18.7.1 General

Broadcasters have been using digital transmission techniques for the distribution of audio and video for some considerable time already, but only in the last four or five years has it been possible seriously to consider digital transmission of high-quality television signals directly to the home at low bit rates, i.e. about 10 Mbit/s or less. This is because of developments in video and audio compression techniques and, already, widespread consumer use of these techniques is established, largely because the complex integrated-circuit developments needed to make the receiver equipment cheap enough for consumer applications have been completed. When the WARC plan of 1977 (see above) was being studied, digital coding was not seriously considered because coding technology was not advanced enough.

For many years broadcasters have been developing (in CMTT and EBU) digital bit-rate reduction techniques for use in transmitting television between studios on telecommunications networks for both conventional and HDTV18,19.

A large number of techniques exist for reducing the bit rates needed to remove inessential information from television pictures in order to reduce the amount of data needed to be transmitted or stored. Differential PCM (or predictive coding) was the first to be used but this was followed by transform coding methods in the mid-1980s, in particular, the discrete cosine transform (DCT). Motion-compensation techniques are now used in hybrid schemes combining predictive and transform techniques, and statistical (Huffman) coding is used to extract any redundancy remaining in the bit stream transmitted to the channel, see...