Before broadcast can start, the cart which is to start the broadcasting sequence must be selected by clicking on it.
Cart selection is shown by a yellow frame :
Click on the "Play" button 
,to broadcast the selected cart : its frame flashes red.
When a cart is on air, a progress bar shows how far the broadcast has gone.
NB: During broadcast, the cartstack scrolls automatically and always sets the on-air cart in the second position.