Sega Mega CD(Review)








The Mega-CD is an add-on device for the Mega Drive video game console, designed and produced by Sega and released in Europe, Australia, New Zealand, Japan, and North America. In North America, it was renamed Sega CD, as the name Mega-CD bore no obvious associative meaning in that market where the console was named "Genesis" instead of "Mega Drive". The device adds a CD-ROM drive to the Mega Drive, allowing the user to play CD-based games and providing additional hardware functionality. In addition, the add-on can also play audio CDs and CD+G discs. The development of the Mega-CD was confidential; game developers were not made aware of what exactly they were working on until the add-on was finally revealed at the Tokyo Toy Show in Japan. The Mega-CD was designed to compete with the PC Engine CD (TurboGrafx-16 CD) in Japan, which had a separate CD-ROM drive. The first version of the Mega-CD sits underneath the Mega Drive console and loads CDs via a motorized tray. A second version places a top-loading CD-ROM drive to the right of the console, and is intended primarily for use with the redesigned Sega Mega Drive 2. Both versions of the Mega-CD are compatible with the initial two versions of the Mega Drive console, but not with the Mega Drive 3 or Genesis 3.



Models
The following models were released:

Mega-CD I (Sega CD I in North America)
Mega-CD II (Sega CD II in North America). Designed for the Mega Drive 2/2nd model Genesis and to reduce manufacturing costs.

Victor Wondermega RG M1JVC Wondermega RG M1 (JAPAN-ONLY (released on April 1, 1992): combination Mega Drive/Mega-CD player with enhanced sound, video and karaoke capabilities and an mechanized CD tray lid. Also included MIDI functionality. Sega themselves released on April 24, 1992 a variation of this unit with slight cosmetic changes, but functionality remains identical – Branded as Sega Wondermega)
JVC Wondermega Model RG M2 (JAPAN-ONLY: a second model with a different case design and wireless controller functionality built in. The mechanized CD tray was omitted in favour of a more conventional spring loaded tray design. Otherwise, functionally identical to the Wondermega I. This model was stripped down released in the US as the X'Eye.)
Sega Multi-Mega (called Sega CDX in North America). A portable CD player that plays both Mega Drive and Mega-CD games, as well as audio CDs, and CD-G discs. Resembling a slightly longer version of the typical portable CD player of the day.
Pioneer LaserActive Sega Mega-CD module, an add-on device available for the LaserActive system


Technical specifications

Mega-CD Motherboard (from the late version with top-loading CD-ROM drive)The Mega-CD specifications are as follows:

CPU
The main CPU is a 12.5-MHz 16-bit Motorola 68000 processor. The Mega Drive has the same processor, but at a lower clock rate of 7.67 MHz (NTSC) / 7.61 MHz (PAL). In the combined system, both processors run concurrently for Mega-CD games, and the Mega-CD processor is idle for Mega Drive games.

Graphics
Graphics Processor: Custom ASIC
Number of simultaneous colors on screen: 64 out of 512
Display resolution: 320 x 224 pixels and 256 x 224, video size from ¼ to full screen
Cinepak video compression scheme, implemented in software
Scaling and rotation effects
RAM
Program RAM: 4 Mbit (upgrade from the Mega Drive's 64kbyte)
Word Ram: 2 Mbit, this RAM is used as a bridge for the two processors
PCM samples: 512 kbit
CD-ROM data cache: 128 kbit
64 kbit Internal Backup RAM (for storing saved games, scores, etc. for CD games)
Storage
500 MB CD-ROM discs (equivalent to 62 minutes of audio data)
CD-ROM drive transfer rate: 150 kB/s (1x)
BIOS
Size: 1 MBit
Used for games, CD player, CD+G and karaoke
Access time: 800 ms

Sega CD development consoleWhen the Mega Drive + Mega-CD is powered on, the Mega Drive CPU gets control first. If a cartridge is inserted, the Mega Drive ROM starts it, and the Mega-CD remains idle, except for the audio mixer, which passes through the Mega Drive audio to its RCA outputs. If the cartridge slot is empty at power-up, a part of the BIOS is mapped to the Mega Drive CPU memory map and is run by the Mega Drive CPU, which decompresses other part of the BIOS onto the Mega-CD memory. Then the Mega-CD CPU runs other code of the BIOS. Both CPU run together a program which checks the disc while displaying a Mega-CD (or Sega CD) title animation with music utilizing the sound hardware of both the Mega Drive and the Mega-CD. If a game disc is inserted, it automatically loads the game after a few seconds, or immediately if Start is pressed on the controller. If a regular audio CD or a CD+G disc is inserted, it waits for button A, B, or C on the first controller to be pressed, then starts the GUI CD/CD+G player software in ROM. It will also run this software on a game disc if A, B, or C is pressed before the game auto-starts. From the CD player, it is also possible to enter a utility to manage items in the game backup RAM and RAM carts, or to start an inserted Mega-CD game disc (without resetting the console). On all second revisions of the Mega-CD boot ROM, if the user has a Mega Mouse or Sega Mouse plugged into control port 2, this may be utilized as a pointing device instead of using a traditional control pad. If no disc is inserted, it prompts on screen for one while playing the title animation and music. Except when a game disc is in the drive, if no button is pressed at the Mega-CD title screen, the animation and music will loop indefinitely.
In the original Mega-CD model which has the motorized front-loading tray, pressing reset while the title screen is displayed will open the tray (eject the disc) or close it (load the disc). At all other times (without a cartridge inserted), pressing reset will return to the Mega-CD title screen, but will not open the tray. It is impossible to open the motorized tray with a cartridge inserted, as the BIOS which controls it will not run then. In the later version of the Mega-CD with the top-loading drive, resetting or powering off may leave the disc spinning, and the only way to stop the disc before opening the tray is to switch off the system and wait 15 to 30 seconds for it to spin down (under the light friction of the spindle motor bearings) or to enter the CD player mode and play or pause the disc, then stop (in BIOS v2.00, maybe others). After the disc can be heard to spin down while the lid is closed, whereas just pointing to Stop and pressing a button will not spin down the disc when the player is not in play or pause mode.


Audio
The Mega-CD adds the Ricoh RF5C164 chip, which gives 8 extra sound channels, all capable of sampled sounds, to the Mega Drive's YM2612 and SN76489 chips (which provide a total of 18 channels, with the YM2612's 6 channels and PSG's 4).

Sound format: Stereo PCM
Clock frequency of source: Up to 12 MHz
Sound channels: 8
Maximum sample rate: 32 kHz (44.1 kHz for CD-DA)
Wave data width: 8 bits
16 bit DAC
8x internal over-sampling digital filter
Frequency Range: 20 Hz – 20 kHz
Signal-to-Noise Ratio: > 90.0 dB @ 1K
Channel Separation: > 90.0 dB
Output: RCA stereo Pin Jack x2 (L/R) / SCART cable

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