In the frequency range of VHF OIRT, for the organization of stereo radio broadcasting, a polar modulation system developed in the USSR was used. In this type of modulation, a signal called a composite stereo signal (CSS) is used, whose low-frequency part (30–15000 Hz) contains the half-sum of the signals from the left and right channels. This allows a listener with a receiver without a stereo decoder to receive a stereophonic transmission as monophonic.
The half-difference of the signals from the left and right channels, required for decoding the stereo signal, is transmitted at an ultrasonic (suprasonic) frequency of 31.25 kHz, which is not perceived by the human ear. In this case, the envelope of the positive half-cycles of the subcarrier frequency is modulated by the left channel signal, and the envelope of the negative half-cycles is modulated by the right channel signal.
Since the amplitude of the initial oscillation of the subcarrier frequency exceeds the sum of the amplitudes of all other components of the composite stereo signal spectrum, most of the transmitter's frequency deviation will be spent on its transmission. This requires reducing the deviation from the low-frequency part of the spectrum, which results in a decrease in the volume of the stereophonic transmission on a conventional receiver. To address this, the subcarrier frequency is suppressed by 14 dB. In the stereo decoder, the subcarrier is restored, the difference signal is detected, and the left and right channel signals are separated.
Related articles: Frequency Modulation (FM), Frequencies of Digital Television Channels in Moscow Frequencies of Television Channels with Subcarriers Frequencies of FM Radio Stations in Moscow