AM stereo in the USA.

Kahn's AM stereo design was later revamped for monaural use and used in the Power-Side system, in which a decreased signal in one sideband is used to improve coverage and loudness, especially with directional antenna arrays. Power-Side became the basis for CAM-D, Compatible AM Digital, a new digital system being promoted by Leonard Kahn and used on several AM stations.

Kahn receiver chips have also been used as an inexpensive method for providing high frequency (world band) receivers with synchronous detection technology.

Belar System

The Belar system was used in limited number of stations, such as WJR. The Belar system, originally designed by RCA in the 1950s, was a simple FM/AM modulation system, [2] with an attenuated L-R signal frequency modulating the carrier (with a 400 µs pre-emphasis) in the extent of +/- 320 Hz around the center frequency, and the L+R doing the normal "high level" AM modulation (usually referred to as plate modulation in transmitters using a tube in the final stage, where the audio is applied to the plate voltage of the tube; in solid state transmitters, various different techniques are available that are more efficient at lower power levels). The Belar system (by the company of the same name) was dropped due to issues with its design though it was much easier to implement than the other systems. It and the Kahn system did not suffer from platform motion (which was a killer for AM stereo at night; platform motion is where the stereo balance would shift from one side to the other and then back to center) but the use of low level frequency modulation did not permit a high separation of L and R channels.

Adoption in the United States

In 1975, the Federal Communications Commission (FCC) started a series of five-year tests to determine which of the five competing standards would be selected. By the end of the testing period, the Belar system was dropped. In 1980, the FCC announced that the Magnavox system would become the standard. This announcement was met with harsh criticism and a series of lawsuits. On March 4, 1982, the FCC revoked their endorsement to the Magnavox standard and let the marketplace decide, meaning that all four standards were allowed. After the 1982 decision, many stations implemented one of the four standards. Initially, all systems remained competitive, but by the later 1980s, Motorola C-QUAM had a clear majority of stations and receivers. Around this same time, Harris Corporation dropped their system and instead endorsed C-QUAM. During this time, radio manufactures either made receivers which decoded just one system, or decoded all four. The multiple systems used greatly confused consumers and severely impacted consumer adoption. As a result of this confusion, and the continued growth of the FM band, interest in AM stereo dwindled.

In 1993, the FCC declared Motorola's C-QUAM system the standard. To ensure that all AM stereo receivers maintained the same sound quality, the National Association of Broadcasters and the Electronic Industries Association started the AMAX certification program.

Global adoption

In the early 1980s, other countries, most notably Canada, Australia and Japan approved and implemented AM stereo systems. Most governments approved a single standard, usually Motorola's C-QUAM, which greatly reduced confusion and increased user adoption.

Following the launch of the American-owned, ship-based pirate radio station Laser 558 off the British coast, there were announcements that another such station, provisionally called Stereo Hits 576, would soon follow, using AM stereo on an adjacent frequency to Laser. Nothing ever came of this project and 576 kHz was adopted by Radio Caroline instead.

In many countries, especially those where the AM band is still dominant, AM stereo radios are still manufactured and stations still broadcast stereo signals.

Current status

Globally, interest in and use of AM stereo has been declining steadily since the 1990s, as many music stations have continued to move to the FM band. As a result, the vast majority of AM stations broadcast news/talk or sports/sports talk formats. Many of the stations that initially implemented AM stereo are clear-channel 50,000-watt stations, and are more concerned with listening range than stereo sound (although there is no proof that use of AM stereo affects listening range)[ citation needed ]. As a result, these stations still have the necessary equipment to broadcast in stereo, but it is left unused (or converted to HD Radio). Also, many former AM stereo stations were bought up by broadcasting conglomerates, which generally discourage AM stereo broadcasting. In the United States, most stations currently using AM stereo are small, independently owned and broadcast a variety or music format.

Surround sound

On February 26, 2010, KCJJ (AM 1630) in Coralville, Iowa, aired a four-hour quadraphonic radio broadcast of the Robb Spewak show. The show spotlighted music from the quadraphonic era on the 40th anniversary of the format's release in America and was engineered by Tab Patterson. All the music was from discrete 4-channel tapes, then encoded into Dolby Pro-Logic II and transmitted using their stereo C-QUAM transmitter.

Decline in use

Radio stations around the world are converting to various systems of digital radio, such as Digital Radio Mondiale, DAB or HD Radio (in the United States). Some of these digital radio systems, most notably HD Radio have "hybrid modes" which let a station broadcast a standard AM signal along with the digital information. While these transmission modes allow standard AM, they are not compatible with any AM stereo system (meaning both cannot be broadcast at the same time).

