Electronic miracle that promises wider choice

The transmission of digital television performs the electronic equivalent of squeezing a Jumbo jet, with its 200ft wingspan, through a living-room window and reassembling it on the other side. But to the viewer, the only obvious difference will be more channels, higher-quality sound and, in time, the choice of channels with high-quality or wide-screen images.

Old televisions will still work, but to view the new channels you will have to buy a "set-top box", as for satellite or cable viewing, and probably a subscription to each service.

The process begins with a standard colour television image. This consists of thousands of points on a screen, each of which has a particular mixture of brightness for three primary colours.

For digital television, the information describing the colours and their brightness at each point is converted into a short series of 0s and 1s. A complete screen, or "frame", can thus be represented by a long series of 0s and 1s. Each second of a television picture consists of 25 frames. When translated into digital format, this takes about 200 million 0s and 1s, or 200 megabits, every second.

But existing digital compression techniques can squeeze this down by a factor of 100 by leaving out duplicate frames, or parts of frames which are the same as previous ones. This creates a stream of data which only requires two or three megabits per second. This can then be encoded on to a UHF signal like high-speed Morse code, and broadcast over a standard television transmitter.

When the signal arrives at the domestic aerial, it is routed to the set- top box which converts the digital stream back into a standard analogue picture. The set top box will probably cost about pounds 300.

Digital radio works in the same way, but has less data to compress, so that six stereo radio channels can be squeezed into 256,000 bits per second, giving sound of compact-disc quality on each. The BBC is starting digital radio broadcasts next month.

The role of the "multiplexer", as the Government calls the companies providing the transmission facilities, is to knit together the digital streams from the broadcasters. The same frequency range of 5.5 megahertz, required to broadcast a single television channel in analogue format, can carry the interwoven digital streams comprising six or more digital television channels.

Each multiplexer will be able to choose which broadcasters to carry. They will also choose to encrypt some or all of the channels so that people have to pay to see them, as BSkyB does with the majority of its broadcasting.

The advantages of digital television should be wider choice, better sound - like the difference between CDs and records - and, potentially, high- definition television with far better picture quality.

However, consumers seem satisfied with picture quality and are pressing for more choice.

But the biggest question that remains is whether the set-top boxes will have a standard form, or whether each multiplexer operator will find ways to make their boxes unique. This could lead to market domination and cause smaller rivals to wither.