GD-ROM is the name of a form of Optical Media created by Yamaha and used extensively and exclusively for Sega for their NAOMI Arcade Board and its Console equivalent: The Dreamcast. THE GD-ROM holds about twice as much data as a regular CD-ROM, but only a fourth of that of a DVD and less than even mini DVDs like the ones used by Nintendo's Gamecube.
The GD-ROM in the Dreamcast works in constant angular velocity (CAV) mode, like the majority of modern optical drives. Sega achieved the higher density by decreasing the speed of the disc to half and by letting the standard CD-ROM components read at the normal rate, thus nearly doubling the disc's data density. This method allowed Sega to use cheaper off-the-shelf components when building the Dreamcast.
There are 3 distinct areas when you look at the bottom of a GD-ROM disc. The low-density inner track (dark gray) contains about 35 Mb of data, which is also accessible by normal CD-Readers. On an ordinary CD player, the track would launch a warning that the disc should be used only in the Dreamcast in various languages. Other variations use the game's character's voices in a lighthearted or humorous message. For example, Skies of Arcadia gives the message "We can't save the world from a CD player! Put us back in the Dreamcast so we can do our job!" The CD section also contains a data segment, readable in PCs. Although most discs include only text files identifying the game, its copyright and bibliography, some contain bonus material for home computer users. For example, Sonic Adventure contains images of Sonic characters to use on the desktop.
The outer, light grey, track contains about 1.2 GB of data, but is written in a high density format which can NOT be accessed by normal CD-Readers.
The black area between the two tracks doesn't contain data and acts like a border. In this ring the following text can be read: "Produced by or under license from Sega Enterprises LTDs or Trademark SEGA". A normal CD-Reader will read the first, inner track and won't read past the black area. Even if it could read past the black area then it can't read the high density track.
The format was developed for Sega by Yamaha. GD-ROM was created because the standard CD-ROM was prone to piracy and reaching the limits of its storage capacity, while implementing the new DVD-ROM technology would have made console production too costly in part because royalties had to be paid to the DVD Forum.
Many first generation Dreamcast Games used one disc, with some RPG using two, but when games became bigger and with more technical details or story elements, more discs were need. Games like Shenmue or Yazuka, with their open world elements use as many as four discs.
As games became bigger and the limits of the format were reached, SEGA reacted by creating more advanced Audio and Video Codec Compression, thus allowing to make more used of the capacities of the disc.
Use in Game Consoles
The GD-ROM was used exclusively by SEGA in both the NAOMI Arcadeboard and the Dreamcast. The GD-ROM even with its limitations was cheap to produce and SEGA was able to provide as many disc as needed, with a limit of five disc per game.