So which do you prefer: Strictly or X-Factor? It's the time of year when we define each other by our Saturday night viewing preferences. At work on Monday morning, Bruce Forsyth's charms are measured against Cheryl Cole's and we put the credit crunch briefly to one side to discuss whose rumba stole the show and if Bad Lashes deserved to go.

But another, much drier debate about British television is going on. It might not attract the watercooler crowd, but because watching TV is still this country's number one pastime, people ought to be aware of it and what might happen as a result.

It goes like this. Technology is rapidly changing broadcasting, making it harder for commercial channels to generate income through advertising. The digital switchover will be complete by 2012 and the spectrum that has constrained the number of terrestrial channels will no longer be a commodity for regulators to trade, in return for commitments that don't make a lot of money, but are important to the public - such as children's television, regional news, and current affairs. What is known as public service broadcasting, PSB, is at risk. Does it matter? Yes, not least because the creative industry plays such an important role in the British economy.

The BBC, ITV, Channel 4 and Five make up the PSB family, and all these broadcasters - although funded and structured differently - contribute to an independent, plural British broadcasting culture.

But commercial pressures are forcing them to make choices. Last week ITV said that it might have to give back its public service licence and withdraw from regional news by 2014. Ofcom - the industry regulator - says that Channel 4 needs financial help, and giving it some of the licence fee is one option.

I don't believe that releasing one or more commercial broadcasters from their public service obligations and expecting Channel 4 to join the BBC in meeting all public expectations is realistic. If we as a nation value the sum of their public service broadcasting parts, ITV, Channel 4 and Five all need to stay in the PSB game. The question is: how can the BBC help?

At the BBC Trust - the body responsible for representing the public interest in the BBC and safeguarding the corporation's independence - we accept that there are real financial pressures for the commercial channels. But we don't believe that the answer lies in undermining BBC independence or its contribution to the economic and cultural life of this country.

Taking some of the licence fee to prop up others sounds a simple solution - but it would have serious consequences. Chief among them would be a threat to the independence of all broadcasters and a dilution of public accountability.

For the BBC, the licence fee provides funding unfettered by government and a clear line of accountability back to those who pay - and research shows that they understand where their money goes. Sharing it with Channel 4 and others would inevitably require a new public body to distribute the money: a single institution that broadcasters would seek to please - rather than their audiences - in return for funding. And even if the BBC was excluded from this new quango, what would happen if the demands of its customers outstripped its supply of ring-fenced licence fee? Could it, would it, resist knocking on the BBC's door for a greater slice?

The future funding of public service broadcasting is a matter for government and Parliament. But I see top-slicing as at best a diversion from the serious business of finding sustainable solutions. So if top-slicing isn't the answer, what is?

In short, we need to leverage the strength and financial security of the BBC for wider benefit in ways that do not jeopardise its and others' independence. The BBC delivers £5billion a year for the creative economy - can it deliver more? We think the answer is yes.

Crucially this requires the BBC to get its relationship with the rest of the industry right and to look for new ways to collaborate and seize the opportunities of new technology. Commercial players are sceptical when the BBC extends a hand of friendship, never certain if they will be squeezed to death by the strength of the corporation's enthusiasm. But the BBC is getting better at working with others. Alongside new governance arrangements, it is implementing radical changes to greater public benefit. New formal arrangements that allow independent and BBC producers to compete for airtime are an example.

The Director-General and his team are developing proposals for a sustainable future for PSB. The BBC Trust has challenged them to be bold and ambitious. We want proposals that prospective partners endorse - and don't rule out seeking partners beyond the PSB family.

We want the BBC to build on its long heritage of technological invention for wider benefit. Since the 1960s when David Attenborough supervised the creation of colour television, to Nicam stereo and Freeview, as well as a continuing commitment to share training facilities, the BBC has demonstrated its public service principles.

The BBC has long been recognised as the cornerstone of PSB. This is a moment for it to show its mettle in helping to sustain a system that adds value to all our lives. At the trust, we are pressing it for just that.

Sir Michael Lyons is Chairman of the BBC Trust

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