Multichannel Television Reception
in Latin America
Broadcast television signals are electromagnetic waves that are sent from a transmitter to one or more television receivers. There is a television standard which is followed by the different manufacturers of television sets and which permits different users to view these signals in the same way. The television standard is defined in terms of certain ranges of the electromagnetic frequency spectrum.
First of all, there is the VHF (Very High Frequency) band of frequencies. There are twelve VHF channels, which are numbered 2 through 13 respectively, and they occupy the frequency range between 54 MHz and 216 MHz (where MHZ means mega-Hertz, or million of cycles per second). Each channel is 6 MHz wide, with 4 MHz used for the transmission of video images and 2 MHz for audio and other purposes (such as program and commercial identification).
With the exception of channels 4 and 5, and channels 6 and 7, the VHF channels are adjacent to each other. Adjacent channels interfere with each other through signal leakage, and so adjacent channels are not used in the same city. Channels 4 and 6, and channels 6 and 7 can be assigned simultaneously, but not both 5 and 6. This means that the maximum number of VHF is 7 in any city.
On our Broadcast Television page, for example, one will find that the VHF broadcast spectrum is Mexico City is occupied by channels 2, 4, 5, 7, 9, 11, and 13. In the case of Buenos Aires (Argentina) and Montevideo (Uruguay), which are sufficiently close to each other as to create interference, we find channels 2, 7, 9, 11 and 13 in Buenos Aires and channels, 4, 5, 10 and 12 in Montevideo. The higher-numbered channels are less powerful and can be assigned simultaneously in this situation.
To increase the number of television stations that can be received, the UHF (Ultra High Frequency) band was introduced. There are 56 UHF channels, which are numbered 14 through 70, in the range of 470 to 806 MHz. The channels are 6 MHz wide with no unused gaps in between. Due to interference, adjacent channels are not assigned simultaneously in the same market. In theory, that leaves as many as 28 UHF television stations per market. In practice, there are seldom more than three of four UHF stations in even the largest cities. At the same transmission power, the high-frequency UHF stations reaches a much smaller area than the low-frequency VHF stations and they are therefore less economical to operate. Furthermore, not all television sets are built to receive UHF frequencies.
If the goal is to provide more television channels, then it is more efficient to use what are known as the multi-channel television modes. The specific choice depends on the economics and operating environment. This is a rapidly growing industry in Latin America. According to the Los Medios y Mercados de Latinoamérica Study, there were 17,932,000 multichannel households in 1998, up from 13,423, 000 in 1994. You can click on these links to find out more about the different types of multichannel television services.
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(posted by Roland Soong on 11/14/99)
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