Compression

Both major Satellite TV service provider in U.S. use the MPEG-2 compressed video format which is same format as movies on DVDs. By compressing the signal with MPEG-2, the provider can reduce the 270-Mbps stream to about 5 or 10 Mbps generally. This is directly improve the transmission bandwidth and made Database System service successful. With digital compression, a typical satellite can transmit about 200 channels. Without digital compression, it can transmit only 30 channels.

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

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

• As an intraframe -In video compression, the coding of the differences between frames. Interframe coding often provides substantial compression because in many motion sequences, only a small percentage of the pixels are actually different from one frame to another. However, it depends entirely on the content. A room full of people dancing will not compress as well as a person sitting in a chair talking.
  
  With interframe coding, a video sequence is made up of keyframes that contain the entire image. In between the keyframes are delta frames, which are encoded with only the incremental differences. Depending on the compression method, a new keyframe is generated based on a set number of frames or when a certain percentage of pixels in the material has changed.

• As a predicted frame - A predicted frame contains just enough information to improve the satellite receiver which requires less computation overhead as compared to conventional systems. Specifically, the encoded digital video stream includes direct mode predicted frames in place of some of the standard predicted frames. The direct mode predicted frames are formed without computing motion vectors for each macro-block of the direct mode predicted frame. During decoding, motion vectors from a co-located macro-block of a preceding predicted frame is copied and applied to each macro-block of the direct mode predicted frame. Because computing motion vectors is generally the most computing intensive task of encoding digital video streams, the use of direct mode predicted frames can greatly reduce the computation requirements for encoding digital video streams.

• As a bidirectional frame - In accordance with the inventive arrangements, a preferred directional orientation can be determined for distributed content. Upon receiving a request for content from the content viewing client, the requested content can be distributed in a directional format consistent with the specified preference of the content viewing client. Specifically, where the preferred directional orientation differs from the native orientation of the requested content, the requested content can be transcoded to rearrange not only the textual portions of the requested content, but also positioning of frames within the content. In this way, one need not store separate versions of the same content to accommodate different preferred directional orientations.

The compression rate depends on the programming algorithm. The encoder can use a lot more predicted frames because most of the scene stays the same from previous frame to the next. In other means of programming, such as action movies and music videos, frames or bitframe change very quickly from one to another, 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.

Encryption and Transmission

After the video is compressed, the provider will encrypt it so that only the subscriber have the athority to view it. Encryption scrambles the digital data and converted back into usable data at the receiver by applying the correct decryption algorithm and security keys.

When the signal is compressed and encrypted, the broadcast center tranmit it in high frequency wave to one of its satellites . The higher frequency it sends the signal, the further the signals can goes. The satellite picks up the signal with a big dish, amplifies the signal and uses a another dish to transmit the signal back to Earth in large geographical area , where viewers can pick it up.