some devices that connect to a tv, such as vcrs, older video game consoles and home computers, output a composite signal. this may then be converted to rf with an external box known as an rf modulator that generates the proper carrier (often for channel 3 or 4 in north america, channel 36 in europe). sometimes this modulator was built into the product (such as video game consoles, vcrs, or the atari, commodore 64, or trs-80 coco home-computers) and sometimes it was an external unit powered by the computer (in the case of the ti-99/4a or some apple modulators) or with an independent power supply.