In the UK the great digital switchover starts in 2008 and is due to finish in 2012. The analogue TV signal will be switched off leaving millions of televisions, video players and equipment obsolete. While the upgrade and migration options for most televisions are clear, the situation with portable televisions is not.

Digital Portable TV may be the toughest challenge in the whole digital switchover.

The whole point of portable TV is you take it with you and watch it where you want. With analogue TV this is pretty straightforward. The set top aerials are able to pick up the analogue signal pretty well everywhere. If you can't get too strong a signal you can still get a picture even though it might have some flicker or interference.

Who hasn't sat in a (beer) tent somewhere trying to watch the Wimbledon singles final on someone's portable TV with what appears to be a snow storm on the screen. The thing is, no matter how poor the signal, you can usually get some sort of picture. And if the event is important enough we are generally prepared to put up with a poor picture as long as we can see something.

With digital the situation is different. There is no gradual deterioration of signal. You either have it or you don't. There is no half way house. If you are in an area of poor reception the chances are you simply won't get a picture at all.

For caravaners and students and others that move around a lot the problem will be obvious from early 2008. If you are staying in the Borders area your portable televisions simply will not work.

All the manufacturers are keen to tout their iDTV (integrated Digital Televisions), HDTV (High Definition Television), PVR (Personal Video Recorders) and of course Sky and Cable Television. So far there is silence on the question of portable televisions.

What does seem certain, with the current low price of portable televisions, is that no manufacturer is going to produce add-on boxes to convert your existing sets to digital. The only viable option is to replace all your portable sets.

Just don't rush out and do it just now ? wait and see what the main manufacturers have up their sleeves. We haven't seen any portable televisions with digital receivers yet.