February 20, 2008 > TechKnow Talk: Satellite Television
TechKnow Talk: Satellite Television
Before the 1970s, most of us had only one choice when it came to television reception. We installed a wire antenna on the roof or atop the TV itself and captured a free signal broadcast by a local station. Most homes in or near urban areas were able to receive perhaps three separate broadcasts with reasonable quality. Since there were only three television networks at the time, this was sufficient.
But this direct reception process has an important limitation. While the long wavelength radio waves that carry the transmission pass through or around trees and buildings with little signal loss they are stopped by more substantial obstacles, notably the earth itself. Thus the receiving antenna must have "line of sight" to the broadcast tower. Even without intervening mountains, the curvature of the earth and signal loss limits the distance from which a quality signal can be received to 100 miles or so, depending on the height and quality of the antennas, atmospheric conditions, and signal strength.
Families living in remote areas were unable to watch television and instead resorted to activities such as conversation and reading. Technology came to the rescue for some in the late 1940s with the invention of CATV, or Community Antenna Television. As the name implies, a rural community erected a large antenna on high ground, capable of receiving and amplifying television signals from distant broadcasts, and delivered those signals to homes via cables.
Following the widespread availability of commercial cable TV in the 1970s and 1980s, CATV has come to be known as Cable Television. The broadcasts are no longer collected "over the airwaves" directly from the broadcaster, but from satellite signals. Because of the sophisticated signal-enhancing equipment in the cable broadcast hub, and because the signal is not sent long distances through the air, the signal quality is much better than direct reception from a broadcaster. But cable TV has a limitation too: the cable itself. It is not cost-effective to run cables to rural homes. Ironically, those who benefited from the original CATV are some of the same people who are now left without cable TV access.
Enter satellite television. It solved the line of sight problem of direct broadcast (since the signal comes from the sky) as well as the need to string cable to the consumer. To get a sense of how successful this has become, one need only travel to remote islands of the South Pacific, the outback of Australia, the bush country of Africa, or the jungles of South America. Satellite dishes are seen in profusion throughout these areas, as well as in the plains and mountains of the U.S. and Canada.
Increasingly, they are also seen on urban homes here in the U.S. and around the world. Satellite TV has become a direct competitor to cable in urban markets. Let's look at some of the technologies that enable satellite TV and the reasons that some consumers prefer it to cable.
As mentioned above, even cable TV comes from a satellite these days. What sets satellite TV apart is Direct Broadcast Satellite (DBS) technology, designed to send a television signal directly to the home of the consumer. The programming may originate nearly anywhere, from major network broadcast centers to local TV stations to small independent broadcasters. These signals are purchased by the DBS provider and delivered to the provider's broadcast center, most commonly via satellite. The broadcast center applies the same signal enhancement techniques used by the cable broadcasters to create a very high-quality digital signal.
These audio-video signals contain a massive amount of data, far more than could be continuously transmitted by the satellite. So the next step is to compress those signals to a manageable size. The current standard for compression is MPEG-4, the same process used to deliver streaming video across the Internet to home computers.
Video compression is a complex process. It employs algorithms that compare each successive frame of the image. Portions of the image that do not change from one frame to the next may be removed from some frames to be re-created after transmission. Similarly, for an object that moves smoothly across several frames, it may be removed from some frames and its position calculated based on data from surrounding frames when it is re-inserted after transmission. As might be expected, programming in which there is little change from one frame to the next compresses more than rapidly moving images.
The compressed signal is then encrypted (or scrambled) in such a way that only subscribers who have the provider's receiver can decrypt it. This prevents unauthorized use of the signal by those who have not paid for it. There are many encryption techniques and they are frequently updated.
The signal is transmitted to the provider's satellite using an uplink antenna. These are typically very large transmission antennas, 30 feet or more in diameter. A transponder on the satellite is tuned to receive the frequency of the uplink signal. It then retransmits it at a different frequency, to avoid potential interference with the uplink signal. These microwave signals use portions of the C-band or Ku-band ranges (about 3.5-6.5 GHz and 11.5-14.5 GHz, respectively). The uplink signal is typically at a higher frequency within these bands than the downlink signal. It is not unusual for a single satellite to carry 20 or more transponders: C-band, Ku-band, or both.
The downlink signal is received by the dish mounted on the consumer's home. The parabolic (curved) surface of the dish gathers the signal and directs it to a collector positioned at the dish's focal point. This device is called the feed horn, the most important component of which is a "low noise block down-converter" (LNB). The LNB improves the signal quality by eliminating extraneous input (noise) and amplifying the signal. The signal is then transmitted to a receiver (an electronic "black box") inside the house.
The receiver decrypts and decompresses the signal and converts it from digital format to an analog or digital format recognizable by the television. The receiver also separates the individual channels so the consumer can tune it to the channel of choice. In addition, the receiver logs pay-per-view activity and "phones home" to the provider periodically with this information for billing purposes. The provider occasionally sends the receiver new encryption algorithms and other updates, embedded in the TV signal.
Since the earth is rotating, and the satellites are moving in orbit, why doesn't the receiving dish lose line of sight contact sometimes? The answer is geosynchronous orbit. In an orbit 22,240 miles in altitude, a satellite circles the earth in exactly 24 hours, matching the earth's rotation and remaining above the same point on its surface. Occasional adjustments are made by thrusters on the satellite to ensure it stays precisely positioned. Once the dish is pointed at the satellite and secured in place, it should maintain contact indefinitely. Geosynchronous communications satellites have been in use since the 1960s.
The competition between cable and satellite TV providers is raging. In the Bay Area, we have several choices. While only one cable provider is available, two full-service satellite providers are competing for our business. Whether cable or satellite TV is preferable depends on one's individual viewing habits.
Satellite TV is perfect for the devoted couch potato, offering more programming choices, especially in high definition (HDTV). On the other hand, satellite packages offering the most channels are more expensive than cable and, unlike cable, satellite reception may be affected by high winds. Satellite TV also requires an initial investment in the dish and receiver, and a separate receiver is needed for each television (though receivers offering multiple outputs are becoming available).
Whether one chooses cable or satellite, today's TV entertainment options are a far cry from those three snowy channels of only 30 years ago.