Stereophonic Radio Broadcasting

radio transmission, mostly of music, in which the listener can not only distinguish the pitch, timbre, and volume of the sound but can also form some idea of the spatial arrangement of the sound sources. Transmissions of this type provide auditory perspective; that is, they create a natural space effect. The broadcasts are carried out in the ultrashort-wave range, which provides sufficiently high fidelity of reproduction and has a low noise level. In two-channel stereophonic transmissions, which are the most common, the carrier-frequency oscillations are frequency-modulated in the transmitter using a complex stereophonic signal. This signal is formed by modulating oscillations with an auxiliary (subcarrier) frequency using two signals whose spectra lie in the audio-frequency range. These signals—a right signal A and a left signal B—are obtained from two groups of spatially and electrically separated microphones having different orientations with respect to the sound sources.

The different systems for stereophonic broadcasting are distinguished by their methods of generating the complex stereophonic signal. A system with polar modulation is used in the USSR. Here, for the formation of the complex signal the positive half-waves of oscillations with a subcarrier frequency of 31.25 kilo-hertz (kHz) are amplitude-modulated using signal A, and the negative half-waves, using signal B. After modulation, the sub-carrier itself is suppressed by 14 decibels (dB). In many countries, a system with a pilot frequency is used. This system utilizes the amplitude modulation of oscillations with a subcarrier frequency of 38 kHz using the difference signal A - B. The complex stereophonic signal is formed by adding the modulated oscillations (having the subcarrier frequency), the sum signal A + B, and the pilot frequency of 19 kHz. The pilot frequency is necessary for the restoration of the subcarrier in the receiver. Among the other methods of stereophonic radio broadcasting are those involving the frequency modulation of oscillations with the sub-carrier frequency.

Receivers for stereophonic radio broadcasting differ from nonstereo receivers in that they have an additional component—a decoder—which extracts signals A and B from the complex signal. Stereophonic receivers also contain, correspondingly, two audio-frequency channels. Stereophonic transmissions must provide normal reception for nonstereo receivers, even though the sense of spatial realism and breadth will be absent.

Prospects for the future in stereophonic broadcasting include the use of quadraphonic (four-channel) systems.