Stereo systems?

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).