[This is preliminary documentation and subject to change.]
The broadcast receiver card must be able to receive both standard broadcast information from broadcast networks and data stream information, as defined by the MSBDN format for data transmission, which supports Multipacket Transport (MPT) and Internet Protocol (IP). For more information on this format, see the MSBDN Receiver section of the Broadcast Architecture Device-Driver Kit (DDK), part of the Windows 98 DDK.
Data streams are likely to be on channels, for example cable and Multichannel Multipoint Distributed System (MMDS) channels, separate from video and audio streams. Similarly, data streams are likely be on transponders, for example transponders for digital broadcast systems, separate from video and audio streams. This separation being usual, the broadcast receiver card requires two paths and signal lines. This section refers to these two paths as the "digital audio-video signal path" and the "digital data signal path." The following sections describe these signal paths separately to clarify their differences.
It is anticipated that hardware vendors will eventually combine both signal paths on the same PCI-bus card and also provide two separate tuners on one PCI-bus card. Such a combination means control, interface, bus mastering, power, and antenna connector resources can be shared. However, early versions of the broadcast receiver card need include only one tuner.
One-tuner versions of the card must receive both data and audio-video signals without user intervention. However, one-tuner cards need not receive both data and audio-video signals simultaneously, unless the signals are broadcast on the same channel or transponder. This reception functionality requires the card to include both network access control circuitry and MSBDN circuitry. The receiver card can also include a smart card as appropriate.
The digital audio-video signal path contains network-specific technology for tuning, demodulating, decoding, error-correcting, demultiplexing, decrypting, and controlling access to digital audio and video signals.
The digital audio-video signal path must be capable of receiving at least four substreams simultaneously, such as video, audio, data, and Program Guide substreams. The design of the receiver card must allow transfer of these streams to computer memory with very low CPU utilization (less than 10 percent). Such a design ensures that the transfer does not interfere with the performance of applications running on the computer. This design implies that the card uses bus mastering or other direct memory access techniques.
Certain network designs mandate that particular access control functionality be resident in hardware. Such access control functionality controls some types of interaction with the consumer, for example preventing certain users from viewing video of specified ratings. In the broadcast client system, the software that verifies access for a particular network is divided into a number of parts. Security functions typically run on a microprocessor on the receiver card. The user interface and any modem interface functions are implemented in the computer. The different verifier portions communicate with each other through Broadcast Architecture driver interfaces; these interfaces manage verifier communication with the user and the network authorization center.
Digital audio-video data is tuned, demodulated, and error-corrected by a module referred to as the transport. The transport then routes the data to circuitry that selects portions of interest from the total stream. These portions may be identified by packet identifiers (PIDs). Information of interest can be passed through MSBDN circuitry for Data Encryption Standard (DES) decryption. After any required decryption processes, data is bus mastered into CPU memory.
Note that not all data is encrypted by the sender. Such data can bypass the circuitry for DES decryption.. It is also possible to take selected data directly from the error correction circuitry and bus master it into memory.
The signal path that receives digital data is similar to the existing audio-video path, but it must meet the extra requirement that the digital data signal path must be able to demultiplex a packet stream into one or more substreams. In other words, it must be able to separate several combined data streams. During this demultiplexing process, the path must be able to filter out unwanted packets based on a field in the packet header.
For a more complete description of the data receiver and of required decryption functionality, see the MSBDN Receiver section of the Broadcast Architecture Device-Driver Kit (DDK), part of the Windows 98 DDK.