Sonic the Hedgehog CD (2011)

Sonic the Hedgehog CD (or Sonic CD) is a 2011 remastered version of the original Sonic the Hedgehog CD released for the Sega Mega-CD. The version was developed by Christian Whitehead and published by Sega. It was released digitally on multiple platforms including PlayStation Network, Xbox Live Arcade, iOS and Android on December 2011, and later on Steam and Windows Phone the following year.

Development
When Sega announced which of their games they would like to release in the iOS App Store, Christian Whitehead showcased a concept demo of the game being played on his iPod touch in 2009. The port is not a direct emulation of the original Mega-CD version but rather, it was developed from scratch using Whitehead's "Retro Engine". He then contacted Sega to tell them about his release plans.

New features

 * With the capabilities of the Retro Engine, the game runs at a consistent 60 FPS, supports widescreen and online leaderboards.
 * Miles "Tails" Prower is an unlockable playable character who has his moveset from Sonic the Hedgehog 3. However, he does not have animated cutscenes and Amy Rose is entirely absent from his gameplay.
 * Both the Japanese and American soundtracks of the game are included, with the player having the option to switch from one soundtrack to the other.
 * The player can switch to using the Spin Dash from the Sega Mega Drive/Genesis games (e.g. Sonic the Hedgehog 2, Sonic the Hedgehog 3).

Changes

 * In Collision Chaos, sound effects have being added when using the pinball flippers.
 * There's a chime before the underwater countdown begins (much like the Genesis/Mega Drive titles).
 * In Tidal Tempest, the air bubbles spawn slightly higher. In the Mega CD version the player could grab them just by standing on top of the bubble vent, but in this version he/she has to jump to reach them, like in the Genesis/Mega Drive titles.
 * In Stardust Speedway, the spotlights in the background in the present move fluidly and do not flicker.
 * During the Metal Sonic race at Stardust Speedway, the Bad Future theme always plays in the background, regardless of which soundtrack is chosen.
 * During the Time Attack Mode, Amy Rose does not show up at the end of Stardust Speedway; she was instead replaced by a flower capsule, most likely to save time.
 * During the final battle against Dr. Robotnik, two "light beams" have been added to each side, which are akin to the ones at the Final Zone, Wing Fortress Zone, and Flying Battery Zone.
 * The final boss during this level now has smooth animation sequences, most notably during the attack with only three "wedges" left. In the original, he jumped between still frames while a loud mechanical sound played with each shift as he rotated: now, the rotation is smooth and Robotnik appears to "float" while using it.
 * The U.F.O.s that carry the Speed Shoes power-up in the Special Stage have been recolored blue, most likely to better differentiate between the purple ones, which carry the Ring power-up. In the original, this difference was only visible from a short distance, with gold stripes indicating rings and silver having the Speed Shoes.
 * The 'warning' sound effect signifying low time starts at 4 seconds remaining, as opposed to the original's 14 seconds.
 * Sonic kicks up dust clouds when he skids, as he does in Sonic the Hedgehog 2 and future games.
 * Rather than pulling further ahead of Sonic when going fast or charging a dash move, the camera stays centered on him like in the Genesis/Mega Drive games.
 * The time taken for time travelling is actually a bit longer. Some objects that activated warping in the original Mega CD game do not do so in this remake.
 * The Stage Select menu and Sound Test can no longer be accessed by code. Instead, they have to be unlocked in the same manner as the D.A. Garden and Visual Mode.
 * The FM portion of the Sound Test is now PCM. All "past" tracks have been moved to "D.A.", along with the rest of the soundtrack.
 * While the debug mode is enabled, power-up monitors can be used more than once.
 * Ring objects are now available to be placed, unlike the original.
 * There is no background music that plays after the "S" monitor has been struck.
 * When the Japanese soundtrack is enabled, the song that plays in the Time Attack menu is completely different from the one in the original game. However, the song is not entirely new: it is an instrumental remake of "Brand-new World" from Sonic the Hedgehog - Remix, a CD that was released in 1994 to accompany the original edition of the game.
 * Entering the code "PCM: 32, DA: 8" on the Sound Test briefly shows a mock-up image of "Desert Dazzle", a stage that was once planned for the remake. It also enables Tails while choosing a stage from the Stage Select menu.
 * Rather than the Time Attack theme being used after entering the code for accessing hidden pictures, the Quartz Quadrant Good Future theme plays instead.
 * In order to access the Special Stage Time Attack, the player has to select the "Next" icon, rather than pushing left.
 * Certain mobile versions of the game have haptic or "rumble" feedback.
 * Tails cannot use the Super Peel-Out.
 * The opening cutscene has been disabled.
 * Rather than a voice clip being used after getting a 1-Up, the Time Stone/Badnik Teleporter sound effect is used instead.
 * As such, neither she or Metal Sonic will show up at Collision Chaos Zone 1, and the spin tunnel path is opened by default.
 * She has been replaced by a flower capsule at Stardust Speedway Zone 3, much like the Time Attack mode.
 * As with the opening, the ending cutscene was also removed, and replaced with a credits screen. Unlike Sonic's game, which lists the staff who worked on the original version, this one lists the staff who worked on the remastered version.
 * If Tails remains idle for three minutes, it does not result the loss of a life.

Reception
Sonic the Hedgehog CD received positive reviews with Metacritic assigning the PlayStation Network, Xbox Live Arcade and iOS versions 80/100 (based on 12 reviews), 82/100 (based on 20 reviews) and 93/100

Videos
2011 Rerelease Trailer = IOS Trailer=

Trivia

 * The achievement name "Paradise Found" is a parody of the 17th Century poem "Paradise Lost" by John Milton. When Sonic achieves a Good Future in any of the zones (via destroying the Metal Sonic projectors and roboticizers), he is essentially "founding" a paradise for the future of that zone.
 * In the 2011 re-release, the animated cutscenes are cropped to have a 16:9 appearance if the game is played in widescreen HD. However, if the game is played in 4:3 SD format, the cutscenes will appear in their original format.
 * There was originally going to be an extra boss that would only have been accessible after the player had collected all of the Time Stones, but it was ultimately scrapped.