A set-top box (STB) or set-top unit (STU) (one type also colloquially known as a cable box) is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of signal, turning the source signal into content in a form that can then be displayed on the television screen or other display device.
They are used in cable television, satellite television, and over-the-air television systems, as well as other uses.
In Europe, a set-top box does not necessarily contain a tuner of its own. A box connected to a television (or VCR) SCART connector is fed with the baseband television signal from the set's tuner, and can have the television display the returned processed signal instead.
Pace Micro Technology DC757X Set top box
This SCART feature had been used for connection to analogue decoding equipment by pay TV operators in Europe, and in the past was used for connection to teletext equipment before the decoders became built-in. The outgoing signal could be of the same nature as the incoming signal, or RGB component video, or even an "insert" over the original signal, due to the "fast switching" feature of SCART.
In case of analogue pay-TV, this approach avoided the need for a second remote control. The use of digital television signals in more modern pay-TV schemes requires that decoding take place before the digital-to-analogue conversion step, rendering the video outputs of an analogue SCART connector no longer suitable for interconnection to decryption hardware. Standards such as DVB's Common Interface and ATSC's CableCARD therefore use a PCMCIA-like card inserted as part of the digital signal path as their alternative to a tuner-equipped set-top box.