AM stereo
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AM stereo is any of a number of mutually incompatible techniques for broadcasting two-channel audio in the mediumwave band in a manner that is compatible with receivers designed for standard amplitude modulation. 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 broadcasting the left and right audio channels. This was not a very practical option, requiring a listener to have two radios tuned to one station. Synchronization was problematic, often resulting in a sort of ping-pong effect between the two channels. Reception was also likely to be different between the two stations—never mind that not many listeners had matched receivers of the same model or brand.
In the early 1980s five competing AM systems were available:
The Belar system although available was the least popular system, and may not have been used at all on any broadcast stations other than testing (perhaps on WJR). The Harris, Magnavox, and Motorola C-Quam systems were fundamentally based on the same principle which phase modulates the primary carrier along with amplitude modulating it. The stereo information is placed on the phase modulated part of the carrier while the standard mono program was amplitude modulated as it normally would be in mono. Each of these systems would do it in slightly different ways. The Harris system utilized synchronous detection, and their initial system had a movable pilot signal, later changed to match the Motorola C-Quam pilot. The Kahn-Hazeltine system worked on an entirely different principle which independently modulated the upper and lower sidebands separately to differentiate the difference in program material. If an AM station were broadcasting AM stereo using the Kahn system, stereophonic sound could be partially enjoyed by tuning two standard radios slightly to either side of the station, one to the right side of best tune and one to the left of best tune. The best way to experience the most stereo separation however was by using an AM Stereo tuner with appropriate circuitry for decoding the Kahn system. The Harris, Magnavox, and Motorola C-Quam systems could only be decoded with an appropriate receiver. In the early 80's some clever electronics manufacturers, such as Sony, developed AM radios capable of receiving all four AM stereo systems. In the end, the two main systems that ended up battling it out o the death were the Kahn system and the Motorola C-Quam system. Eventually the C-Quam system became the FCC Standard for AM Stereo broadcasting. The Kahn system is still approved and in use today; however, not for stereophonic broadcasting though. Experimentation by Leonard Kahn revealed that by using the Kahn system to put more high end on one sideband and more low end on the other sideband you can improve the perceived coverage in the protected nulls of some directional AM signals. In some cases Kahn Powerside worked well, and in other cases it made no noticeable difference. It is interesting to note however that the Kahn system didn't disappear, but rather evolved into serving a different purpose. The C-Quam AM Stereo system is also still in use to this day, but on a shrinking number of stations. As many AM stations have switched their program formats from music to talk, the need for stereophonic sound has given way to the need for better signal to noise ratio which is more easily attainable in mono. The future of AM Stereo is up for debate. The Suits of Corporate radio would like for AM Stereo to be present in a new Hybrid Digital system called HD Radio but also known generically as "In Band On Channel" IBOC. What IBOC does is insert a digital signal In Band On Channel along with the analog audio. A Special HD Radio receiver will be needed to hear the digital audio. The method is patented, but it basically works as a quasi-cross between the Kahn System and the C-Quam system, the difference being instead of modulating the sidebands and phase modulating the primary carrier with the analog stereo decoding information, it is modulating these sidebands with digital information. Recently, engineer Leonard Kahn has also devised a new analog/digital AM stereo system known as "CAM-D" (see wrathofkahn.com for details).
- Meduci (AMAX)
AMAX was a standard adopted by receiver manufacturers and broadcasters. There was a certain level of technical requirements for broadcast equipment manufacturers and AM Stereo receiver manufacturers to hold up to in order to attain AMAX certification. It was sort of a stamp of approval indicating that a consumer was buying a quality piece of AM Stereo receiving equipment. The AMAX program was developed to try and resurrect AM Stereo in the mid 90's once the FCC finally adopted C-Quam as the standard system for AM Stereo. Some auto manufacturers built quality AMAX AM stereo radios, and a Sony portable meeting AMAX AM stereo was available as well. General Electric offers a mono AM radio, the GE SuperRadio III, which easily meets the bandwidth requirements of AMAX, but does not offer stereo AM audio.
AM stereo timeline highlights:
- 1960 - AM Stereo first demonstrated on XETRA-AM, Tijuana, MX, using the Kahn ISB system.
- 1963 - WHAZ runs a stereo program on eight AM stations, four on each channel.
- 1980-ish - The FCC begins AM Stereo testing with five systems. Initially they select the Magnavox system as the official AM stereo standard. Their research is immediately accused of being flawed and incomplete. The FCC revokes the Magnavox certification as the AM stereo standard.
- 1982 - After another series of lawsuits and accusations, the FCC decides that the marketplace should decide. The marketplace continues fighting it out, and this becomes a death blow to the possibility of AM stereo becoming commonplace across the board.
- 1985 - AM stereo officially begins in Australia, with the C-QUAM standard. [1]
- 1993 - FCC makes Motorola's C-QUAM the AM stereo standard, and also requires extended band stations to broadcast in stereo. Canada also endorses the C-QUAM system.
A famous European transmitter using AM stereo is Villebon sur Yvette on 864 kHz, which transmits France Bleu.
AM stereo is popular in Japan because of the limited number of FM stations in that country, and in Australia possibly because AM stations are more suited to covering large sparsely populated regions than FM stations (and the fact that unlike the American FCC, Australia was quick to pick C-QUAM as its standard). After some short lived experiments, the Independent Broadcasting Authority decided not to adopt AM stereo in Britain. Canada still has some quality AM stereo music stations on the air, and AM stereo receivers were more readily-available in Canada and Australia than the USA for some reason.
Some enthusiasts claim that AM stereo gives better stereo separation than FM stereo, but under skywave reception conditions, some AM stereo systems suffer from "platform motion" on first-generation AM stereo chipsets, but this was corrected by the third-generation chipset.
