GD-ROM (an abbreviation of "Gigabyte Disc Read-Only Memory") is a proprietary optical disc format originally used for the Dreamcast video game console, as well as its arcade counterpart, the Sega NAOMI and select Triforce arcade board titles. Developed by Yamaha, Sega intended to use the format to curb piracy common to standard compact discs and to offer increased storage capacity. It is similar to the standard CD-ROM except that the pits on the disc are packed more closely together, resulting in a higher storage capacity of 1 gigabyte, a 42% increase over a conventional CD's capacity of 700 megabytes.
The underside of a GD-ROM disc
|Media type||Optical disc|
Sega's use of the format instead of the new DVD-ROM technology supported by the PlayStation 2 and Xbox has been considered a mistake that contributed to the Dreamcast's early demise.[by whom?] The DVD-ROM format allowed for larger storage capacity than the GD-ROM, as well as support for DVD Video playback, which by then became the accepted standard format for home video. Furthermore, the use of GD-ROMs was the Dreamcast's only method of anti-piracy protection, which contributed to the system's rampant piracy problem once its support for MIL-CDs was discovered and exploited. Despite the failure of the Dreamcast, Sega continued to use the GD-ROM format in arcades with the Sega NAOMI 2, Sega Chihiro and Triforce.
The format was developed for Sega by Yamaha, and first commercially appeared with the Dreamcast's Japanese launch in November 1998. 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). The use of the GD-ROM format, however, contributed to a high failure-rate of Dreamcast laser pickups, because they had not been designed to handle GD-ROMs efficiently. In addition, the Dreamcast did retain the ability to read standard CD-ROM discs, and thus still suffered from software piracy as bootleggers managed to fit certain games on CDs and exploit the Dreamcast console's compatibility with the MIL-CD format.
Before the Dreamcast was released, Sega "confirmed that Dreamcast owners will one day be able to upgrade the GD-ROM drive to DVD," as information indicated Sony's anticipated successor to the PlayStation, the PlayStation 2, would use the DVD format with its much larger capacity 4.7 GB single-layered up to 8.5 GB double-layered discs compared to the 1.2 GB capacity of the GD-ROM. Despite displaying a Dreamcast DVD display unit at E3 2000, the plans for a DVD add-on or fully separate unit never materialized during the significantly shorter production run of the Dreamcast. The GD-ROM format is considered one of the causes that led to the failure of the Dreamcast, since the PlayStation 2 initially sold well for providing DVD video playback at much lower cost than dedicated DVD players of the time, even though the PlayStation 2 launch games were lacking compared to the Dreamcast's. Sony was able to provide a larger install base due to DVD, despite the costs being higher to manufacture.
GD-ROM was also made available as an upgrade for the Dreamcast's arcade cousin, Sega NAOMI and later Sega NAOMI 2, providing alternate media to its cartridge-based software. It is also used for the Sega Chihiro and Triforce.
There are three data areas on a GD-ROM disc. The first is in conventional CD format, and usually contains an audio track with a warning that the disc is for use on a Dreamcast, and can damage CD players. These vary by region.
The CD section also contains a data segment, which is only 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). There then follows a separator track which contains no data except for the text Produced by or under license from Sega Enterprises LTD Trademark Sega (Similar to the Sega Saturn, it was believed that the security key was stored in this area to prevent piracy). The final (outer) section of the disc contains the game data itself in a higher density format. This section is 112 minutes long, with a data size of 1.2 GiB.
A normal CD-reader will not read beyond the first track because, according to the CD table of contents (TOC), there is no data there. With modified firmware that looks for a second TOC in the high-density region it is possible to read data from the high-density region even on a normal CD-reader. One can also utilize a "swap-trick" by first letting the CD-reader read the TOC from a normal CD with a large track and then swapping that disc with a GD-ROM in a way that avoids alerting the CD-reader that a new disc has been inserted. It is then possible to read as much data from the high-density region as indicated by the TOC from the first disc.
The most popular way to access data from GD-ROMs, however, is to use the Dreamcast itself as a drive, and copy the data to a computer by means of a "coder's cable" or a Dreamcast Broadband Adapter; another alternative is modding the Dreamcast to add a USB connector. Sega has discontinued production of GD-ROM media. Later CD-ROM and DVD-ROM drives allowed PC software to offered on CDs using only the high-density region as part of the DRM.
The first section (in conventional CD format) usually contains a message informing users that the disc can damage AV equipment. Different discs, usually varying by region, contain different messages.
NTSC-U discs usually contain this message: "Warning! This disc is only for use on Sega Dreamcast." or this message: "This is a Dreamcast game disc, the first track contains game data, please do not play it on a normal CD Player." Some discs contain light-hearted or humorous messages from the game's characters (for example, Skies of Arcadia gives the message "We can't save the world from a CD player, so just put us back in the Dreamcast so we can do our job!"). This is also common on Japanese NTSC-J discs.
PAL region discs contain this message: "This is a Dreamcast disc and is for use only on a Dreamcast unit. Playing this disc on a Hi-Fi or other audio equipment can cause serious damage to its speakers. Please stop this disc now." The message is also repeated in French, German, Spanish and Italian. Some PAL region games however, use the NTSC messages.
The GD-ROM in the Dreamcast works in constant angular velocity (CAV) mode, like the majority of modern optical drives. Very old CD-ROM drives read with a constant linear velocity (CLV) design, however (usually 12× or slower). 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.
The NetBSD project has developed a GDRom driver for NetBSD. A port of that driver for Linux exists, though due to licensing issues and the poor compatibility of that driver with Linux kernel interfaces, a new Linux driver is under development.
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