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).
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 favor 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.
History 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.
Timeline 1924: WPAJ (now WDRC) broadcast in stereo from New Haven, Connecticut, using two transmitters: one on 1120 kHz and the other on 1320 kHz."Stereoscopic or Binaural Broadcasting in Experimental Use at New Haven", Cincinnati Enquirer, March 22, 1925, Section 6, page 6. However stereo separation was poor, to preserve compatibility for mono listeners. In the 1950s, several AM stereo systems were proposed (including the original RCA AM/FM system which later became the Belar system in the 1970s) but the FCC did not propose any standard as AM was still dominant over FM at the time. 1960: AM stereo first demonstrated on XETRA-AM, Tijuana, Mexico, using the Kahn independent sideband system. 1963: WHAZ runs a stereo program on eight AM stations, four on each channel. 1980: After five years of testing the five systems, the United States Federal Communications Commission (FCC) selected the Magnavox system as the official AM stereo standard.Popular Electronics, pages 57 & 58; August 1980 issue; retrieved June 26, 2022. The FCC's research is immediately accused of being flawed and incomplete. 1982: After a series of lawsuits and accusations, the FCC decides to let the marketplace decide and revokes the Magnavox certification as the AM stereo standard for political reasons. Belar had dropped out of the AM stereo race due to receiver distortion problems, leaving Motorola C-QUAM, Harris Corporation, Magnavox, and the Kahn/Hazeltine independent sideband system. 1984: General Motors, Ford, Chrysler, and a number of import automakers begin installing C-QUAM AM stereo receivers in automobiles, beginning with the 1985 model year. Harris Corporation abandons its AM stereo system and puts its support behind C-QUAM (Harris continues to manufacture C-QUAM equipment today). 1985: AM stereo broadcasting officially begins in Australia, with the C-QUAM standard. 1988: Canada and Mexico adopt C-QUAM as their standard for AM stereo. 1992: Japan adopts C-QUAM as its standard for AM stereo. 1993: The FCC makes C-QUAM the AM stereo standard for stations in the U.S., and also grants "stereo preference" for radio stations requesting to move to the AM expanded band (1610–1700 kHz), although such stations have never actually been required to transmit in stereo. 1993: The AMAX certification program begins. This was to set an official manufacturing standard for high-quality AM radio receivers, with a wider audio bandwidth for higher fidelity reception of strong signals, and optionally C-QUAM AM stereo. Despite the availability of AMAX receivers from companies like Sony, General Electric, Denon, and AMAX-certified car radios from the domestic and Japanese automakers, most electronics manufacturers did not wish to implement the more costly AMAX tuner design in their radios, so most AM radios today remain in mono with limited fidelity. 2006 to present: AM stereo gains new life through the support for C-QUAM decoding in most receivers designed for HD Radio. These new digital radios receive AM stereo signals, although the AM transmitters are now limited to 10 kHz audio bandwidth and the HD receivers flip left and right channels in decoding C-QUAM stereo.
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. As with FM stereo, all of the AM stereo designs used pilot tones (unheard parts of the broadcast signal) to alert the receiver electronics that a stereo signal was coming in and to switch the receiver into the proper decoding mode. 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.
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 changed 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.
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 some other parts of the world where it was declared the official standard such as Canada, Japan, and Australia.
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.
Kahn-Hazeltine 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 later used the Motorola C-QUAM system instead before reverting to mono.
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