The Sega CD is a video game console add-on for the existing Sega Mega Drive. It was originally released as Mega-CD. It allows users to play CD-ROM games.
Sonic the Hedgehog CD was exclusively released for the Mega-CD, although it has since been released as part of compilation games for PCs. The Mega-CD has many other games, such as the controversial Night Trap and Time Gal video games.
Sega also released a combined, semi-portable Genesis/Sega CD unit called the Sega Genesis CDX (Sega Multi-Mega in Japan and Europe). This unit retailed at a lower price than the individual Genesis and Sega CD units put together, but is incompatible with some games. The CDX features a small LCD screen that, when the unit is used to play audio CDs, displays the current track being played. The CDX can be used as a standalone CD player powered by 2 AA batteries, but must use an AC adapter when being used to play video games.
- Sega CD Model 1 - Original model, constructed as a base for the Mega Drive/Sega Genesis Model 1, although it is compatible with the Model 2 as well. The Model 1 has an automated CD tray, although this is prone to hardware failure.
- Sega CD Model 2 - Revised model constructed for increased compatibility with the Mega Drive/Sega Genesis Model 2, although it is compatable with the Model 1 as well. This model relocates the CD-drive from underneath the console to the console's side, and features a spring-loaded lid rather than an automated CD tray, greatly increasing its hardware reliability.
- Sega CDX - a combined, semi-portable Genesis/Sega CD unit called the Genesis CDX (Multi-Mega in Europe) bundled with Sonic CD, Sega Classics Arcade Collection, and the Sega CD version of Ecco the Dolphin.
- Victor Wondermega - a combined, non-portable Genesis/Sega CD unit released by third-party company, JVC in Japan in 1992. The only officially-released Genesis console to feature S-video output. Later released in North America as the JVC X'Eye without S-video Output in 1994.
- LaserActive - a Laser disc player released by Pioneer in 1993. The device featured modules such as the Sega Genesis and CD modules that allowed it to play said titles. Unlike other units, the LaserActive is not compatible with the Sega 32X.
- Unlike the Sega Genesis and 32X, which largely had region-free games with a few exceptions, Sega CD games are always region-locked.
- There are 220 games released for the Sega CD. There are six titles that were also released in formats that used a combination of the Sega CD and the Sega 32X.
- The Sega CD contains a small amount of internal memory for saving games. This internal memory is too small for saving a large amount of games, leading Sega to eventually release a memory back-up cartridge to handle additional save files.
- The Sega CDX's compatibility with the Sega 32X is very nebulous. Instructions packaged with the 32X depict compatibilty using an additional adapter piece, which never saw release. Even without the adapter piece, the 32X is indeed compatible with the Sega CDX, although some sources claim voltage issues make this a risky combination. JVC's Wondermega/X'Eye is also compatible, although the 32X will obstruct the console's lid from opening.
- The Sega CDX's shell is compatible with the Sega Power Base Converter, allowing the console to play Sega Master System games. Although the Power Base Converter can functionally work with JVC's Wondermega/X'Eye, it cannot physically attach to the unit due to the shell's construction causing obstruction.