The Sega Mega Drive version of Sonic the Hedgehog 2 and Sonic the Hedgehog CD are thought to have at one point been the same thing, likely separating at an early point in development when a disgruntled Yuji Naka and several members of the original Sonic Team migrated from Sega of Japan's offices in Tokyo to Sega of America (or specifically, the Sega Technical Institute) in California - travelling to where Sonic the Hedgehog was actually popular, as opposed to staying in Japan where the reception had been more lukewarm.
The entirety of the story is unclear, and many former developers seem unaware of the situation, however Sega spent a large part of 1992 promoting four different versions of Sonic 2 - the Sega Mega Drive, Sega Master System and Sega Game Gear ports, all of which shipped, and a Sega Mega-CD version which did not. Development on Sonic 2 CD is thought to have began with an aim of launching after the aforementioned versions, however the project was axed, potentially as a response to Japanese Sonic 2 sales, and became an entirely separate game, Sonic CD.
Slightly enhanced Mega-CD ports of Mega Drive games were not uncommon during the early 1990s, and Sonic 2 CD appears to have followed a similar pattern - the addition of CD quality audio, cutscenes between levels and the potential for more stages were all announced by various sources, though very little seems to have been demonstrated and may relate soley to the Japanese-led Sonic CD project.
No gameplay of Sonic 2 CD was ever demonstrated, however during mid-1992 members of the gaming press appear to have been given access to some sort of unconventional prototype or rolling demo. The footage bears very little resemblence to what is found in Sonic 2 - it may have been intended for loading screens, or to act as a simple test for Mega-CD hardware.
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Brief mention of a Mega-CD version of Sonic 2, scheduled for an Australian release in early 1993.
After an interview with Yuji Naka, Sega Visions claims that a Sega Mega-CD version of Sonic the Hedgehog 2 is coming, with extra levels and "special bonus rounds" between zones.
|"Preview: Sonic the Hedgehog 2 - Sonic II Sur Mega CD"|
|"Mega CD Sonic Update"|
Short article describing the game as an enhanced version of Sonic the Hedgehog 2 called "Super Sonic".
|"Top 100: Sonic the Hedgehog"|
While Mega explains its decision to rate the original Sonic the Hedgehog as the third best Mega Drive game available in the UK, it briefly alludes to Sonic the Hedgehog 2 and a Mega-CD game, Super Sonic.
Later in the issue, Mega briefly predicts a Mega-CD port of Sonic 2, but is likely speculating at this point.
|"CD ROM Showcase: Sonic the Hedgehog 2"|
|"Sonic the Hedgehog 2"|
A brief mention at the back of this Sonic 2 preview, suggesting Sonic 2 CD exists.
Mega reminds its readers of Super Sonic. The magazine pairs this with a screenshot of the original Sonic the Hedgehog, though does not otherwise suggest the two games are linked. The publication merely confirms "high quality" music is on the cards.
Short article about the game, still called "Super Sonic" at this point, and how voice acting will be included.
|"Gutter Talk!: Sonic CD Slips Up"|
UK magazine Sega Force reports that sales of Sonic the Hedgehog 2 in Japan have fallen below expectations, leading to a rethink in Sonic CD's development. No longer will it be an enhanced version of the Mega Drive game.
Brief mention of the game, still referred to here as "Super Sonic", said to be released in Japan "any week now".
|Sonic games for the following systems|
|| 1991 Sonic the Hedgehog | Sonic Eraser 1992 Sonic the Hedgehog 2 1993 Sonic the Hedgehog CD | Dr. Robotnik's Mean Bean Machine | Sonic Spinball 1994 Sonic the Hedgehog 3 1995 Chaotix | Sonic Classics 1996 Sonic 3D: Flickies' Island |
Prototypes Sonic 2 | Sonic CD | Sonic Spinball | Dr. Robotnik's Mean Bean Machine | Sonic 3 | Sonic & Knuckles | Knuckles in Sonic 2 | Chaotix | Sonic 3D
|Unreleased Sonic the Hedgehog Games|
Sonic's Edusoft (Master System)
Sonic Sports (Sega 32X)