The BBC, which ultimately broadcast high-definition television on Freeview in the DVB-T2 format adopted in 2009, initially considered an alternative method utilising spare capacity at night to allow the download of high definition programmes for later replay. In addition the BBC's Research and Development team invented a transmission method that doubles the effective bandwidth that could currently be delivered by each multiplex using "spatial multiplexing". While conventional analogue and digital terrestrial television signals are transmitted either vertically or horizontally polarised (providing protection against interference from other distant signals which may be transmitted on the same frequency, but will usually be planned to use the opposite polarisation), the proposed technique would have involved using the same frequency to transmit two different signals at once, one vertically and one horizontally polarised. A special receiving aerial, containing both vertically and horizontally polarised elements, could receive both signals simultaneously and feed these to a suitable set top box, but would require new transmitters and receiving equipment and aerials; the technique was ultimately not adopted.