What people want is to potter around in a back-garden of trivia, rich with the compost of youth
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Behind whatever a CD-Rom delivers to the screen, there is a database and an engine for moving chunks of it around, making CD-Roms encyclopaedic by nature. When the information they carry is valuable to a professional market, they can be priced in three or four figures. The trickier area is the consumer market, for it pays less and expects to be entertained.

The most flattering CD-Rom reference-work to be given for Christmas would be a general title, such as Microsoft's Encarta 97 Encyclopedia (pounds 49.99, Windows only). Encarta's cool, considered interface suggests serious enquiry; its texts allay concerns about the erosion of educational standards.

While encyclopaedias suggest an open mind that likes to range widely, cultural trends of the 1990s indicate that what people really want is to potter around in a little back-garden of trivia, rich with the compost of their youth. CD-Roms like Music Central 97 (Microsoft pounds 29.99) have the capacity to hold thousands of your own personal back pages, and - unlike nostalgia TV - allow you access to the memory of your choice. An eerie feature of Music Central is a gallery of clips from the Old Grey Whistle Test, presented in a miniature window on to the old grey past.

You could also use it to broaden your horizons, though, exploring unfamiliar genres, or as a guide to purchases; just as the companion Cinemania 97 (Microsoft pounds 29.99) can be used as a guide to movies on video or TV. You don't have to turn to your back pages just because they are in print again.

Mainstream CD-Rom publishing tends to stick to safe bets like movies and music, but that leaves a lot of other niche markets untapped. There are some two-and-a-half million coarse fisherfolk in this country, and probably about a million people who enjoy watching birds. All have relatives who need to choose them Christmas presents, though admittedly fewer may have multimedia computers.

A hint of the potential for growth in these areas is given by The CD- Rom Guide to British Birds (Birdguides pounds 49.95). Max Whitby, its executive producer, had made videos with birder Dave Gosney. Whitby believed CD- Roms would make an ideal format for birdwatchers' guides, and took the gamble of producing the original version last year. He proved his point. Though the disk's presentation is plain, using simple windows like a word- processing program, it works brilliantly. With the ability to present drawings, text, video clips and recordings of bird song all together, it makes printed guides look poor flightless creatures. The exercise has worked commercially, too. Birdguides now sells more CD-Roms than videos.

The serious birder is given design without frills: perhaps coarse anglers are a more frivolous bunch, or maybe the lusher look of The Art of Coarse Fishing (Eagle Eye Interactive pounds 29.99, Windows only) reflects its origins as a magazine partwork. It seems a well executed production, though I didn't know computer software could make me feel queasy till I saw the sequences on bait, or how to unhook a pike so that "both the fish and your hands remain intact".

Stamp collectors might be expected to have mixed feelings about a technology that will render the objects of their passion obsolete. But a stamp collection is a species of database, so philatelists and software developers are kindred spirits. As well as programs designed to assist cataloguing, developers in a number of countries have produced CD-Rom stamp encyclopaedias. One of these is China Stamp Forest (details on request), produced in the People's Republic. It features "cartoon film" depicting events from China's history since 1878, when the country began to issue stamps. This approach may seem reminiscent of Flann O'Brien's observation that the year 1921 was a momentous one in Ireland's history, being the one in which the Westinghouse brake pump was introduced on Irish railways. But a stamp collection is a narrative of national history. Stamps mark the birth of nations, changes of regime, territorial struggles; they make statements about who and what a nation considers important from one year to the next.

Those who mock such collections may also be missing the real story, because a stamp album does not include context. Multimedia encyclopaedias are the ideal vehicle for adding dimensions to what are otherwise merely very small pieces of paper. Derek Yardley, of the Philatelic Traders Society, showed me a recent example from Barcelona, Piezas Maestras de la Filatelia Espanola (details on request). It appeared to be well organised, and has a baroque facade to rival any production aimed at the general market. The screen takes the form of a desk blotter; a feature on airmail has a little Fokker triplane as an icon. Dramatically, this section opens with balloon post from the Prussian siege of Paris.

As far as the CD-Rom market at large is concerned, the interesting thing about stamp-collecting, fishing and birdwatching is that they share a U-shaped age profile. People take them up in childhood, abandon them when diverted by puberty, and are then prevented by the demands of real life from returning until they retire. Comfortably off Third Agers have the potential to act as patrons of what Andreas Whittam-Smith calls the "third screen", the third screen-based medium after cinema and television. Unlike the rest of the population, they have both the money to spend on expensive cultural commodities and the time to play with them. A reliable stream of revenue from older consumers could help propel CD-Rom publishing out of the choppy waters it now finds itself in, and encourage it to make titles that are genuinely for adults, rather than adolescents.

The hitch, of course, is that the older age group is also the one least comfortable with computers. But each year, a new cohort of people leaves the workforce accustomed to spending their days in the same room as a PC. If they are offered products which speak to their special interests, that may be the incentive they need to share their retirement with a computer as well.