Polar modulation (PM) is a special type of amplitude modulation, in which the upper and lower envelopes of the modulated oscillation differ and vary according to the transmitted signals. This modulation is used as the primary method for dual-program or stereophonic broadcasting.
In the frequency range of VHF OIRT, stereo broadcasting was implemented using polar modulation developed in the USSR (author: L. M. Kononovich, regular broadcasting since 1963). In this type of modulation, a complex signal called the composite stereo signal (CSS) is used. Its low-frequency part (30–15000 Hz) contains the half-sum of the left and right channel signals. The half-difference of the left and right channel signals, required for decoding the stereo signal, is transmitted at an ultrasonic (suprasonic, inaudible) frequency of 31.25 kHz. This value is numerically equal to the frequency of the second harmonic of the horizontal sweep frequency of televisions (15,625 Hz), since it was intended to later use this system for stereo sound transmission in television. 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 — by the right channel signal. The resulting signal is called a polar-modulated oscillation (PMO).
Because the amplitude of the original oscillation of the subcarrier frequency exceeds the sum of the amplitudes of all other components in the spectrum of the composite stereo signal, the main part of the transmitter's frequency deviation is spent on its transmission. This requires reducing the deviation from the low-frequency part of the spectrum, which results in lower volume of stereo 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.
A disadvantage of this method is the incomplete separation of stereo channels, which is why most commercial stereo decoders used a circuit with sum-and-difference signal conversion in a resistive matrix.
At present, although broadcasting in the VHF OIRT range continues in Russia and the CIS countries, stereo broadcasting using the polar modulation system has almost ceased, as receivers capable of receiving such stereo programs have not been manufactured for many years.
Related articles: FM, Frequencies of digital television channels in Moscow Frequencies of television channels with subcarriers Frequencies of FM radio stations in Moscow.