A set-top box is a device which plugs into a television and provides content. Generally speaking, it refers to devices for broadcast content rather than recordable media such as DVDs and videotapes. The name set-top box is somewhat outdated, thanks to the change in the size and shape of televisions in widespread use today.
Historically, the most common set-top boxes were those provided by cable and satellite companies. The main job of these boxes was to deal with the fact that these systems carry more channels than a television can assign to its own channel numbering system. The box therefore takes a signal carrying the data for multiple channels and filters out the particular channel the viewer wants to watch, usually sending the signal to an auxiliary channel on the television. The box can also decode scrambled content, either for a premium channel or for events on a pay-per-view basis.
Modern set-top cable boxes take advantage of the two-way communications available with cable to provide interactive services. At its simplest, this can allow users to order a pay-per-view event without needing to telephone the cable company. At its most complex, this can allow almost complete access to the Internet.
Set-top boxes are now used in many companies even for terrestrial or "over-the-air" broadcasts received through a TV aerial. This is because these broadcasts are available in digital format. As with cable and satellite television, the digital terrestrial system carries multiple channels in the "space" previously assigned to a single channel.
A number of set-top boxes now include some form of digital recording system, which records broadcasts on a hard drive, thus removing the need for recordable media. Such systems are available both as standalone boxes and built-in to a cable, satellite or digital terrestrial receiver. Models vary from those which simply record programs selected from an electronic program guide to those which can automatically record an entire series or even suggest programs the viewer may enjoy.
Modern game consoles can be seen as a form of set-top box. As well as playing games, the consoles offer services such as an Internet connection or playing video content from a linked computer. In some countries, consoles can even carrying live television services via the Internet.
Movie rentals are becoming increasingly common through a set-top box. This offers convenience to the viewer and allows rental firms the ability to cut the costs of producing and distributing discs. Devices including cable boxes, games consoles and standalone rental boxes are all used for this purpose.