I would like to post a clarification to Installment 008 (http://thedigitallifestyle.com/cs/blogs/custom/archive/2009/03/27/the-custom-integrator-show-installment-008.aspx) regarding the architecture for the Digital Cable Tuners.  In the section labeled THE ARCHITECTURE, the process for determining if the customer is subscribed to a certain channel is not quite right (just below the DRI on USB diagram).  The following clarification explains it more appropriately:

DCT/CableCARD Operation

    1.  When a CableCARD is present, the Virtual Channel Table (VCT) gets downloaded into the DCT through the separate OOB/FDC tuner.  The VCT contains a lookup of Virtual Channel Numbers (VCNs) and actual frequencies used by the associated channels.

    2.  When the CableCARD is present, the tuner no longer tunes by frequencies passed in via the tune request.  Instead, Windows Media Center passes a Virtual Channel Number to the tuner, which then gets mapped to a frequency, modulation, and program ID based on information in the VCT.

    3.  The CableCARD then determines if the service is included in customer’s subscription based upon the additional information provided by the Entitlement Control Messages (ECMs) and Entitlement Management Messages (EMMs) sent down from the head-end.  If the channel is Subscribed, the CableCARD decryption is enabled for the selected program identified in the Transport Stream (TS).

As an FYI, the EMMs (“Hits”) are received via the OOB/FDC tuner and tie the ability to view encrypted subscriptions to the Host tuner.  The ECM is sent inside of the individual channel’s Transport Stream.  The ECM itself is the key needed to decrypt the channel, which is rotated every few seconds to eliminate hacking.  When the CableCARD attempts to decrypt a channel, it first decrypts the ECM to get the key that allows it to decrypt the MPEG-2 stream.  Naturally, this is integrated closely with the headend supplying the streams and the necessary keys.

I apologize for any confusion.  I like to make sure the information I provide is accurate.



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