I drew up a AM Stereo time-line [LINK] years ago but never really dug into the subject. It's time to revisit the war between Motorola and Kahn. First I should admit that there were more than two players. In D.C. we have two major parties, but there are a number of smaller groups that bear mentioning. It was the same with AM stereo. The top five were as follows:
- Motorola (C-QuAM)
- Kahn-Hazeltine (Independent Sideband AM aka "ISB")
- Magnavox (PMX)
- Harris Broadcast (V-CPM)
- Belar System
Motorola's C-QUAM was invented in 1977 and was used widely in both the US and Canada and showed early signs of becoming a standard. But fast forward to 2014 and you can count on your fingers the number of American stations still using it: WLS-AM, WNMB-AM, WBLQ-AM, WIRY-AM, WAXB-AM and WLAD-AM. It's patents have expired and it is generally accepted as the international standard for AM Radio broadcasting despite being incompatible with IBOC. Despite those poor North American numbers... if there was a winner it was Motorola. The FCC finally adopted it in 1994.
Leonard R. Kahn's system didn't go quote as far. But he was a true believer. He was an AM radio evangelist. He once actually said "There is no limitation to the fidelity of AM radio. From a mathematical standpoint, AM does better in frequency response than FM." This is complete malarkey of course. Because an AM radio station has a maximum bandwidth of 20 kHz, it is patently and inherently inferior to an FM station with 200 kHz of bandwith. On that comment he was wrong by a full exponential order. More here and here. But Kahn had more than 100 patents and his AM stereo patent was granted in 1958.He tested the technology on WQXR-AM in new York in 1956.
US4172966 cites Kahn... twice Always the spoiler, his own ISB system was a compromise between double sideband (DSB) and single sideband (SSB) [ I know I am ignoring vestigial sideband (VSB) systems. Some of the same ideas re-appeared when he debuted CAM-D, his Compatible Amplitude Modulation-Digital system to compete with IBOC.] Still, Kahn did everything in his power to fight C-QUAM.
He criticized the system for it's "platform motion" effects. While later revisions diminished the problem,it's stereo audio was susceptible to skywave interference.The result was a moving stereo balance with the audio "center" moving left and right randomly. It was true and even in the FCC order that declared it the standard they admitted that the problem was never completely resolved. But he also made a legal attempt to block Motorola from using its C-QUAM in the US entirely. In 1986 he filed a complaint claimed that C-QUAM violated FCC emission bandwidth specifications: specifically 73.44. He was right, but he was nit-picking. In the end C-QUAM won because they had the most radios on the market, and the majority of stereo AM broadcasters were using their system. The FCC adoption specifically mentioned other countries using the system. C-QUAM did not win because if technical superiority. It was a lesson that Ibiquity took to heart a decade later.