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Components of CA
Common Interface scheme
DVB-Receiver with Common Interface module

The Common Interface is an extensible digital interconnect found in the digital TV market.

All Common Interface equipment must comply with the EN 50221-1997 standard.

The interconnect is formed between a host and a module. Examples of a host are a digital television or digital set-top box. The EN 50221 specification allows many types of module but only the Conditional Access Module (CAM) has found popularity because of the Pay TV market.


Conditional Access Module (CAM)

The host sends an encrypted MPEG transport stream to the CAM and the CAM sends the decrypted transport stream back to the host. The CAM often contains a smart-card reader.

Technical Specification

The common interface shares many features of the PC Card Standard (PCMCIA). By reducing the widths of the address and data busses it has been possible to include a bi-directional parallel transport stream interface.


Transport Stream Interface (TSI)

The transport stream format is specified by IEC 13818-1 and is the MPEG 2 TS format.

Command Interface

In addition there is a command interface for communication between the host and module.

This communication is in the form of a layered protocol stack which allows the host and module to share resources. For example the module can request the current date and time from the host, or can make the host display messages on the TV screen and can then read keypresses from the host remote control. The resource-sharing feature allows the CAM to request and receive PIN numbers as well as allowing the module to implement an interactive TV service.

The Command Interface is extensible and there are several specification documents available which describe these extensions. For example the host could contain a modem for communication over a telephone line allowing the CAM to implement pay-per-view. However these extensions have often not proved popular with manufacturers.

See also

External links


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