Gemstar is the brand name for the extended data service hardware format
that sends out the tv show information and tv guide information on
specific tv channels during the blanking interval.

Television sets that include Tv Guide plus have all the Gemstar
features installed. Tv sets with the XDS only have the extended data
feature enabled that gives the show title, run time, and maybe some
other minimal information.

The Gemstar circuit can be implemented in a separate module, within the
system controller ic, or on a separate data controller ic near the
system control ic.

The problem is the Gemstar implementation has some software bugs that
are expensive to fix and each new implementation seems to have a new
software bug. Add the fact that the tv stations sometimes send out
incorrect data on line 21, or it gets distorted on the signal, and
there is a large chance for error.