What is Satellite TV?

Conceptually, satellite television is a lot like broadcast television. It's a wireless system for delivering television programming directly to a viewer's house. Both broadcast television and satellite stations transmit programming via a radio signal (see How Radio Works for information about radio broadcasting).

Broadcast stations use a powerful antenna to transmit radio waves to the surrounding area. Viewers can pick up the signal with a much smaller antenna. The main limitation of broadcast television is range. The radio signals used to broadcast television shoot out from the broadcast antenna in a straight line. In order to receive these signals, you have to be in the direct "line of sight" of the antenna. Small obstacles like trees or small buildings aren't a problem; but a big obstacle, such as the Earth, will reflect these radio waves.

If the Earth were perfectly flat, you could pick up broadcast television thousands of miles from the source. But because the planet is curved, it eventually breaks the signal's line of site. The other problem with broadcast television is that the signal is often distorted even in the viewing area. To get a perfectly clear signal like you find on cable, you have to be pretty close to the broadcast antenna without too many obstacles in the way.

Satellite television solves both of these problems by transmitting broadcast signals from satellites orbiting the Earth. Since satellites are high in the sky, there are a lot more customers in the line of site. Satellite television systems transmit and receive radio signals using specialized antennas called satellite dishes.

The television satellites are all in geosynchronous orbit, meaning that they stay in one place in the sky relative to the Earth. Each satellite is launched into space at about 7,000 mph (11,000 kph), reaching approximately 22,200 miles (35,700 km) above the Earth. At this speed and altitude, the satellite will revolve around the planet once every 24 hours -- the same period of time it takes the Earth to make one full rotation. In other words, the satellite keeps pace with our moving planet exactly. This way, you only have to direct the dish at the satellite once, and from then on it picks up the signal without adjustment, at least when everything works right.

Digital broadcast satellite transmits programming in the Ku frequency range (12 GHz to 14 GHz) however, early satellite television was broadcast in C-band radio -- radio in the 3.4-gigahertz (GHz) to 7-GHz frequency range. These satellite systems were developed primarily for television networks to distribute their content to local stations. As satellite reception systems became less expensive and more pervasive, these large dishes began appearing in peoples back yards. Once you installed one of these 10 meter dishes, it was common in those days to watch satellite TV free - that was before compression and encryption.

Compression, Encryption, Transmission

The two major providers in the United States use the MPEG-2 compressed video format -- the same format used to store movies on DVDs. With MPEG-2 compression, the provider can reduce the 270-Mbps stream to about 5 or 10 Mbps (depending on the type of programming). This is the crucial step that has made DBS service a success. With digital compression, a typical satellite can transmit about 200 channels. Without digital compression, it can transmit about 30 channels.

At the broadcast center, the high-quality digital stream of video goes through an MPEG-2 encoder, which converts the programming to MPEG-2 video of the correct size and format for the satellite receiver in your house.

The MPEG encoder analyzes each frame and decides how to encode it. The encoder eliminates redundant or irrelevant data, and extrapolates information from other frames to reduce the overall size of the file. Each frame can be encoded in one of three ways:

As an intraframe - An intraframe contains the complete image data for that frame. This method of encoding provides the least compression.

As a predicted frame - A predicted frame contains just enough information to tell the satellite receiver how to display the frame based on the most recently displayed intraframe or predicted frame. This means that the frame contains only the data that relates to how the picture has changed from the previous frame.

As a bidirectional frame - To display a bidirectional frame, the receiver must have the information from the surrounding intraframe or predicted frames. Using data from the closest surrounding frames, the receiver interpolates the position and color of each pixel.

This process occasionally produces "artifacts" -- little glitches in the video image -- but for the most part, it creates a clear, vivid picture.

The rate of compression depends on the nature of the programming. If the encoder is converting a newscast, it can use a lot more predicted frames because most of the scene stays the same from one frame to the next. In other sorts of programming, such as action movies and music videos, things change very quickly from one frame to the next, so the encoder has to create more intraframes. As a result, something like a newscast generally compresses to a much smaller size than something like an action movie.

After the video is compressed, the provider needs to encrypt it in order to keep people from accessing it for free. Encryption scrambles the digital data in such a way that it can only be decrypted (converted back into usable data) if the receiver has the correct decryption algorithm and security keys.

Once the signal is compressed and encrypted, the broadcast center beams it directly to one of its satellites. The satellite picks up the signal with an onboard dish, amplifies the signal and uses a another dish to beam the signal back to Earth, where viewers can pick it up.

A Satellite Dish System

There are five major components involved in a direct to home (DTH) satellite system: the programming source, the broadcast center, the satellite, the satellite dish and the receiver.

Programming sources are simply the channels that provide programming for broadcast. The provider doesn't create original programming itself; it pays other companies (HBO, for example, or ESPN) for the right to broadcast their content via satellite. In this way, the provider is kind of like a broker between you and the actual programming sources. (Cable television companies work on the same principle.)

The broadcast center is the central hub of the system. At the broadcast center, the television provider receives signals from various programming sources and beams a broadcast signal to satellites in geostationary orbit.

The satellites receive the signals from the broadcast station and rebroadcast them to the ground.

The viewer's dish picks up the signal from the satellite (or multiple satellites in the same part of the sky) and passes it on to the receiver in the viewer's house.

The receiver processes the signal and passes it on to a standard television.