Set Top Box - Online Article



With the onset of cable television and satellite television came a demand for another type of small computing device called a set-top box. A set-top box is the device that connects a television to a cable signal or satellite signal received from a service provider. Set-top box technology has evolved with the increasing demand by consumers for television and related services such as email and video on demand. The first-generation set-top box had a simple function: it received a scrambled analog television transmission from the service provider, unscrambled the signal, and sent the unscrambled signal to the television.

The next generation set-top box enabled two-way digital transmissions between the service provider and the consumer. A digital incoming signal from the service provider contained the television signal, and the outgoing signal from the customer carried requests for service, such as pay-per-view access to programming.

The latest in set-top box technology enables service providers to offer fully interactive services that include video on demand, interactive advertising, TV-centric applications, email, and Internet access. Set-top boxes have progressed from being a simple embedded signal decoder to a dedicated personal computer.

There are three categories of set-top boxes: broadcast TV, enhanced TV, and advanced services. A broadcast TV set-top box provides traditional broadcast television and has no return channel. Signals are received in an MPEG-formatted data stream. An enhanced TV set-top box is similar to a broadcast TV set-top except the enhanced TV set-top box has a return channel from the customer to the service provider. An advanced services set-top box is basically a dedicated personal computer that has sufficient computing power to provide rapid processing for interactive, multimedia services.

Inside Look at a Set-Top Box

All modern set-top boxes perform five common operations. These are to decode a digital signal received from the service provider, authenticate access rights, transmit a signal to a television, transmit audio information to create surround sound, and provide interactive services such as access to the Internet and email.

Multiple signals are received by the set-top box from the service provider, each of which is transmitted over its own communications channel. The tuner circuit within the set-top box filters all communications channels except the channel selected by the consumer.

The signal from the selected channel is sent to the demodulator circuit. The demodulator circuit is a chip that converts the signal into binary data before sending the binary signal to the demultiplexer chip.

The demultiplexer has a number of functions within the set-top box. First, with the assistance of the built-in security system, it determines whether the consumer has the right to access the service transmitted on the selected channel. The consumer is notified if access to the service is denied; otherwise the demultiplexer separates the binary signal into a video signal and an audio signal and then forwards each to the proper decoder circuit. The decoder circuit transforms each signal into a signal used to display the video and replay audio.

Set-top boxes are controlled by an operating system that is usually proprietary, although some manufacturers use a third-party operating system for their set-top box.

The most commonly used operating systems are PowerTV OS, VxWorks (also used in cellular phones and car navigation systems), pSOSystem, DAVID OS-9, Windows CE, ChorusOS, JavaOS (also used for automobile computers and private telephone systems), and Linux.

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