The BBC, ITV, Channel 4 and Five have begun their joint trial broadcast of HDTV over DTT.

The six-month technical trial is playing out to a closed audience of 450 homes in range of the Crystal Palace transmitter in London.

The BBC's offering will be a simulcast of its new HD channel - also available on satellite and cable - while ITV will broadcast its World Cup games, select dramas and classic films in HD.

Channel 4 plans to offer US imports Lost and Desperate Housewives plus movies from FilmFour. Five will air CSI, original programmes such as Tim Marlow at MOMA and movies such as Cocktail.

Feedback from the triallists, who have been provided with special HD DTT set-top boxes, will help determine whether - and how - to roll out a full HD service via DTT in the future.