In 1961, the Federal Communications Commission (FCC) authorized radio stations to begin broadcasting stereophonic frequency modulation (FM) programs. The last technical difficulties had been solved, and with them went the final obstacle to clear, static-free radio reception in stereo. FM stereo (also known as FM “multiplex”) represented an engineering triumph: the marriage of the clean FM signal with the realism and fidelity that two channels of audio could provide.

However, deployment of the new technology by broadcasters proceeded very slowly. Though receiver manufacturers of the day rushed to put stereo-capable receivers on the market, the listening public was similarly slow to accept the new broadcasts. Until the middle 1970s, for example, automobiles still carried monaural FM receivers as standard equipment. As more broadcasting stations converted to stereo transmission, though, the public began to take notice, and soon the mono FM receiver was a thing of the past.

But how might it be possible to broadcast two channels of audio information successfully? This problem began to occupy broadcast engineers soon after stereophonic sound became popular with the public in the 1950s. Also, station owners, noting a decline in FM listening, regarded stereo broadcasting as a possible way to increase revenues, giving an impetus to the research.

Radio engineers tried first to combine an ordinary amplitude modulated (AM) receiver in the same box with an FM receiver. Unlike today’s systems, these experimental receivers were designed to receive both AM and FM signals simultaneously. At the broadcasting station, one channel of the stereo program was broadcast by the AM transmitter, and the other channel by the FM transmitter. This scheme was unsuccessful, since the AM program could never be of the same quality as the FM program, and the difference in audio quality was quite noticeable. Also, the FCC had mandated that FM stereo transmissions had to be compatible with existing single-channel (monaural) FM receivers, which the dual AM/FM system most definitely was not.

The problem was finally solved by multiplexing the two channels of information in one signal (hence the term multiplex used, as noted above, to refer to the stereo broadcast). A stereo FM signal, as broadcast by the transmitter, contains four components: a pilot frequency, a subcarrier frequency, and two channels of modulated audio:

  • A 19-kilohertz (kHz) pilot frequency for multiplex reference (this will be used to create a steady 38-kHz frequency in the receiver);
  • A 38-kHz subcarrier frequency to carry one of the audio channels;
  • The sum of the two audio channels, left (L) + right (R), in phase with the signal;
  • The difference of the two audio channels, L – R, out of phase with the signal.

The audio sum component, L + R, modulates the main frequency of the FM signal as broadcast (which would be in the typical 88 – 108 megahertz FM broadcast range), and the audio difference component, L – R, modulates the 38-kHz subcarrier.

At the receiver, the incoming signal is applied to a demodulator to recover the L + R audio signal, while the 19-kHz pilot frequency is doubled to 38-kHz and used to regenerate the subcarrier. This in turn is used to demodulate the L – R signal. At this point, the FM stereo receiver has all it needs to recover both channels of audio.

The L – R signal is applied to a phase inverter circuit to produce a – L + R signal, which is then fed into a summing circuit with the L + R signal, resulting in cancellation of the L components and producing the original right channel audio. The left channel audio signal is similarly recovered, by sending the L + R and L – R signals through another summing network. The two audio signals can then be sent to an audio amplifier, resulting in two channels of clear stereophonic sound. This technique has an added advantage in that a monoaural FM broadcast will be received it as if it were a stereo signal, but both channels will contain the same information.


RCA staff, RCA Receiving Tube Manual. Harrison, New Jersey: RCA Electronic Components, 1971.
Cole, H. Edgar, "A Brief History of FM," Radio Forum Newsletter.<> (December 2003).
Quest, Steve J, "Theory of AM, FM, and FM Stereo." <> (December 2003).

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