The Sega CD was the first planned add-on released for the Sega Genesis. The Sega CD allowed the users to play Sega CD games and audio CDs. The drive is a 1x speed disc drive. The CD drive is accessible when the cartridge slot doesn't contain a Sega Genesis game within it. The Sega CD was an attempt to compete with the Turbo-Grafx-CD, and the concept of using CD's for the media was conceived in order to get around the space limitations of the cartridges that had been used up to that point. With CD's developers could put much more game data on, as well as include full-motion video clips and CD quality audio soundtracks. The Sega CD contained internal memory (64Kb, or 125 Game Save Blocks) in order to save games.
The original Sega CD was designed to fit with the Genesis 1 console and sat underneath the system and had a motorized disc tray on the front. The second version fits with the Genesis 2 side by side and featured a manual top loading door that allowed the user to insert discs. Both versions pass the video for the console through to bottom of the Genesis so there is no need for a separate video/audio cable cable to connect the two. The Sega CD however does require it's own power source which means having and additional AC Adapter in order to play Sega CD games. Both versions also work with the Genesis model 1, but the second Sega CD model doesn't fit as nicely on the system.
The big selling point for the system was the full motion video games, aka interactive movies. Sega used these games to showcase what the system could do, and initially consumers and reviews praised these titles. However as the amount of these games increased and the novelty wore off many reviewers changed their initial impressions and these games are no commonly remembered for the poor quality of the videos which were heavily dithered and usually limited to a rectangle in the center of the screen about 1/4 of the full screen size. Many factors contributed to the low quality video including the system's limited 512-color palette and the processor not being well-suited for video, which resulted in grainy images and a low framerate as well as a low resolution in most games.
Despite a couple titles really using the additional space to cram in extra features and a new soundtrack as the system was intended for, many developers simply used the Sega CD to resell Genesis games with very minor improvements.
Sega also released a console entitled the CD-X which was meant to be a portable version of the Sega CD. It resembled a portable CD player like the Sony Discman that was popular around that time and could even be used as a portable CD player as it featured all the CD controls as buttons, a headphone jack, and ran on batteries. However the system was quite bulky compared to a discman as it also featured the standard console hardware including the Genesis cartridge slot, AC adapter and composite video output, as well as two controller inputs.
Later Sega allowed JVC to produce the JVC X'Eye systems that would play Sega Genesis and Sega CD games.
Several accessories were created for the Sega CD. The most important being a memory card named the “CD Backup RAM Cart” that allowed the users more rooms to save games and the ability to move games back and fourth from the cart and the consoles internal memory. The card could hold 1 Mbit, or 2045 Game Save Blocks. Sega also released a light gun named "The Justifier" bundled with Lethal Enforcers. This was supported by few Sega CD games including Lethal Enforcers 2, and Snatcher.
In 2006 a third party developed a new cartrdige which could hold saved games, other data, and allowed a system to play games from any region. This was called the Mega Cart.
Getting a Sega CD.
It's no longer easy or cheap to find any version of the Sega CD. The seperate add-ons themselves are quite tough to find and when a Genesis/CD bundle is found it usually goes for well over $100. The CDX and JVC X'Eye also go for even more usually being closer to $200.