Digital channel CBBC has already launched

The birth of BBC2 was preceded by test transmissions and a publicity campaign employing a kangaroo with a baby in its pouch - representing the new channel.

The launch day was 21 April 1964, a day later than scheduled thanks to a power cut, and the first programme broadcast on the station was Play School at 1100.

The new channel was intended to be a place for more demanding cultural programming, documentaries and other experimental programmes.

There was also Newsroom, a half-hour news programme - then a first.

'Think'

Advance publicity for BBC Four echoes some of the ideals that went in to the creation of BBC Two. Controller Roly Keating has told viewers to expect "the best in contemporary documentary, music, theatre or international cinema".

And the new channel's slogan is "Everybody needs a place to think".

Monty Python began on BBC Two in 1969

But digital TV is still only available to a minority of UK households.

Similarly, when BBC Two launched relatively few households had sets which could receive the 625-line system.

And by the end of the 1960s only 2% of households could receive colour pictures - another innovation which BBC Two introduced in 1967.

In 1969 all ITV studios switched over to 625-line production, signalling the impending demise of the 405-line system.

Analogue

But in a precedent which may worry the proponents of digital TV, it was not until 1985 - 21 years after BBC Two's launch - that the 405-line system was finally switched off.

The government maintains that it still hopes to switch off analogue TV signals by 2010 at the latest.

Already some 40% of UK homes - around eight million in all - are able to access digital TV by cable, satellite or terrestrial means.

But the BBC Two experience suggests that eight years to convert the rest of the country may be a rather tight deadline.