A Compatible System of Stereo Transmission by FM Multiplex
A method of applying modulation to the channels of a stereo FM multiplex transmitter is described which provides a -compatible- reception for the monaural listener. By transmitting an additive combination of the microphones on the main channel of the FM transmitter and a subtractive combination on the sub-carrier channel, the main channel transmits a balanced program without the deterioration which would be brought about if the output of only one microphone were transmitted on the main channel. At the receiver the combinations are separated so that the microphones feed the individual loudspeakers directly. Additional advantages of the system include a 6 db signal-to-noise gain for the sub-carrier channel. This gain is highly desirable because the relatively poor signal-to-noise on the subcarrier channel curtails the distance range of the stereo transmission. In addition, the signal-to-noise ratio appearing in the receiver channel is made identical so that disturbances due to noise appear symmetrically in both loudspeakers. This results in less annoyance due to noise. A discussion of relative signal-to-noise ratios on the main channel and sub-carrier channels is given, together with recommendations for optimum performance of a stereo transmission.
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