Some call it the biggest revolution in television since colour. Others go much further and say that digital television is the greatest media revolution since the invention of television itself.

Whatever degree of hyperbole one chooses, there is no doubt that within 15 years all television will be digital.

Digital television simply means converting the signal into ones and zeros - the language of the computer. Digital signals can be squeezed, giving far more channels. Where there is one analogue - or conventional - channel now, there can be up to 10 digital channels.

Digital also gives far better picture and sound quality, simply because digital cannot recognise anything but the ones and zeros, so it cannot transmit interference.

It can, in some instances, allow for "inter-activity" or two-way communications, as the viewer responds along the same route the signal took getting to the television.

It allows new ways of watching television. Already digital services in Germany offer sport viewers a choice of camera angles, with each camera transmitting direct to the viewer on different channels. The viewer then changes channels to watch the match from different angles.

More channels mean more films and sport, as TV executives try to find material to satisfy the insatiable appetite of hundreds of channels.

There will be more specialist channels, dedicated to golf, soccer, films, repeats or - as exists in Japan - one that shows a goldfish swimming. This, apparently, is meant to relax the viewer.

Digital television is being driven by commercial concerns, so it is apt that the viewer in the digital world is a consumer. Channels will be trying to grab our attention, trying to get their voices heard in a crowded environment.

This is why some fear television standards will drop as the old adage of not being able to underestimate public taste becomes the yardstick for the schedulers.

Digital television can be received by cable, satellite or terrestrially. The first offers the most "inter-activity", allowing viewers to respond, call up films and buy from home shopping channels.

Digital satellite can offer huge numbers of channels, but a reduced amount of inter-activity. Rupert Murdoch is talking of 200 channels by the end of next year.

Terrestrial digital is the cheapest system and is the most favoured by the British authorities. It offers a smaller number of channels. But experts point out that even where there is huge choice viewers tend to stay with a few trusted channels anyway.

Terrestrial also has the least inter-activity. But it is believed that cable viewers would rarely use all the inter-activity for which they would be paying.

When digital arrives it will take time before we all start using it. Those channels currently available will be "simulcast", or transmitted in traditional analogue format and digital.

The main public service broadcasters such as RTE will maintain analogue transmission for at least 15 years.

First we will have a set-top box decoding the signal for our existing TV. In time cheap digital television sets will be available.

They will also offer wide-screen viewing. A wide-screen digital television, which can receive analogue signals, costs nearly £2,000 at present.

It is predicted that high street retailers might be offering them for £600 by the end of the year.