Digital terrestrial television (DTT) is a way of sending digital signals - pictures and sound - to an aerial. A process called "digital multiplexing" squeezes the signal so that it takes up less space than a traditional analogue signal.
A set-top box on the viewer's television set decodes the digital signal received by the aerial so it can appear as pictures and sound. The sound and picture quality should be better than it is with an analogue signal; the households participating in the Government's trial reported high rates of satisfaction.
DTT will be rolled out to households across EU member states by 2015 in order to free up the analogue spectrum for broadband internet and other technology.
In Ireland, the Government plans to switch off analogue services in 2012. From that point on, television services will no longer be available through an aerial without a set-top box.
People who do not already pay for satellite and digital cable services will be left staring at a blank screen if they do not get a set-top box.
So that viewers don't lose out, RTÉ is to run a free-to-air digital multiplex, which will eventually hold up to eight channels. The Broadcasting Commission of Ireland has awarded a licence for three commercial multiplexes to Boxer DTT, a consortium backed by businessman Denis O'Brien.