A set-top box (STB) or set-top unit (STU) is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device. Set-top boxes are used in cable television and satellite television systems, to transform the signal from the cable or satellite to a form that can be used by the television set or other receiver. It also enhances the quality of signal from cable or satellite.

Before the mid-1950s, all British television sets tuned only VHF Band I channels. Since all 5 Band I channels were occupied by BBC transmissions, ITV would have to use Band III. This meant that all the TV sets in the country would require Band III converters which converted the Band III signal to a Band I signal. By 1955, when the first ITV stations started transmitting, virtually all new British Televisions had 13-channel tuners, quickly making Band III converters obsolete.