CGA and EGA monitors are digital, rather than analog like televisions
and more modern monitors, usually making them incompatible with TV.
Television signals contain all colour information along with syncs on
one conductor. In addition, there are two types of television signals
- the RF that comes in from cable or an antenna, and composite. The
line-in/out on a VCR is a composite signal, and doesn't contain all of
the different channel information that an RF cable signal does.
The original CGA monitors accept a composite signal, but it is TTL,
which uses a different voltage from composite. Some CGA (and
perhaps EGA?) monitors have composite-in jacks and circuitry inside
them to display a composite signal. If you have one of these, then
you can feed it a composite video signal from a VCR, laser disc
player or other composite video source.
Since the VGA/SVGA monitor was introduced, computers have used an RGB
video signal, with separate horizontal and vertical syncs. This means
that five separate wires are used to carry the video signal from the
computer to the monitor. In order to display a TV signal on a VGA
monitor, signals for all five wires have to be derived from one, the
so-called composite TV signal. This involves some electronic circuitry,
so it can't be accomplished simply by attaching all of the wires
Because of the demands of higher pixel addressabilities and refresh
rates, VGA and newer monitors run at horizontal refresh rates of
30 kHz or higher, which is double that of composite video (15.7 KHz).
Basically, these newer monitors are unable to sync to a low enough
frequency to display broadcast (NTSC or PAL) video. The end result
is that it is not feasible to use a VGA or better monitor to display
a television signal. The only real alternative is to purchase a
TV card for your computer which allows you to display a television
signal on your monitor. Personally, I'd rather spend the money on
a small TV rather than look at a four inch window on my already
cramped computer monitor.