compression

Due to compression, TV audio and video require less bandwidth and multiple digital TV channels can fit unto one satellite transponder as oppose to a single analogue channel occupying the whole transmission line.

A TV signal is captured by a camera and then manipulated during program production. At this point the video must be at its highest quality and full bandwidth for recording, editing and special effects purposes. Then the TV signal needs to be compressed for economical transmission and storage. The possible efficiency of compression depends on a couple of factors. If a signal will be further edited and manipulated in the receiving studio it must maintain a relatively high quality and therefore can’t be compressed as much as a signal that will be sent directly to the viewer’s TV set. Also, the extent to which a signal can be successfully compressed depends on the type of program (E.g. movies can be compressed more than sports). Nevertheless, a typical program mix will fit up to 10 digital television channels on one transmission line.

Furthermore, compression techniques that fall into two categories: lossless (reversible, no data loss) and lossy (irreversible, greater data reduction). Effective compression is best achieved with a combination of data reduction techniques such as bit rate reduction and compression. The difference is that bit rate reduction eliminates unnoticeable data, and compression removes unnecessary and excessive data through mathematical algorithms. Due to different forms of redundancy and the fact that the human visual system is unable to detect certain details, information can be altered or removed causing changes that are imperceptible to the human eye or brain.

After compression, the structure data, audio and video must be multiplexed. A number of compressed TV signals are combined by a multiplexer and put unto a shared transition medium. This is done by one of the two possible kinds of multiplexers that result in either a transport or a program stream, which is suited for secure transmission paths since it can contain large amounts of information. In addition multiplexing can be done using various methods. Time division multiplexing allocates a distinct time interval for each channel in a set; with the help of synchronization and a fixed interval order the channels take turns using the common line.