Recording satellite or digital cable signals on a digital video recorder can be more complex than recording analog signals or broadcast digital signals. A number of different transmission schemes exist, and the video streams are usually encrypted to prevent people from viewing the content without paying subscription fees.
A satellite or cable set-top box does two things. First, it decrypts the signal. Second, it decodes the MPEG stream into an analog signal for viewing on the television. In order to record cable/satellite digital signals you must get the signal after it is decrypted, but before it is decoded (between steps one and two); this is how DVRs built into set-top boxes work.
Cable and satellite providers often offer their own DVRs along with a service plan. These DVRs have access to the encrypted video stream, and will generally enforce the provider's restrictions on copying of material outside the confines of the box.
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