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 was developed by Harris Corporation, a major manufacturer of radio/TV transmitters. Harris is the successor to the pioneer Gates radio line. Harris was the early leader in the AM Stereo wars. It was implemented by a large number of stations in the 1980s, but the FCC temporarily rescinded their approval of the Harris system, causing most to switch to Motorola's C-QUAM system. This Harris system eventually changed their pilot tone to be compatible with C-QUAM. 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 upgraded 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 long time 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.
QUAM uses Quadarature Phase and Amplitude: the phase of the audio is rotated ahead or behind the carrier and the Amplitude of each phase is also changed; thus giving 16 points for reference (used also in dialup modems to get past the 9600 baud limit on analog lines). The QUAM signal (L-R info) is then phase modulated on the transmitter (the QUAM exciter replaced the crystal in the AM transmitter) and the L+R still modulated the transmitter as it had in the past. C-QUAM is a modified QUAM and thus called Compatible (C- in C-QUAM).
C-QUAM has 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 was developed by American engineer Leonard 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.
However, the Kahn system suffered from lower stereo separation above 6 kHz (reaching none at 10 kHz where as FM stereo has 40 db or more separation at 15 kHz) and the 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 (also known as 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.
Belar System
The Belar system was used in limited number of stations, such as WJR. The Belar system was a simple Phase Modulated/AM modulation system, with the L-R phase modulating the carrier (with a 400uS pre-emphasis) 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). 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 phase modulation with no QUAM information did not permit a high separation of L and R channels.