Psion, in one of the last creative gasps of its glory days, was one of the first UK companies to dabble with the idea of using Digital Audio Broadcasting (Dab) to deliver data to handheld computers. Early Dab chips have been too heavy on batteries, but the technology is otherwise ideal for mobile use because it was designed to improve reception in cars - it takes advantage of ghost reflections that normally cause fading among buildings.

A Dab signal, which is time-sliced to carry several services at once, passes around 1.5Mbits/sec of which up to 20 per cent can be used for data under present rules; error correction will further reduce the bandwidth, but there is enough left to carry even video to tiny handheld screens.

Services envisaged initially for the data channel being developed by Digital One ( see 'Birth of a new medium story') are modest in scope and similar to those touted for 2.5 and 3G mobile: clips of goals, music videos, and location-based information.

Digital One chief executive Quentin Howard pointed out: 'There is a fundamental difference between a broadcast and a cellular network. If a million people want a video clip you can broadcast it easily. But you cannot deliver anything to a million people at the same time over mobile phones.'

Some think Dab broadcasts could increase traffic on mobile phones, which will be used as a back channel for adverts and service requests.

NTL has envisioned using Dab as an extension to Avant Go-type services: you put your device on a cradle to download an electronic newspaper, say; Dab can 'trickle charge' this with updates during the day.

The BBC is treading cautiously over Dab multimedia - indeed, hardly treading at all. Simon Nelson, controller of Radio and Music Interactive, said that the flexibility of Dab is one of its key strengths but spectrum was allocated into it in the UK primarily for sound radio.

He said the BBC was pleased to see other countries experimenting with multimedia, but there was currently only modest scope for it in Britain, although the BBC would continue to explore the possibilities. He added: 'Sound radio services will remain the core feature of the BBC's Dab offering, and any other service possibility will be primarily assessed on its ability to enhance, for its radio audiences, that central objective.'

But the company is exploring a similar multimedia mix from another direction - the convergence of TV and computing. Researchers at the BBC labs reckon that Media Center PCs and personal video recorders that allow you to time-shift TV mean people are viewing television more like they read a book - in chunks, over several sessions.

The labs have been experimenting with adding meta-data to the broadcast stream that will split up a programme into 'chapters' to facilitate this practice; it also allows the picture to be screened in a window surrounded by text and hypertext links.

The result, demonstrated at last month's Mediacast show, looks similar to a multimedia book. The demonstration was on a big TV screen but the BBC has experimented with doing the same thing with a Tablet PC, which is more suitable because text is better read close up. In future, people may use a Tablet viewer in conjunction with a TV to combine the advantages of what pundits call two-foot and six-foot information. The work is being done in conjunction with the Digital TV Group, which includes major service providers and receiver manufacturers, with the aim of establishing a standard for the metadata using an experimental platform called the TV-Anytime (TVA) testbed [www.tv-anytime.org].

But, as the BBC man at Mediacast said, these are very early days. The word 'metadata' - data about data - puts the system firmly in its current place as an addition to traditional programs rather than a new format in itself. Indeed, it is only half a step away from what many people already do: read related material on the web as they watch a programme. But it is moving TV viewing from being largely a passive occupation to a much more complex interaction... more like reading, allowing you to backtrack, ponder, and look things up.

Whether or not the emerging multimedia interface is a new medium of expression is perhaps a matter of semantics. There have been suggestions that reading and writing will be displaced altogether by audio-visual communication, but that seems to me to be unlikely.

What does seem certain is that, as generations of users mix text, audio, stills and moving images on paper-quality screens, there will evolve an interface that makes best advantage of all content types and amounts to something rather more than the sum of them.