AM stereo arrives, but who will hear?

Within a year some radio listeners tuned to the AM band will be able to use both ears: Broadcasts will be in stereo.

With this shift to stereo, many of the nation's 4,650 amplitude modulation radio stations hope to spin a new tune heralding growth in listenership. For over a decade AM radio stations across the country have been losing listeners to FM, or frequency modulation, stations, which could offer stereo.

According to Chris Payne, a spokesman for the National Association of Broadcasters, AM stations have been losing 3 percent of the radio market a year for the last decade and currently FM has the dominant market share in the battle over the airwaves.

Notes Harry Jessel, associate editor of Broadcasting magazine, ''Many of the AM stations are switching to all news and information to try to keep listeners.'' This is the case with WABC in New York, which is abandoning its contemporary radio sound after 22 years and turning to news and information. Now , as a result of an order issued by the Federal Communications Commission (FCC) last week, AM stations can begin installing equipment to broadcast in stereo.

Many stations say they can't wait.

C.G. Perry, vice-president at Group W, part of the Westinghouse Corporation, says it has already outfitted three of its stations, WBZ in Boston, KDKA in Pittsburgh, and WOWO in Fort Wayne, Ind., with stereo broadcast equipment. Mr. Perry says the stations will begin broadcasting as soon as possible, even though consumers will be unable to pickup the AM stereo sounds right away. The broadcasts, he said, would have ''good promotional value'' and stimulate the market.

John Gehron, station manager of WLS, an American Broadcasting Companies station in Chicago, says the station will be broadcasting in stereo as soon as the FCC gives final approval, which is expected shortly. ''It will be great to be able to offer stereo AM to the consumer,'' he says.

Unfortunately for the broadcasters, their initial broadcasts will be a station manager's bad dream: Most consumers won't be able to pick up the stereo sounds right away. Manufacturers of receivers have yet to begin producing radios using the silicon chips that will permit AM radios to receive stereo sounds. This is largely because of the FCC has for years had difficulty in making a choice among competing stereo systems offered by about five manufacturers.

The FCC in its decision decided not to pick a single AM stereo broadcast system. Instead, it decided to allow the competing and different systems of the five companies to battle it out in the marketplace.

The industry, says Mr. Payne, would have preferred a single standard because the various systems proposed are not compatible. In the instances of FM stereo and color television the FCC picked one system for each medium, and this was subsequently licensed out to other manufacturers. Thus, with future AM stereo broadcasts a consumer may need a separate receiver to listen to different stations using different systems.

Many of the receiver manufacturing companies plan to sit on the sidelines waiting to see which system is ultimately used in the marketplace. A spokesman for Delco Electronics, which makes nearly 40 percent of all car radios, says the company is evaluating which direction to go. He says it will take the company at least a year to switch its radios to accept AM stereo - once it decides which system to gear them to.

This has prompted a major battle between the companies that manufacture the equipment. These companies are Magnavox, a subsidiary of North American Phillips Corporation; Motorola Inc.; Harris Corporation; Belar Electronics Laboratories; and Kahn Communications Inc. and Hazeltine Corporation, which have jointly developed a system. Within the past week most of the companies have issued press releases proclaiming their systems the best and announcing - presumably to their competitors - they would ''aggressively market'' them. This marketing has included listing the number of radio stations currently signed up to use the systems. Gene T. Whicker, vice-president and general manager of Harris Corporation, said his company has 150 contingent orders from AM stations. Kahn/Hazeltine announced it has signed up all-music stations that ''cover virtually all major population centers in the US.''

The manufacturers as well as the receiver producers and the FCC, will gather at the NAB's annual convention in Dallas in April. Interest in the AM stereo conference is expected to be so great that the NAB has hired a larger hall for the session.

The manufacturers expect stereo AM to spread quickly. Ed Onders, a patent lawyer for Kahn/Hazzeltine, estimates 50 percent of all stations will switch to AM stereo in the first year. Eventually, 75 percent will use AM stereo equipment. Harris Corporation figures stations will ultimately invest $100 million in equipment. One semiconductor manufacturer estimates it will add an extra $20 to the cost of a new stereo to include AM stereo.

Not everyone thinks stereo will mark a renaissance for AM radio. Bob Sherman, executive vice-president for NBC's radio division, says, ''Radio well done, especially in programming, will get the listener. Stereo will just wipe out a disadvantage.'' NBC has asked its parent company for the funds to convert three of its four stations to stereo.

One of the disadvantages the AM broadcasters have to surmount is the discrepency in quality between AM and FM radios. Henry Kavett, a spokesman for ABC's radio division, notes, ''With the bulk of listening on the FM side of the band, the FM section is excellent. The AM section is far inferior. Even a $40 FM radio sounds better than a typical AM radio.'' Part of this quality discrepancy has to do with the way AM stations are tightly packed together on the radio band.

Said technical expert Chris Payne of the NAB, ''The AM radio has always been the cheapest product available. It's possible to get AM sounding like FM to the typical consumer.'' But, he added, ''to the audiophile or hobbyist, there will always be a difference.''