AM stereo
AM Stereo is a term given to a series of mutually incompatible techniques for 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 Kahn; and quadrature amplitude modulation (QAM) multiplexing systems (conceptually closer to FM 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. This system was not very practical, as it required the listener to use two separate radios. 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
- 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. 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 drops out of the AM stereo race, 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 requires Expanded Band AM (1610-1700 kHz) stations to broadcast in AM stereo (although that rule was never actively enforced).
- 1993 - The AMAX certification program began. 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 with limited fidelity due to the design of the HD Radio chipset.