A digital transport stream typically comprises scrambled audio/video/data content packets and scrambled conditional access messages (ECM, EMM) which have to be processed in order to extract control data (CW); the latter being necessary for the multimedia unit to descramble the content packets relating to the event (e.g a broadcasted program or a set of programs). Conditional access messages include two kinds of information, namely Entitlement Control Messages (ECM) and Entitlement Management Messages (EMM). The ECM is information relating to content packets, whereas the EMM is information dedicated to an individual end user (subscriber) or a group of end users. The ECM contains the access conditions for the current broadcast event together with control word (CW) for unscrambling this event. The Control Word (CW) is a key used for decrypting content packets of a packetized event broadcasted within the data stream. Thus, each ECM is specific to each event (e.g. a TV program). The EMM is a message used typically by the security module to set, reset or change product access entitlements, credit, etc. . . . Thus EMM refers to the rights (access data) of the subscriber for accessing to the content (events).
There is two main types of set-top-boxes known by the prior art. The first type comprises set-top-boxes that can generally receive broadcast unencrypted events, and which therefore do not require any access control. As internet access is widespread at user homes, such set-top-boxes are increasingly provided with an IP-connection which allows the end user to use his set-top-box together with his television screen as a terminal for internet facilities.
The second type refers to set-top-boxes provided each with a security module. These set-top-boxes are typically used by Pay-TV systems for processing selective access to broadcast services on a per-subscriber basis managed by a conditional access system (CAS). Such a security module typically refers to a smart card (chip card) which can be inserted into a slot of a conditional access reader. This card reader can be connected to the multimedia unit or be lodged directly within this unit. Thus, the security module becomes a necessary device for decrypting the scrambled content packets of the digital transport stream. To this end, the conditional access messages contained in the transport stream are routed to the security module which has the keys to decrypt the ECM in order to extract the control word (CW) which then will be used by a descrambling module (within the multimedia unit) for descrambling the content packets. This process is achieved only if the conditional access rights (provided by the EMM) are valid and checked by the security module.
The problem with set-top-boxes of the first type is that they are often unsuitable for processing conditional access messages required for descrambling audio/video content packets within a Pay-TV environment. This is true even if they are provided with an IP-connection. Thus, a user wanting to subscribe a subscription for having access to Pay-TV programs has to change his set-top-box with another one that supports conditional access processing.
On the other hand, the set-top-boxes of the second type, provided with a security module, are dependent on this module for descrambling the content packets of the transport stream. It means that the security module of the set-top-box is required for descrambling purposes. If this security module becomes unavailable (for instance because the required smart card has to be changed to a renewal or for any other reasons), this set-top-box becomes at least temporarily unavailable for receiving Pay-TV programs.
Accordingly, there exists a need for improving processes and multimedia units (e.g. set-top-boxes) in order to optimize the processing of control data for descrambling the content packets of the transport stream, for instance by adapting this processing to different circumstances, depending on the availability and the adequacy of the network and/or depending on the material configuration at the end user.