Sonic CD

Sonic CD is a game released in 1993 for the Sega CD, also known as the Mega CD outside of North America. 2 different soundtracks exist, one for North America and the other for Europe and Japan.

Hidden Sound Test pictures
5 hidden pictures.

Sound Test
At the title screen, press Down, Down, Down, Left, Right and A to go to the sound test.

Secret Special Stage
When setting all three selections to 07 will load this special stage with a difficult layout as well as an Eggman head as it's background. Completing this special stage will show a secret credit screen.

Unused sprites
TBA

Staff's time attack scores
Press Right, Right, Up, Up, Down and C at the title screen and this pops up.

Unused Objects
TBA

Clock
When broken, it stops the moving backgrounds for a few seconds. Can be found in a hidden area with Debug Mode after defeating the boss on Palmtree Panic, but this was taken out on the 2011 remake. Used in the v0.02 prototype.

Blue Ring
In the 510 prototype, it has the same effect as the S monitor. In the 712 prototype, it acted as an invisible shield that can stack with a regular shield. It was redrawn as the Hyper Ring powerup on Sonic Mania.

S
Breaking this monitor gives Sonic invincibility, power sneakers and a shield all in once. It was also found on Debug Mode on the 2013 remakes of Sonic 1 and 2, and also found on an hidden area near the Palmtree Panic boss fight on the original Sega CD version.

Unused sounds
Some unused sounds.
 * On the 712 prototype or if the game was booted into a CD player on the 2nd track, the prototype time warp sound will play.
 * The unused ding sound was used on the Hyper Ring powerup on Sonic Mania.
 * All of the unused sounds can be heard on the sound test.
 * The unused warning bell was used on the 2011 remake, but in a lower pitch.
 * The unused 'Yeah' sound was used as a 1up sound on the 712 prototype.

Coming Soon screen
Used in the 510 prototype. Also used in the final version when 'COMMING' is selected on the level select.

CX room
Near the past end of Quartz Quadrant Act 1, there is a room with the letters CX on it.

Regional differences
Almost the entire soundtrack was redone for the American version by Spencer Nilsen because Sega of America thought the soundtrack of the Japanese and European versions sounded too identical to the electronic dance soundtracks being produced at the time, so the American soundtrack has more of a rock feel. One example of this is the theme song: instead of "You Can Do Anything", the American version has "Sonic Boom". The only songs that weren't altered were the Past themes, since those were played through the PCM chip and not Redbook. The Past themes are also stored directly on the ISO file on track one as opposed to being separate tracks on the game disc. In addition, there are a few other differences in the American version compared to the original Japanese and European games: All of these changes are applied to all regional versions of the PC version, though the 2011 re-release reverts the Metal Sonic racing theme so it always plays Stardust Speedway "B" Mix and mostly restores the original ending videos' exclusive audio when played with the Japanese soundtrack.
 * Some of the music placement was switched around. For example, the "Little Planet" theme now plays on the Time Attack screen (rendering the title theme copy unused), and the music played during the Metal Sonic race is now the normal mix of Stardust Speedway's current Future and not always the "B" Mix.
 * The player can restart the stage at any time by pressing a button while it is paused, eliminating some potential time warp issues. This is at the cost of one life.
 * In order to remove Engrish, "YOU ARE GREATEST PLAYER" was changed to "YOU ARE THE GREATEST PLAYER"
 * There are no longer sound effects at the beginning of the ending. Due to the extended "Special Edition for North America" credits, the ending theme plays over the entire video and mutes other audio (including the post-credits).
 * The bad ending's "TRY AGAIN AND FREE LITTLE PLANET FOREVER" text was shortened to simply "TRY AGAIN".
 * The copyright text at the title screen was altered slightly.

Sonic 1 CD and Sonic 2 CD
See Sonic the Hedgehog and Sonic the Hedgehog 2.

Currently missing/under construction

 * Gallery of unused objects
 * Gallery of unused Sonic sprites