Analog broadcast copy protection

Many mass-produced consumer DVRs implement a copy-protection system called CGMS-A or Copy Generation Management System—Analog. This encodes a pair of bits in the VBI of the analog video signal that specify one of the following settings:

* Copying is freely allowed
* Copying is prohibited
* Only one copy of this material may be made
* This is a copy of material for which only one copy was allowed to be made, so no further copies are allowed.

CGMS-A information may be present in analog broadcast TV signals, and is preserved when the signal is recorded and played back by analog VCRs, which of course don't understand the meanings of the bits. But the restrictions still come into effect when you try to copy the tape onto a PVR.