Sega's FM Sound Unit for its Mark III console is a fine example of a manufacturer augmenting the capabilities of its systems post-launch. Making use of the Yamaha-made YM2413 FM chip, it added nine extra mono sound channels to the output from the console's built-in SN76489 PSG chip, massively improving its audio prowess.
Sadly, Sega decided against including the FM Sound Unit in the international version of the Mark III, the Master System (although it was built-in on the Japanese variant of the Master System) – so the wonders of FM audio have been largely confined to that part of the world – until now, that is.
Profliic modder Tim Worthington recently recreated the module via a Complex Programmable Logic Device (CPLD) and published the schematics online. Now, Mathijs Nilwik of SYF, a company which produces replacement parts and upgrades for Sega and Nintendo consoles, has turned those schematics into mass-produced reality – say hello to the GGFM, a mod which allows you to benefit from FM audio when playing Master System games on your Sega Game Gear.
The GGFM fits on top of the GameGear cartridge slot. The FM audio is then 'injected' using the TV tuner input of the cartridge. Notable games which make use of the unit include Phantasy Star, After Burner, Double Dragon, R-Type, Thunder Blade and Ys: Vanished Omens.