On 20th April 1964, the BBC launched 'BBC2' on analogue television in London. It subsequently became the first channel to regularly broadcast in colour. Now, just under 48 years later, the analogue BBC Two signal in London is about to be turned off for good.
Just after midnight, and likely to be at the end of the programme about Wikileaks at 00:20, the BBC Two analogue service will be taken off air. Shortly thereafter, ITV1 London in analogue will resume on the BBC Two frequency until switchover is completed at stage 2 on the 18th. The frequency used by ITV1 London in analogue will be closed down and reused for high powered digital transmission of the BBC channels.
Meanwhile, engineering work at the Crystal Palace transmitter will take place so that by no later than 6am, the new high powered BBC service is up and running.
Unfortunately, it is not possible to just switch the analogue transmitter off and the digital transmitter on. Cables and pipes connected between the transmitting antenna and the transmitter equipment (itself consisting of large units) need to be worked on, and signals rerouted correctly at the appropriate powers. Additionally, the old equipment needs powering down, and the new equipment powering up. This means that there will inevitably be breaks in normal terrestrial TV service - both analogue and digital - overnight. Channel 5 on analogue may well get away with the least disruption, as it is broadcast from neighbouring Croydon transmitter.
For full information about London's Digital Switchover - see the special subsection of this website, plus a full timeline of events.