Film, Radio and TV - 29 |
|
Cable and SatelliteTV Services
.
The Beginning of U.S. Cable Television An appliance dealer living in Lansford, some 80 miles away, was having trouble selling TV sets because of poor reception in his area. He reasoned that he could sell more TV sets if he put up a tall antenna in his community to pick up the TV stations from Philadelphia. (Recall from Module 17 that TV signals are line-of-sight and are blocked by mountains and tall structures.) Using coaxial cable lines he distributed the TV signals from his antenna But "cable" not only imports distant signals. Viewers in large cities where TV signals are very strong often have problems with ghosts, or multiple images in their pictures. (Note the TV tower construction photo on right.) You may recall that ghosts -- at least the TV kind -- are caused by multi-path reception, or receiving multiple TV signals after they bounce off of tall structures in the immediate area. (Note illustration below.) To address this problem, AT&T started installing intercity coaxial cable, in 1952. Coaxial Cable Note the central wire is surrounded by a white electrical insulator. This in turn is surrounded by braided wire and metal foil shielding. This shielding serves to both confine the internal signal to the cable and keep external RF (Radio Frequency) energy from nearby appliances from interfering with the signal. Although coaxial cable has been used for decades to conduct TV signals, it has a number of problems. Topping that list is the need to constantly re-amplify signals (with the various problems that introduces) as they are sent through cables. You often see CATV cables and amplifiers attached to utility poles.
|
In 1950, CATV, or community antenna television, was born in a small town in near Philadelphia, Pennsylvania.
to the homes of customers
-- and, thereafter we assume sold many TV sets.
cutaway view of the single copper wire inside.
Fiber Optic Cable The medium of transmission is light. Light waves have an extremely high frequency and travel at 300,000-km (186,000-miles) per second. A single OF cable can theoretically carry trillions of bits of information every second. The thickness of an optical fiber is only slightly larger than a human hair. The photo on the right shows a light-conducting OF strand going through the eye of a needle. Compared to coaxial cable, optical fiber has ten advantages:
As cable and telephone companies continue to move toward optical fiber, eventually, even home-to-home video transmissions will become as simple as dialing the right number. Satellite Services
Signals reflected from the center element will hit the dish and then be sent upward on their 36,000-kilometer (22,500-mile) path to the satellite.
As illustrated on the right, once the signal is received, it is amplified, the frequency is changed, and then it is sent back to the earth. The "footprint" (coverage area) of the returning signal covers many thousands of square kilometers or miles of the earth's surface.
The signal from the satellite is collected in a dish and directed toward a receiving element, as shown on the right. This signal is then amplified thousands of times and fed to your TV receiver.
Satellite Distribution of Programming Once they arrive on the East Coast, they are recorded, commercials are typically added, the programs is scheduled into the network agenda, and then beamed back up to satellites at intervals appropriate to the time zones across North America. Cable (CATV) companies also receive much of their programming from satellites. This includes both TV and audio services. Many TV and audio services (satellite "stations") are not broadcast over the airwaves, but are only available directly from satellites. Satellite-to-Home Services There are two basic divisions of satellite services: the essentially free satellite services that use the C-band and Ku-bands, and subscription services such as the DISH and Direct-TV. The latter services, which offer hundreds of channels and the ability to record programs, have seen a steady growth rate over the last few years. Note graphs below.
Although many C- and Ku-band satellite services are also subscription based, there are several hundred free TV services ("stations") available. These include:
Even with the dozens of channels of free programming, the C-band and Ku band services have been declining in popularity. This is primarily because
desirable programming is more readily available on DISH and Direct-TV, the large dishes involved,
especially with C-band reception, and the problem of regularly having to
readjust your receiving dish to one of the dozens of satellites. Free to Air Services FTA programming varies in different parts of
the world and may or may not be scrambled and subscription based. Satellite Audio Services Some are standard radio stations that also distribute their signal by satellite. Examples are CBM-AM in Quebec and WQXR-FM in New York. Others, which cover everything from classical to hard rock music, are music services available only by satellite. Satellite audio services, such as XM-Radio
and Sirius radio, were discussed in an earlier section.
Flyaway ENG Satellite Links
|
is fiber optics (also called optical fiber or OF) .
The information carrying capacity of OF is hundreds of times greater than a normal copper wire. (Note on the comparison in this photo between an optical fiber link and a telephone cable with hundreds of wires.)
Geosynchronous satellites rotate at the same speed as the earth and end up being stationary in relation to the earth's surface. This obviously simplifies the job of keeping them within the range of both the uplink and downlink dishes on the earth. (See illustration on the left.)
are sent to the East Coast for network distribution.
|
Use Limited to direct Internet access from CyberCollege® or the InternetCampus®
|
