RADIO AND TELEVISION, transmission and reception of audio and video information by means of electromagnetic waves. This article gives a brief overview of the state of broadcasting and reception.
The electromagnetic waves used for radio and television broadcasting are modulated by the signals of the programs being transmitted. In radio broadcasting these signals consist of sounds, and in television broadcasting they consist of sounds and images. Radio stations are usually divided into AM and FM by modulation method, but the type of program material transmitted does not depend on the modulation method.
Most broadcasts are intended for the general public in the specific service area of the country where the station is located; others broadcast across national borders. Stations intended for international broadcasting are usually located in border areas; they broadcast at high power levels or rebroadcast via satellite.
Some television programs are reserved for subscribers only. Such programs are transmitted over cable networks or microwave terrestrial lines, as well as using coding; such broadcasting is called targeted broadcasting.
The aforementioned broadcasting systems are one-way; they do not provide opportunities for the listener or viewer to make his or her opinion known. Most radio systems, on the other hand, are two-way, i.e., they are designed to exchange messages.
In accordance with international agreements, several frequency bands are allocated for terrestrial broadcasting.
The International Telecommunication Union (ITU) has divided the earth’s surface into three geographical regions (Region 1 – Europe and Africa; Region 2 – New World and Region 3 – Asia and Australia). Agreements within each region deal with the allocation and use of frequency bands. There are also bilateral and multilateral agreements between neighboring countries. Radio conferences are held periodically to discuss treaties and agreements. Among the tasks usually included in the radio conference program are improving spectrum allocation, allocating frequencies for the needs of new or improved technologies, and providing services. Technical criteria include signal levels, quality and coverage, and protection from excessive interference. Advance notice procedures facilitate the process of harmonious development of broadcasting in regions. Political, military, and commercial factors are usually important in negotiations because of existing competition for spectrum, markets, etc.
The geographic service area that is brought up for discussion is determined by the range of frequencies, services, and distances involved. For example, a satellite agreement requires the participation of all interested countries in the world, whereas for terrestrial FM or television broadcasting agreements, the participation of neighboring countries is sufficient. Each country regulates domestic broadcasting in accordance with its own laws and regulations.
TV BROADCASTING IN THE VHF AND UHF BANDS.
The channel spacing in these bands is 6 MHz. In North America, color television is modulated to NTSC standards. Most of the Region 1 countries use the PAL system. The CIS countries use SECAM and Japan uses NTSC. These three systems are incompatible; the main differences between them relate to the modulation processes used to encode and transmit color information. International agreements now provide for the use of standard conversion methods.
SIGNAL DISTRIBUTION
Mid-wave band signals are propagated by the terrestrial (surface) wave during the day and night, and also by the ionospheric (spatial) wave at night.
The currents produced by horizontally propagating radiation (surface wave) typically penetrate up to 15 m below the ground surface at 530 kHz and 1.5 m at 1,700 kHz. If the topsoil were a perfect conductor, then the attenuation of the signal regardless of its frequency was proportional to the distance it traveled. Seawater and fertile prairie land give a signal attenuation close to this, but in general the earth never behaves as a perfect conductor. Attenuation increases with frequency and is usually much greater at 1700 kHz than at 530 kHz.
Electrical conductivity is measured in mmo/m (mOhm-1hm-1) or milliSiemens/m (mSm/m). A conductivity of 1 mmo/m is considered low, 6 mmo/m is medium, and 40 mmo/m is high.