All MPEG standards exist to promote system interoperability among your computer, television and handheld video and audio devices. They are:
- MPEG-1: the original standard for encoding and decoding streaming video and audio files.
- MPEG-2: the standard for digital television, this compresses files for transmission of high-quality video.
- MPEG-4: the standard for compressing high-definition video into smaller-scale files that stream to computers, cell phones and PDAs (personal digital assistants).
- MPEG-21: also referred to as the Multimedia Framework. The standard that interprets what digital content to provide to which individual user so that media plays flawlessly under any language, machine or user conditions.
Satellite TV Signal
Satellite signals have a pretty long path to follow before they appear on your TV screen in the form of your favorite TV show. Because satellite signals contain such high-quality digital data, it would be impossible to transmit them without compression. Compression simply means that unnecessary or repetitive information is removed from the signal before it is transmitted. The signal is reconstructed after transmission.
Standards of Compression
Satellite TV uses a special type of video file compression standardized by the Moving Picture Experts Group (MPEG). With MPEG compression, the provider is able to transmit significantly more channels. There are currently five of these MPEG standards, each serving a different purpose. DirecTV and DISH Network, the two major satellite TV providers in the United States, once used MPEG-2, which is still used to store movies on DVDs and for digital cable television (DTV). With MPEG-2, the TV provider can reduce the 270-Mbps stream to about 5 or 10 Mbps (depending on the type of programming).
Now, DirecTV and DISH Network use MPEG-4 compression. Because MPEG-4 was originally designed for streaming video in small-screen media like computers, it can encode more efficiently and provide a greater bandwidth than MPEG-2. MPEG-2 remains the official standard for digital TV compression, but it is better equipped to analyze static images, like those you see on a talk show or newscast, than moving, dynamic images. MPEG-4 can produce a better picture of dynamic images through use of spatial (space) and temporal (time) compression. This is why satellite TV using MPEG-4 compression provides high definition of quickly-moving objects that constantly change place and direction on the screen, like in a basketball game.
In the next section, we will see how satellite tv signals are encoded for transmission.