Televisions in India have, over the past week, dished out a fare that pales in comparison to what viewers have been accustomed to, Star Plus, Sony, Zee, HBO, Star Sports, ESPN and Cartoon Network, to name a few, have been yanked off the air. Under the Conditional Access System (CAS), you need a set-top box (STB) to receive these pay channels. This is a big change as far as TV viewing habits go. It also comes at a stiff upfront outlay to install the STBs. Also, you get each pay channel at Rs. 5/-.

A conditional access system (CAS) is a system by which electronic transmission of digital media, especially satellite television signals through cable, is limited to subscribed clients. This is called conditional access. The signal is encrypted and is unavailable for unauthorised reception. A set-top box containing a conditional access module is required in the customer premises to receive and decrypt the signal.

A conditional access system comprises a combination of scrambling and encryption to prevent unauthorised reception. Encryption is the process of protecting the secret keys that are transmitted with a scrambled signal to enable the descrambler to work. The scrambler key, called the control word must, of course, be sent to the receiver in encrypted form as an entitlement control message (ECM). The CA subsystem in the receiver will decrypt the control word only when authorised to do so; that authority is sent to the receiver in the form of an entitlement management message (EMM). This layered approach is fundamental to all proprietry CA systems in use today.