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
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
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.
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.