HDMI modulators convert HDMI signals into RF frequencies for distribution through a TV aerial system. An HD modulator will utilise a vacant channel within the Freeview HD spectrum and add the HDMI input so that it can be viewed on any Freeview HD TV or set top box, just like any normal off-air transmission. Once the TV is tuned to the new channel, you can view whatever HDMI source you wish to distribute, such a Blu-Ray, DVD Player, Sky HD etc. The modulator can be connected to a single TV or it can be routed through your existing aerial system so that it is available to view on any TV in the house. An AV modulator does the same but has only AV inputs rather than HDMI and so produces SD images.