`No daytime DJ can come in in the morning and put on some great band he heard the night before. Unless they're on the playlist'

Who decides what records get played? What informs their decisions?

Over the past few weeks, Radio 1FM - still the nation's favourite radio station, but by no means as favourite as it used to be - has been running a series of full-page ads for itself in the papers. These have featured arty black-and-white shots of company personnel at all levels: the DJs in the studios, the librarian in the basement, the bloke on the door - the human faces of Radio 1. And each of these people has offered us a little bit of rather alarmingly casual chat about the place where they work. The bloke on the door recounts his cheery exchanges of a morning with the controller, Matthew Bannister; Chris Evans points out that he gets up at 4am to pee, so he might as well do the breakfast show. And so on.

As a result of all this, we may now feel more intimate with Radio 1 than we have ever felt. We've seen a photo of the wastepaper basket. We know that those crazy guys and girls round the 1FM office favour decorative touches stopping only a fraction short of posters reading "You don't have to be mad to work here..." And yet, strangely, the station's central business remains opaque to us. Like, how does it come to play the music it plays? Why does it choose that music? Who is in control of these decisions and what informs them?

The advertisement featuring the DJ Simon Mayo came closest to mentioning this. He was pictured in a moodily darkened studio beside a quotation which read: "If I only played my favourite records, it would be boring. The trick is to play everybody else's favourites." Let's leave aside the slightly unguarded implication that a man paid to provide the nation with songs has a boring taste in music. The truth is, Simon Mayo doesn't have much choice about what he plays anyway. In fact, there is just one record per day on his show that he gets to choose himself, in a special slot which he calls (and no wonder) "the Big One". The rest is pre-determined by the station's "playlist" system. There are mysteries here which it would take more than a month of expensive advertising to get to the bottom of, but we can attempt to make a start.

Radio 1's playlist - a selection of records which the station commits itself to playing in rotation - is determined largely by performances of records in the singles chart and updated weekly by computer. The idea, according to Geoff Graham, executive producer at Radio 1, is "to ensure a consistent and evenly balanced sound for the station". The theory - one worth pondering - is that if you allow a DJ to follow Earth Wind and Fire's "After the Love Has Gone" with a suite for pile-driver and power- drill by an obscure group of German art-noise terrorists, people will be switching off in bafflement.

There are three playlists, A, B and C. The A list features the current top chart hits and the biggest of the new releases and ran last week to 24 records in all. These might be heard up to 25 times weekly. On the B list, which is a similar size, sits a selection of hits on their way up to, or down from, the A list and some of the lesser new releases, any of which might hope to be heard 15 times in a week. And on the C list is a small selection of album tracks and obscure releases chosen by taking soundings from programme producers. (A Radio 1 DJ told me that "DJs would be the last person they would consult on this".) The station runs 14 hours of playlist programming on weekdays. The playlist does not govern the music played on most evening and weekend shows, where the selection of records is accordingly more ambitious.

In the early days of playlisting, DJs would simply work from boxes of singles marked A and B. You'd play two As for every B, or whatever the set ratio was, and put the single at the back of the box when it was finished. Daytime DJing then must have been about as rewarding as stacking shelves at Spar. Now it's all computer driven, and possibly less rewarding still.

Gareth Davies, a record plugger with the Beer Davies agency, who touts on behalf of record companies for the slots on the playlist not determined by chart performance, went down to the BBC last Thursday morning, as he does every week, to pick up his copy of the week's new list, which includes an analysis of the previous week's performances. Then he talked me through it.

Some of the statistics deviated somewhat from Radio 1's official estimates on likely turnaround. The most played single between 3 June and 10 June was Pulp's "Common People" with 35 plays. The U2 single got 29 plays. Ali Campbell and Montell Jordan and Michelle Gayle went around 26 times.

"The problem with computer-generated programming," Davies said, "is that you take out the spontaneity. No daytime DJ can come in in the morning and immediately put on some great band he saw the night before. Unless they're on the playlist."

Radio play generates record sales and also works out well for songwriters, whose royalty for one three-minute play on Radio 1 is, according to the Performing Rights Society, pounds 46.10. (Jarvis Cocker of Pulp, then, will have earned in excess of pounds 1,600 last week from Radio 1 alone.) So what's a place on the playlist worth? In the early years of pop radio, this would have been a simpler question to answer: a place on the playlist would have been worth one satchel of cocaine to the DJ, or a fully escorted evening in one of London's more exclusive nightspots.

But overt corruption seemed genuinely to vanish from the record-plugging trade in the late 1970s - to be replaced by altogether less interesting covert corruption in the form of supposedly harmless promotional "gifts". At this point, the value of a place on the playlist seems to have plummeted to the level of an A-ha tour jacket or a Feargal Sharkey wall-clock. And since then there has been still further attrition, to the extent that many of today's pluggers rate a place on the lists at no more than a handful of poorly designed T-shirts. When the playlist began to be arranged chiefly by computer, the novelty of the novelties wore off.

Similarly, the great age of the promotional stunt has been ended by automation. Perhaps there are only so many attention-seeking devices one can dream up; only so many times you can deliver your new single in a pizza box at lunchtime. There was a time when, if you were touting a new single by, say, the Godfathers, it was beholden on you to visit Radio 1 in a hired, period car, wearing a pinstripe suit and spats and carrying a violin case. Gareth Davies once saw a 6ft-something leprechaun at the BBC: a plugger promoting the Waterboys. When this kind of activity was at its flood, the foyer at Broadcasting House must have been like the green room at the Moulin Rouge. Nowadays, apparently, pluggers stay in their bomber jackets and their stone-washed jeans and a sizeable portion of the decision- making process which puts a record on the playlist will be taken up with consideration of, heaven forbid, the actual music.

Playlist systems operate more strictly on commercial stations. Stations like Capital FM in London and Heart FM in Birmingham pick only selectively from the charts and then add in older material according to a vision of their listeners' tastes. Which is why they always seem to be playing the same song whenever you turn on. Radio 1, still looking to the principle of something-for-everyone, refuses to refine its playlist in this way.

This is big-hearted of it but is, perhaps, the station's main problem right now. When a chart-based playlist system was designed, it was possible to assume that everything in the top 20 was popular. But as pop music has diversified and fragmented and become polarised, so the chart has become less easy to read as a gauge of the nation's pulse. Which means that Radio 1 now finds itself committed to a way of picking music which a huge slice of its audience no longer shares. So maybe it's time to turn off the computers and shake up the playlist. It's not the guy on the door people are worried about: it's the stuff in the CD players.