In the 1990s significant advances in digital video compression technology have led to a number of proposals to transmit digital television signals by satellite. It is now possible to compress conventional television signals to bit rates in the range 4 to 10 Mbit/s with comparable picture quality to that achieved by PAL. The compression techniques most commonly used are based on motion compensated, frame difference Discrete Cosine Transform (DCT) signal processing techniques. An international standards committee called the Moving Picture Experts Group (MPEG) is developing standards for the video compression technique. Most proposals for digitally compressed satellite television systems plan to conform to MPEG standards or variations that are conceptually very similar to MPEG standards.
One of the best known proposals is by a company known as DirecTV in the USA. It plans to provide up to 150 channels of satellite television using this compression technology. This system commenced operation in 1994.
In Australia the satellite payTV system, which commenced operation in January 1995, uses a similar form of digital compression technology.
The modulation format to be used in both the DirecTV system and the Australian Satellite PayTV system is QPSK modulation. This modulation format has been chosen because of its insensitivity to amplitude compression distortion.
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