Analog television
Analog television in NTSC, PAL or SECAM formats, analog cable, or regular VHS
tapes use a signal that is fed directly to the electron beam within the
television set. There are a number of details on how this is done, but in
essence each line in each frame corresponds to a specific fraction of time
within the signal.
To record an analog signal a few steps are required. A TV tuner card tunes into
a particular frequency and then functions as a frame grabber, breaking the lines
into individual pixels and quantizing them into a format that a computer can
comprehend. Then the series of frames along with the audio (also sampled and
quantized) are compressed into a manageable format, like MPEG-2, or WMF, usually
in software. Some TV tuner cards like the DVR-250/350 or the TiVo chip deliver
an MPEG-2 or other compressed stream directly to the computer, performing both
the frame grabbing and compression in hardware. This greatly reduces the load on
the CPU allowing an overall cheaper implementation.
Analog broadcast copy protection
Many mass-produced consumer DVRs implement a copy-protection system called CGMS-A
(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.
