How the News Multiscreen works

BBC Parliament is only available on Freeview in quarter-screen format because it shares its video stream with the BBC News Multiscreens. Channel 45 carries a video broadcast, shown below, made up of three separate streams. The two on the left are for the news service, whilst the larger video on the right is for Parliamentary coverage:

When you tune to BBC Parliament on channel 45, the two news screens are "masked" by a layer of MHEG – digital text – to show this screen:

To show the News Multiscreens correctly, your Freeview box will load a different "mask", showing the two news screens instead. The correct audio stream for each channel is selected by the code in the MHEG layer. Channel 703 does not carry any video itself, only MHEG: