Satellite TV offers subscribers literally
hundreds of channels to choose from.
In fact, it's satellite TV's diversity and scope that makes its service so appealing to subscribers. But
it's difficult not to be a little fascinated by satellite TV - how is it
that this little dish is able to deliver so many programs?
Satellite services compress their video programming. In fact, the services use similar
compression software as the type that is used to store movies on DVDs. This compression format, called MPEG-2,
can reduce the storage space from 270 Mbps to about 5 or 10 Mbps. The satellite service has an encoder (a
program the uses a form of encryption to make the signal impossible to
pirate). The signal is compressed and encoded and sent to the satellite.
From there, the satellite receives the signal and broadcasts the signal
to the subscriber's satellite dish. Satellite receivers contain a decryption unit that translates the
signals to viewable programming.
Satellite compression works by examining each frame of the programming. The
compression allows redundant or irrelevant data to be eliminated from the
digital stream. The encoding works in one of three ways:
- The complete frame - the method, known as intraframe, takes all the image data for the
frame. There is very little compression with intraframe.
- A predicted frame - with the predicted frame method, just enough information to tell the
satellite how to display the frame is sent to the satellite. Essentially, the frames are
'predicted,' any irrelevant data is eliminated.
- A bi-directional frame uses information from the intraframe or predicted frame to interpolate
the position and color of each pixel.
Compression rates will change with the type of programming involved. For instance, in the case of a
newscast, the encoding can contain many predicted frames because the set or
scene will stay the same. In the instance of an action movie, where the set and scenes change rapidly, predicted
frames would be difficult to use, so instead the programming is encoded using
intraframes. So, programming the uses little compression will take up far more of the signal than a program that
uses predicted frame compression.
Because of this compression of the signal, satellite companies are able to transmit more data to the satellite, and
ultimately the user. The compression allows satellite transmission of 200 channels to subscribers. Without compression, the satellite
service could broadcast only about 30 channels to subscribers.