Wired Broadcasting

a system of sound broadcasting in which, unlike radio broadcasting, the sound (speech and music) is transmitted to a large number of listeners (subscribers) by means of electrical oscillations over a wired network—either an independent broadcasting network or a telephone network. There are both single-program and multiple-program types of wired broadcasting.

Single-program wired broadcasting was first developed in the USSR. The initial system, with a power of 40 watts, was set up in Moscow in 1925; it accommodated 50 loudspeakers located along the streets. By the beginning of the Great Patriotic War (1941–45), there were more than 11,000 subsystems in the independent broadcasting network, serving approximately 6 million subscriber loudspeakers; on Jan. 1, 1974, there were more than 20,000 subsystems and approximately 55 million subscriber loudspeakers.

Single-program wired broadcasting is conducted at audio frequencies. Figure 1 depicts a block diagram for a single-program wired broadcasting subsystem that covers a major city. The program for the broadcast is fed to a central amplifier station from a local radio studio or through a station of the longdistance telephone communications service from a radio studio of the oblast or republic or from a studio in Moscow. The operating voltage varies in the various portions of the distribution network. It is usually 960 volts in the main feeder lines, 240 volts in the distribution feeder lines, and 30 volts in the subscribers’ lines. All amplifier stations and transformer substations of the major wired broadcasting subsystems are automatic and are remotely controlled from the central amplifier station. In smaller cities and urban areas, a wired broadcasting subsystem may consist of only three, two, or even one section (see Figure 1). High operating reliability in the subsystem is ensured by having backup amplifiers, by supplying transformer substations over two feeders from different support amplifier stations, and by providing a system for detection of failures in the network.

Figure 1. Block diagram of a district wired broadcasting subsystem in a major city: (CAS) central amplifier station, (SAS) support amplifier station (these are situated in various districts In the city and are equipped with powerful audio-frequency amplifiers for supplying the distribution network), (TS) step-down transformer substation, (ST) step-down subscriber transformer, (SS) subscriber loudspeaker, (CL) connecting line, (MF) main feeder line, (DF) distribution feeder line, (SL) subscriber line

Figure 2 shows a diagram of a rural wired broadcasting subsystem. The broadcast program is fed to an amplifier station in a district center via a long-distance broadcast channel, or it is picked up by a radio receiver at the station. In addition to a distribution network of feeder lines for the district center, the station is connected to long (up to 40 km) feeder lines leading to neighboring populated areas. The reliability of these lines and the quality of the sound in the programs being transmitted is somewhat lower. Consequently, in those locations where radio reception is reliable at ultrashort waves, automated rural wired broadcasting subsystems are often installed. The stations of such subsystems receive broadcast programs from the nearest radio transmitting station, and the subsystem is switched on and off automatically by a coded command transmitted from this same radio station.

In cities, the distribution networks for wired broadcasting usually use overhead lines of steel or bimetallic (steel-copper or steel-aluminum) wires. In rural areas, networks use both overhead lines and underground cables, chiefly with aluminum current-conducting cores.

Figure 2. Block diagram of a district (rural) wired broadcasting subsystem: (ASD) amplifier station for the district subsystem, (DF) distribution feeder line, (FT) step-down feeder transformer, (ST) step-down subscriber transformer, (SS) subscriber loudspeaker

During the 1930’s, a number of countries, including Germany, Austria, Italy, and Switzerland, developed systems for multiple-program wired broadcasting over the telephone networks. In the USSR a three-program system of wired broadcasting was started in cities in 1962. In this system, two additional programs are transmitted over an existing independent wired network using amplitude-modulated carrier frequencies of 78 and 120 kilohertz. The corresponding transmitters are installed at the support amplifier stations of a single-program network (Figure 3). All three programs are transmitted simultaneously. The sound is reproduced by means of a special three-program loudspeaker consisting of a set of electric filters, a detector, an amplifier, and the loudspeaker itself. More than 14 million subscribers can now receive three-program wired broadcasts.

Figure 3. Block diagram of a three-program wired broadcasting subsystem: (T₁) and (T₂) transmitters, (ROS) rack-mounted output switch with a step-up transformer, (CET) connecting equipment for transmitters, (TSBD) transformer substation bypass device for passing on signals of additional programs, (TL) three-program loudspeaker, (CL₁), (CL₂), and (CL₃) connecting lines, (SAS) support amplifier station, (MF) main feeder line, (TS) step-down transformer substation, (DF) distribution feeder line, (ST) step-down subscriber transformer, (SL) subscriber line

The principal qualitative factors in wired broadcasting are as follows: the frequency band reproduced is 50 to 10,000 hertz in cities and 100 to 6,000 hertz in rural areas with a variation of less than 6 decibels in frequency response, harmonic distortion factor is less than 6 percent at the lower frequency for a nominal line voltage, and the signal-to-noise ratio is not less than 50 to 55 decibels.

The principal merits of wired broadcasting are its excellent economy and reliability, the high quality of the transmission, and the ease in handling a subscriber loudspeaker. Possible development prospects include the further extension of three-program service in cities and the introduction of such service in rural areas, as well as the creation of joint cable networks for television and sound broadcasting.