RCA's AM Stereo System, Circa 1959

Photo of WNBC's RCA-50B transmitter
Photo of RCA/WNBC AM Stereo Exciter RCA developed an early AM Stereo system, and field tested it in 1959 on WNBC's 50 kW RCA-50B transmitter in New York. WNBC's RCA-50B transmitter, whose design dates back to 1930, is pictured in the 1940 photo shown above, when the station call letters were WEAF. The RCA AM stereo system transmits the L+R stereo sum signal as normal AM, and transmits the L-R stereo difference signal as FM on the main AM carrier. A deviation of about 1,000 Hz is used for the FM, and the L-R audio signal is pre-emphasized using a "modified" 100 usec. time constant.

To the left is a photo of the equipment rack holding the RCA AM Stereo equipment installed at the WNBC transmitter, to convert it for AM Stereo operation. Of special interest are the "Matrix", "FM Exciter", and "Monitor", which are explained below.

The block diagram below gives an overview of the operation of RCA's AM Stereo system. The Left and Right audio signals from the studio first pass through a "matrix" which changes them to a L+R stereo sum signal, and a L-R stereo difference signal. The L+R stereo sum signal is feed to the normal AM modulation input of the transmitter, through the customary limiting amplifier. Using the sum of the Left and Right audio signals to Amplitude Modulate the transmitter insures that a listener with a monophonic radio will receive a balanced, distortion free program. The "matrix" also includes a time delay circuit in the L+R signal path to compensate for the additional time delay the "FM exciter". The L-R signal is feed to the "FM exciter" through another limiting amplifier, whose gain control is slaved to the L+R limiter. The "FM exciter" uses a serrasoid modulator operating at a frequency of 110 kHz, which is one sixth of WNBC's final operating frequency of 660 kHz. After FM modulation, the 110 kHz signal passes through frequency multiplier stages which multiply its frequency by a factor of six, to the 660 kHz carrier frequency signal that drives the RCA-50B transmitter, in place of the original crystal oscillator unit. The "Monitor" is a high quality receiver/demodulator that is used to measure, and monitor, the performance of the AM Stereo transmitter.

To learn more about the legendary RCA 50B Transmitter,Click Here
Block Diagram of RCA AM Stereo System

Photo of RCA AM Stereo Receiver

This is a photo of a prototype AM Stereo Receiver built by RCA, to receive the 1959 AM Stereo Broadcasts on WNBC. This prototype AM Stereo Receiver is based on the chassis of an RCA Model XF-2 radio receiver. To view the schematic of the RCA AM Stereo Receiver, Click Here

I would be interested in hearing from anyone that has any additional information on the RCA AM Stereo system, and the field tests at WNBC in 1959. I can be reached by email at byrnsj@sbcglobal.net.


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