We've talked about it for a long time. We've speculated about what might be the impact. But now I think we can really say that we are actually experiencing the full force of the collision of digital technology with the television industry. Now at last there is all the space anyone could possibly need to broadcast every kind of subject imaginable. What a wonderful world this should be.

This would appear to be the holy grail for public service broadcasting, or PSB to use the fashionable acronym. Each genre can have its own channel where it can flourish. Niche broadcasting has finally arrived in the living room, with channels devoted to the arts, to natural history, to children's programmes, to biography, even to the weather.

Sport and feature films are great rating winners, but it has proven to be a different matter for the less popular genres, the ones that PSB is concerned with. A typical natural history programme on one of these niche channels will be lucky to get 1 per cent of the audience it would attract on BBC1. Arts programmes, shown on a channel devoted exclusively to them fare even worse. The long tail gets rather thin at the end.

So why have these new digital channels not done better? I would argue that the notion that great numbers of people, tired after a hard day's work, come home and flip through 50-odd programme channels to decide what to view is, in fact, largely illusory.

The overwhelming majority of people have their own favourite mainstream networks, such as the BBC or ITV, and they look to them first to engage their interest. So for me, in practice, niche broadcasting is hardly relevant when discussing the provision of PSB. The B, I needn't remind you, stands for broad-casting, not narrow-casting. This is an important point now - as policy- makers hold the fate of public service television in their hands... yet again.

Ofcom, the media regulator, has suggested that another way of financing PSB programmes should be found. A slice of the licence fee, some suggest, should be diverted away from the BBC to subsidise public service content placed on commercial channels.

This is not the way forward. Think of the fate of such programming, struggling for its position there. Commercial channels would not want their income-earning programmes placed anywhere near a PSB production, because of the smaller audience it is likely to pass on. Unless there were regulations to stop it, PSB programmes would inevitably be pushed out of peak hours. They would be retained under sufferance - tucked away where they would do least harm to the network's income.

In this debate, PSB cannot be interpreted as selecting public service programming here or there, financing them from some outside source and then foisting them upon commercial networks.

Public service broadcasting, watched by a healthy number of viewers, with programmes financed in proportion to their intrinsic needs and not the size of the audience, can only effectively operate as a network. A network whose aim is to cater for the broadest possible range of interests, popular as well as less popular. A network that measures its success not only by its audience size but also by the range of its schedule.

Is that what the BBC does? I would like to think so, since I have worked for it and it alone throughout my broadcasting career. But there are moments when I wonder. Do we really require so many celebrity chefs? Is it not a scandal that there seems to be no place for continuing series of programmes about science or serious music - or thoughtful in-depth interviews with people other than politicians? But these, I am happy to say, are things that are changing.

The BBC is still strong. Of course, audiences for established networks have all diminished because so many new channels have appeared. But the great proportion of people, it turns out, are still primarily loyal to one or two networks. Television is a miraculous advance, still not a century old, that allows a whole society, properly used, to see itself and to talk to itself. It can enable people, no matter who they are and where they are, to share insights and illuminations, to become aware of problems and collectively consider solutions. It is one of the wonders of our age.

PSB should not be editorially controlled by governments nor used exclusively for commercial purposes. Let's not sell it off, or shut it down like a library or local swimming baths. It should be a place where all kinds of people, with all kinds of interests and insights, can share them with society as a whole. That, I maintain, cannot be achieved with a few individual programmes, dotted here and there on networks whose aims and basic functions have some other ambition.

It can only be done by a coherent network, one that measures its success not only by the size of the audience it manages to gain for an individual programme but also by the width of the spectrum of interests it manages to represent. A network, in short, that is dedicated primarily to the service of the public.

Sir David Attenborough has been making award-winning nature documentaries for the BBC since Zoo Quest was first broadcast in 1954

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