which company manufactured them?
Sega.
Sega CD
The Sega CD, released as the in most regions outside North America and Brazil, is a CD-ROM accessory for the Mega Drive/Genesis designed and produced by Sega as part of the fourth generation of video game consoles.
what is 'sega cd'? A  CD-ROM accessory for the Mega Drive/Genesis .
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Sega CD [SEP] Introduction
The Sega CD, released as the in most regions outside North America and Brazil, is a CD-ROM accessory for the Mega Drive/Genesis designed and produced by Sega as part of the fourth generation of video game consoles. It was released on December 12, 1991 in Japan, October 15, 1992 in North America, and April 2, 1993 in Europe. The Sega CD plays CD-based games and adds hardware functionality such as a faster central processing unit and graphic enhancements like sprite scaling and rotation. It can also play audio CDs and CD+G discs. The main benefit of CD technology was greater storage, which allowed for games to be nearly 320 times larger than Genesis cartridges.