AM stereo

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.

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.

This system, known as V-CPM for Variable Angle Compatible Phase Multiplex, was developed by Harris Corporation, a major manufacturer of radio/TV transmitters.

CKLW in Windsor, Ontario, Canada (also serving nearby Detroit, Michigan) was among the first stations to broadcast in Harris AM stereo.

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.

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.

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.

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.

In September 1980, the FCC rescinded its decision on Magnavox and started all over again, putting two senior technical consultants to work on the problem full-time for five months.

Most governments approved a single standard, usually Motorola's C-QUAM, which greatly reduced confusion and increased user adoption.

[citation needed] 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.

In the United States, most stations currently using AM stereo are small, independently owned and broadcast a variety of music format.

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.