SET TOP BOX ARCHITECTURE

Firstly, We start with defining that what actually is a Set top Box.

Definition:-

A set-top box (STB) or set-top unit (STU) is an information appliance device that generally contains a tuner input and displays output connects 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.

Architecture:-

The STB selects the appropriate broadcast TV information by tuning to one of many input channels. The signal is digitally modulated using Quadrature Phase Shift Key (QPSK) for satellite applications, Quadrature Amplitude Modulation (QAM) for cable and Orthogonal Frequency Division Multiplexing (OFDM) for terrestrial. The information in the selected RF channel is then processed by the demodulator to produce an MPEG-2 Transport Stream (TS) containing the audio, video and other information that relates to the selected TV programme.

The STB generally also contains some form of modern to allow it to send and receive interactive data. Conventional telecommunication modems are typically used in satellite and terrestrial STBs while cable STBs generally have a cable modem. DOCSIS cable modems use QAM demodulator for the downstream data whilst out-of-hand DAVIC cable modems use QPSK demodulator. In both cases, a QPSK modulator is used to transmit the upstream data, though DOCSIS also have a 16 QAM mode.

In general, digital TV information in the MPEG-2 TS may be encrypted to present customers who have not paid for a particular service from being able to view it. The MPEG demultiplexer selects

and decrypts the compressed audio and video for the particular programme that the viewer wishes to watch, using decryption keys supplied by the Conditional Access Sub System (CASS). The MPEG decoder then compresses the audio and video information for the selected programme. The Central Processing Unit (CPU) controls the whole operation and performs specific data manipulation function. It generally uses a Real Time Operating System (RTOS) on top of a hardware abstraction layer for the management of the resources and processes of the STB directed by the higher level software.

It is thus obvious from this diagram that, the front-end, which contains the tuner and the demodulator, will be different for the three transmission media. Hence if a STB is to be made interoperable across all the three transmission media it should be fitted with switchable front ends.

Whenever an MPEG-2 TS carries encrypted (Or scrambled) services, the TS also carries two types of messages called EMM (Entertainment Management Message) and ECM (Entitlement Control Message). An EMM carries a list of Pay TV services which the owner of that STB is entitled to view and also the date upto which he is entitled to receive them. The ECM on the other hand carries a data element called control word (CW), which is used by the ‘descrambler’ in the STB to descramble the picture and make it intelligible again. Both these messages are carried in the TS in an ‘encrypted’ form. Whereas DVB has standardized the scrambling algorithm (known as DVB common scrambling algorithm, DVBCSA), algorithms used for ECM/EMM encryption are not standardized for obvious reasons.