A wonderful thing that every
games console should have. Since Europe uses the
PAL TV system (50Hz), and America and Japan use
NTSC (60Hz), PAL games consoles must run 20% slower than their NTSC counterparts. This majorly annoys everyone in Europe, and persuaded them to find ways of making their consoles run at the propper speed. In some systems (such as the original
Mega Drive,
Sega Saturn, and
Neo-Geo) the speed is set by a block of
jumpers, so the change can be made simply by cutting one link, and making another. Others, such as the
SNES, and
Mega Drive 2 require that a pin on a chip be lifted, and either tied to
ground or
+5V. Once this mod is made, you have a fully functional 60Hz console. However,
Nintendo UK, being the bastards that they are, decided it would be fun to make games detect the speed at which the console ran, and refuse to work if all was not as expected. To counter this, the mod is made reversable by means of a
switch, often mounted on the consoles casing. (In all fairness, it wasn't just Nintendo that had this bright idea. Mega Drive games by
Konami, and some by
Sega have this nice "
feature".)
Consoles of recent years (eg Sony PlayStation, and Sega Dreamcast) use software switches to choose the speed. Therefore, the speed at which your system runs is determined by the games. This is why it's such a good idea to by all PS games on import. Sega have been nice enough however, to include a 60Hz mode as a selectable option in many (but not all unfortunately) UK Dreamcast games, lending them some extra favour with gamers.
See also: RGB mod and lock-out bypass.
As a source for these mods (until I steal some of 'em), Game Station X http://www.gamesx.com/ is highly recomended.