AM Stereo

History -

AM stereo has been around for a long time. In August 1924, WPAJ (now WDRC, Hartford) transmitted in stereo from New Haven, CT. using 2 transmitters. One transmitter was on 1120 KHz and the other on 1320 KHz. AM stations were allowed to freely experiment with AM stereo until the FCC decided that one system should be used. Based on their tests, they picked a system developed by Magnavox. The other manufactures of AM stereo equipment protested and on March 4, 1982, the FCC approved their systems for use as well, leaving it up to the marketplace to decide who would survive. After 10 years of confusion for the consumer, Motorola finally won the battle....but at this point not many people cared. AM radio no longer had the programming to support a stereo format. Many AM stations have gone to "talk radio" while most of the music-oriented programming is on FM.

How it works -

In order to maintain compatibility with existing mono AM radios, the left plus right information must be transmitted by conventional amplitude modulation. It's how the left minus right stereo information is handled that differs between the various systems.

The Magnavox system -

This is the original system selected by the FCC. It uses a very simple method for transmission, which is why it was probably chosen at first. The left minus right stereo information is transmitted by low level phase modulation of the carrier along with the conventional left plus right mono amplitude modulation. In the receiver, an envelope detector recovers the AM while a PM detector demodulates the left minus right stereo information. The PM detector is preceded by several excellent stages of limiting to remove all the AM components. Gain tracking is used between the AM and PM detectors to keep the signals in proper proportion to each other. The outputs of the detectors are fed through an audio matrix, recovering left/right stereo. A 5 Hz pilot tone, transmitted along with the left minus right information, is used to identify a station transmitting in stereo.

The Kahn system -

Kahn Communications of Freeport, Long Island was one of the early AM stereo pioneers. Kahn also used left minus right phase modulation in his system but by adding the proper phase shifts to the audio it is possible to generate an independent sideband signal with right on the upper sideband and left on the lower. The left plus right signal is phase shifted -45 degrees while the left minus right is shifted +45 degrees (a total of 90 degrees). The AM and PM sidebands interact producing an independent sideband signal much like a phasing type sideband rig. Two receivers off tuned to either side of the signal will produce stereo with some degree of separation. A radio tuned to the center of the signal will hear mono. Detecting the AM and PM signals, undoing the phase shifts and feeding the signals through an audio matrix is another way to recover the stereo audio.

The Motorola system (CQUAM) -

The Motorola Compatible Quadrature AM system (CQUAM) is the system in use today for AM stereo. This system transmits stereo by using 2 phases of the RF carrier 90 degrees apart. Each phase of the carrier is fed to a balanced modulator. The balanced modulator that is in phase with the original RF signal receives left plus right mono audio. The balanced modulator that is 90 degrees out of phase receives the left minus right stereo information. The balanced modulator outputs are summed together with the original in phase carrier and then passed through a limiter so only the phase information is retained. This signal is then modulated by the conventional means in the transmitter, producing a quadrature AM signal that is compatible with mono AM radios. Stereo identification is provided by a 25 Hz pilot tone transmitted in with the left minus right information. This system may be decoded using a chip like the Motorola MC13020P.

Other systems -

Belar, a manufacturer of broadcast modulation monitors, had a system that was almost identical to the Magnavox system. It used FM instead of PM and had no stereo pilot tone. This system was an early dropout in the marketplace race.

The system developed by Harris Corporation was another quadrature AM system. It was the most complicated of all the systems and had the additional problem of producing up to 4.3% distortion, during full stereo modulation, in conventional mono receivers. Harris later joined Motorola in the production of CQUAM equipment, offering retrofit kits for their exciters and modulation monitors.

WA2FNQ in AM Stereo -

Back when all the systems were battling it out, I decided to try some form of AM stereo at WA2FNQ. A Magnavox type system seemed the easiest to do because all it required was the addition of an FM or PM component of the proper characteristics. FM modulating the Heathkit HG-10B in the transmitter was simple. Graphic equalizers were used to help match the response of the left plus right mono and left minus right stereo information. Any error in response or phase (such as that caused by a poor modulation transformer) would result in the loss of stereo separation.

In late 1982, one of the biggest problems was finding something to listen to AM stereo on. I was able to obtain 2 samples of the LM1981 stereo decoder chip made by National Semiconductor. The LM1981 was designed to decode Magnavox AM stereo signals but with some minor circuit changes and adjustments could decode CQUAM signals as well. With my homebrew LM1981 decoder installed at the I.F. output of an old AM tuner, I began listening around the broadcast band. There were no Magnavox or CQUAM signals to listen to. WNBC in New York was broadcasting in Kahn AM stereo. Due to the phase-shifted left minus right information, they had an interesting stereo effect but no true left or right channels. Putting a Gonset converter ahead of the receiver I was able to listen to my own transmitter with reasonable stereo separation.

There have been some AM stereo receivers on the market but very few are available today. Most are high-end receivers or car radios. At one time Sony made a portable radio, the model SRF A-100. Manufactured during the marketplace race, it can receive any of the AM stereo formats as well as FM. This is a good radio to have, if you can find one.

The main station receiver at WA2FNQ is a Gonset Super 6 converter ahead of a Radio Shack TM-152 AM stereo tuner. This tuner, built around the Motorola MC13020P, was designed to only receive CQUAM AM stereo signals. By placing the Sony SRF A-100 in the vicinity of the Gonset converter and TM-152, enough signal can be coupled so I can be use it to receive other modes as well.

Conclusion -

Due to the marketplace confusion created primarily by the lack of one standard being set in a timely manner, AM stereo never had a chance to reach its full potential. The Motorola system has become the standard for AM stereo in this country as well as many others. Although there are stations transmitting in stereo, inexpensive receivers are not commonplace. There is also a lack of programming needed to support the format. However, experimentation will continue at WA2FNQ.

See a block diagram of the system at WA2FNQ

AM Stereo pictures

Block diagrams of the different systems

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