So just how bad could things get in commercial broadcasting? And when will it start to affect the programmes and employees? The answer at the moment is that nobody knows, but everybody is worried.

Last week saw a spate of announcements from media companies alerting shareholders to the recent fall in advertising revenue, and uncertainty about what the future may hold.

All four of the largest radio groups - Capital, Emap, GWR and Scottish - said advertising had fallen in different periods in the past nine months by anything from 3% to 16%.

Not surprisingly in some cases this will affect the groups' profits, and some share prices went down as a result.

'Positive'

But other companies' shares actually rose, presumably on the basis that things from now on could only get better.

So while GWR's shares, which at one point last year were selling for over �11 apiece, fell to less than �1.60, Emap (whose magazine interests have been just as badly hit by the ad slump) saw its share price rise by 38p to more than �5.20.

None of the groups would hazard a guess at when things might start to get better.

Capital said the long-term prospects for radio advertising remained "very positive", but GWR said it believed "the timing of a return to sustainable revenue growth will happen only when confidence on the political and economic fronts returns".

Mistake

The events of 11 September have made the prospect of a deep recession more likely.

They have also had some direct costs. The GWR stations, for instance, ran without advertising for 24 hours following the attack on the World Trade Centre.

That cost around �100,000 in lost revenue. It might have been more, but most of the ads were simply rescheduled.

Significantly however the message from both Capital and GWR is that cutting the programme budget would be a mistake.

Squeezed

Senior executives say the lesson of previous recessions is that companies which keep spending on programmes and promotion emerge stronger at the end of it.

At GWR Roger Lewis, chief executive of Classic FM, says the start of the downturn was evident late last year: the company cut costs some months ago, notably in its online operations.

Keith Pringle, group programme director at Capital, says the group will shortly announce some big syndicated shows - but they were planned some time ago as a way of increasing audiences and sponsorship revenue; the fact that they will also cut costs is an incidental bonus.

But the problem for all publicly-quoted broadcasters is that they have shareholders to satisfy.

Pressure

Bold talk now of continuing to spend on programmes is commendable, but may prove unsustainable if profits get seriously squeezed.

All the groups will be looking for cuts in "non-core" operations; all may be looking for assets to sell.

The same is true in television, where expenditure on programmes is even greater and some of the major players are under considerable pressure from shareholders - a subject I'll return to next week.

A version of this column appears in the BBC magazine Ariel.