Stereophonic AM broadcasting in the USA uses the C-Quam system of encoding the stereo signal. This system has been the official standard since 1993.
A very brief chronology of the events leading to the adoption of the AM stereo standard in the USA follows.
1958-62. AM stereo was first proposed to the Commission in 1958 and 1959 separately by Philco Corporation, Radio Corporation of America, and Kahn Research Laboratories, Inc. On September 27, 1961 [21 RR 1616c], the Commission denied these requests for a rulemaking proceeding on the subject, stating:
Contrary to the circumstances that motivated the rule amendments authorizing and standardizing stereophonic transmissions by FM broadcasting stations, there is little evidence of public need or industry desire for similar rule changes with respect to stereophonic transmissions by standard broadcast stations. Additionally, the pattern of operation of the nearly 4,000 stations now licensed, the needs and purposes served and the very nature of the service itself are such that the beneficial effects of innovations of this nature are clearly de minimis. . . . Preliminary consideration . . . indicates that providing a dual channel transmission system with the requisite separation and without deleterious effects in an AM system such as standard broadcasting, presents an appreciably greater problem than in the case of FM.
The FCC denied an appeal of this decision on January 12, 1962 [21 RR 1616e].
1980's. In 1979-1980, the AM stereo issue arose again. The Commission originally sought to choose a single AM stereo system as the standard for the AM broadcast band. On July 31, 1980, the Commission issued a Further Notice of Proposed Rulemaking in Docket 21313 seeking more information about the five competing systems (Harris, Magnavox, Motorola, Belar, and Kahn and Hazeltine). Kahn and Hazeltine filed appeals with the Commission advocating a "marketplace approach" in which the various systems would compete for broadcasters to use their systems. Subsequently, the Commission agreed to allow the marketplace to decide whether a single AM stereo system would prevail as the standard, or whether different systems could coexist (e.g., receivers could have a multisystem decoding chip to decode the various systems). See the Report and Order, MM Docket 21313, 51 RR 2d 1, 47 FR 13152, released March 18, 1982, and 51 RR 2d 1285. As the Commission indicated, this procedure was a departure from its traditional procedures and the new approach was a "bold, new step for the Commission to take."
What followed was a series of protracted lawsuits, primarily between Kahn and Motorola as well as other parties. The competing systems faded out. As neither broadcasters nor receiver manufacturers wanted to invest in what could be a losing system, effective implementation of AM stereo in the USA was delayed.
1992-94. On October 27, 1992, the President of the United States signed the Telecommunications Authorization Act. A provision of that Act required the FCC to adopt a single stereophonic standard for AM stations within one year of the signing date. The Commission initiated a rulemaking proceeding to accomplish this result. In the Notice of Proposed Rulemaking in ET Docket 92-298, 8 FCC Rcd 688 (1993), the Commission noted that of the stations then broadcasting in AM stereo, 591 used the Motorola C-Quam system, 37 used the Harris system, and less than 20 used the Kahn system. The NPRM indicated that 26 manufacturers incorporated the Motorola C- Quam system in at least one model of radio; none used Kahn's system. The NPRM proposed to adopt the C-Quam system as the standard, noting that 24 million receivers had already been sold using that system. The Commission officially adopted that system in the Report and Order in ET Docket 92-298, 8 FCC Rcd 8216 (1993). See also the Supplemental Order to ET Docket 92-298, 9 FCC Rcd 1907 (1994).