This article is from the Satellite TV FAQ, by Gary Bourgois email@example.com with numerous contributions by others.
ABC is almost entirely scrambled, with some feeds in the clear. NBC is
Presently totally unscrambled, as is FOX. CBS is usually scrambled, but
they still have quite a bit in the clear. The scrambling used
by the networks is such that you can not receive it with a home decoder.
Totally different methods are used. CBS uses VIDEOCIPHER _ONE_ (completely
different than the VCII used in home systems). ABC uses LEITCH for its
scrambled feeds. Networks are scrambled to protect the affiliates. Your
local station does not want you watching the network shows direct from
the birds with 400+ lines of resolution and discrete stereo sound, which
anyone would do given half a chance. They want you to watch the LOCAL
commercials, so convinced the nets to scramble their feeds. For those
who can not receive the networks over the air, the only choice is to
subscribe to NETLINK's DENVER FIVE package or The Atlantic Three or
PRIME TIME 24.
Interestingly, network NEWS feeds are usually in the clear! If you are a
news hound, this will be GOOD news for YOU.
The PRIME TIME ENTERTAINMENT NETWORK (PTEN) backhauls its programs
several days before they are broadcast on the over the air TV channels.
These transmissions are in the clear, as are most SYNDICATED feeds.
PARAMOUNT feeds (Deep Space Nine) are not scrambled. Same with all other
UPN (Star Trek Voyager, and other prime time programming) is all in the
clear, and they also run pre feeds.
WB (Warner Brothers Network) is not scrambled.
PBS is not scrambled on their C band feed.