Pilot-tone modulation is a special type of amplitude modulation where the upper and lower envelopes of the modulated waveform differ and change according to the transmitted signals. This modulation method is adopted by the Western VHF broadcasting standard and is used for stereophonic broadcasting in the USA and Western Europe.
In the frequency band VHF CCIR, pilot-tone modulation is applied for stereo broadcasting. This technology shares many features with polar modulation.
In this type of modulation, a complex signal called a composite stereo signal (CSS) is used. Its low-frequency portion (30–15000 Hz) contains the half-sum of the left and right channel signals. To ensure compatibility with monophonic receivers, the frequency range of 30–15000 Hz also carries the sum signal (L+R), while for stereo reception the frequency range of 23–53 kHz contains the difference signal (L-R), obtained through amplitude balanced modulation of an auxiliary subcarrier at 38 kHz.
In balanced modulation, to reduce the transmitter's emitted power, the carrier signal is completely suppressed, leaving only the two sidebands of the modulated radio signal at the modulator output. This significantly improves transmitter power efficiency since the carrier does not carry information. However, balanced modulation creates another issue: to recover the original AM spectrum at the receiver, an additional pilot signal at half the subcarrier frequency (19 kHz) must be transmitted. In the stereo decoder, the 38 kHz carrier is restored by doubling the pilot frequency.
An essential component of a stereo decoder in a pilot-tone system is a notch filter for the 19 kHz pilot tone (MPX filter). This is required because when recording stereo programs from a receiver to cassette decks equipped with Dolby noise reduction, an unfiltered pilot tone will be interpreted by the noise reduction circuit as a useful audio signal, inevitably causing improper operation of the noise reduction system. The problem is so significant that, despite the presence of such a filter at the receiver's output, nearly all cassette decks with Dolby noise reduction still include their own MPX filter.
Related articles: FM, VHF CCIR, Digital TV Channel Frequencies in Moscow, TV Channel Frequencies with Subcarriers, FM Radio Station Frequencies in Moscow.