Digital AM broadcasting systems, such as HD Radio have been criticized by supporters of AM stereo[ who? ] as sounding "harsh" and "artificial", but supporters of Digital systems argue that the extended frequency response, increased dynamic range, lack of noise and lower distortion make up for the compression artifacts. However, HD Radio also increases adjacent channel noise due to the digital sidebands, which pose serious problems for nighttime broadcasts. Some have proposed to use HD Radio in the daytime and AM stereo at night. Many HD radios are based on a common chipset that decodes C-QUAM.[ clarification needed ]

Related Research Articles

Frequency modulation encoding of information in a carrier wave by varying the instantaneous frequency of the wave

In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.


In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands carry the information (modulation) transmitted by the signal. The sidebands consist of all the Fourier components of the modulated signal except the carrier. All forms of modulation produce sidebands.

Medium wave

Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz, using channels spaced every 9 kHz, and in North America an extended MW broadcast band ranges from 525 kHz to 1705 kHz, using 10 kHz spaced channels. The term is a historic one, dating from the early 20th century, when the radio spectrum was divided on the basis of the wavelength of the waves into long wave (LW), medium wave, and short wave (SW) radio bands.

AM broadcasting radio broadcasting using amplitude modulation

AM broadcasting is a radio broadcasting technology, which employs amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave transmissions, but also on the longwave and shortwave radio bands.

In-band on-channel (IBOC) is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency.

A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.

Digital Radio Mondiale Digital radio broadcasting standard

Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for analogue radio broadcasting including AM broadcasting, particularly shortwave, and FM broadcasting. DRM is more spectrally efficient than AM and FM, allowing more stations, at higher quality, into a given amount of bandwidth, using various MPEG-4 audio coding formats.

Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services.

Multichannel television sound, better known as MTS, is the method of encoding three additional channels of audio into an analog NTSC-format audio carrier.

HD Radio is a trademarked term for Xperi's in-band on-channel (IBOC) digital radio technology used by AM and FM radio stations to transmit audio and data by using a digital signal embedded "on-frequency" immediately above and below a station's standard analog signal, providing the means to listen to the same program in either HD or as a standard broadcast. The HD format also provides the means for a single radio station to simultaneously broadcast one or more different programs in addition to the program being transmitted on the radio station's analog channel. It was originally developed by iBiquity. In September 2015 iBiquity was acquired by DTS bringing the HD Radio technology under the same banner as DTS' eponymous theater surround sound systems.. It was later acquired by Xperi in 2016.

The Crosby system was an FM stereophonic broadcasting standard developed by Murray G. Crosby. In the United States, it competed with, and ultimately lost to, the Zenith/GE system, which the FCC chose as the standard in 1961.

FM broadcasting

FM broadcasting is a method of radio broadcasting using frequency modulation (FM) technology. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM 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. Theoretically wideband AM can offer equally good sound quality, provided the reception conditions are ideal. 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.

CBOF-FM is a French-language Canadian radio station located in Ottawa, Ontario. CBOF's studios are located at the CBC Ottawa Broadcast Centre on Sparks Street.

Compatible Amplitude Modulation - Digital or CAM-D is a hybrid digital radio format for AM broadcasting, proposed by broadcast engineer Leonard R. Kahn.

AMAX is an American certification program developed by the Electronic Industries Association (EIA) and the National Association of Broadcasters (NAB) in 1993. This quality control program addressed both consumer receiver developments and air chains of broadcast AM transmission stations. Tuners and receivers offering AMAX Stereo were designed to capture the widest audio frequency response and stereo separation of AM stereo broadcasts, where available.

KBME AM is a sports-talk radio station in the Houston, Texas metropolitan area. It is currently owned by iHeartMedia, Inc. The station airs local sports-talk and carries nationally syndicated Fox Sports Radio programming. KBME is also the flagship radio station for the Houston Rockets, Houston Astros and Texas Longhorns. The station's studios are located along the West Loop Freeway in the city's Uptown district. The transmitter site is located at what is now the southwest corner of Fallbrook Drive and T.C. Jester Boulevard, near Greenspoint in unincorporated Harris County.

KYZS is a terrestrial radio station, relayed by an FM translator, broadcasting a Sports radio format. Licensed to Tyler, Texas, United States, the station is currently owned by Gleiser Communications, LP and currently features programming from ESPN Radio.


  1. Mehrab, Gerald J. (2008-02-01). "AM Stereo". WA2FNQ web site. Northport, New York . Retrieved 2010-09-26.
  2. AM Stereo articles from Radio-Electronics Dec77, Popular-Electronics Dec78, and Popular-Electronics Aug80