Sonic CD is just your average Sonic the Hedgehog game on a CD... that's what you're told, anyway.

Palmtree Panic Zone

Unused Color Palettes

Salad Plain's palette data from the v0.02 prototype still exists in R11A__.MMD at offset 0x000754 , alongside its palette cycling data at offset 0x0001F8 .

Sonic 1's title screen palette data from is leftover in R11A__.MMD at offset 0x0005D4 .

Sonic 1's level select screen palette data from is leftover in R11A__.MMD at offset 0x000654 .

Unused Objects

Object 04

This object moves downwards quickly for a bit and then disappears. As it moves down, it overwrites the level blocks with a singular block. There's this particular piece of unused graphics located at 0x03695A in R11A__.MMD:

It seems to fit the sprite mappings for this object perfectly:

So, it is very well possible that this object was intended to generate a waterfall, like so:

Level Select

Press Up, Down, Down, Left, Right, B at the title screen. Note that you will be taken back to the title screen upon completion of the selected level or upon attempting time travel. If the level was ended by attempted time travel, entering the Level Select again and choosing another level will start the level as if you had traveled through time, keeping any rings and accumulated time from the last level.

The level select screen skips from level 1 to level 3. Level 2 was also missing in the PC version, known as R2 because of the way the PC version stored the levels in R# folders.

Sonic CD Staff's Time Attack Scores

Press Right, Right, Up, Up, Down, C at the title screen to see the best times that the team behind Sonic CD got in Time Attack mode for each stage and act. High-Speed Shoes music plays in the background.

Sound Test/Debug Mode

At the title screen, press Down, Down, Down, Left, Right, A to go to the sound test. By entering the following codes, you can get the corresponding image, debug mode, and/or extra after pressing Start.

Image Code Description FM NO.40

PCM NO.12

DA NO.11 Tails holding goggles next to a fourth-generation Lotus Seven, with the words "SEE YOU NEXT GAME" above it. This code also activates the debug mode: like the previous Sonic games, press B to change from Sonic to an object, A to cycle through objects, and C to place an object on the screen. Drawn by Yasushi Yamaguchi (also known as Judy Totoya/Toyota, as seen here), character designer for Tails, and special stage designer for Sonic CD. The "Little Planet" theme plays here. The Tails unlock screen on the 2011 remake also says "SEE YOU NEXT GAME", likely as a reference to this screen. FM NO.42

PCM NO.04

DA NO.21 An unusual Batman tribute. Drawn by Takumi Miyake, landscape and visual designer for Sonic CD. The final boss music plays in the background. FM NO.42

PCM NO.03

DA NO.01 Sonic, Metal Sonic, and Eggman gettin' funky. Drawn by Kazuyuki Hoshino, character/special stage/visual designer, and illustrator for Sonic CD. Metallic Madness Present music plays in the background. FM NO.44

PCM NO.11

DA NO.09 A cute Sonic with grey eyes. The Palmtree Panic Good Future music plays here. The message says "ゆーあーくーる by さんちゃんず" (You are cool, by Sanchanzu). Drawn by Masahiro Sanpei, landscape designer and animation visual director for Sonic CD. FM NO.46

PCM NO.12

DA NO.25 A rather creepy wallpaper collage featuring multiple Sonics with a freakish humanoid face. The boss music plays. The Japanese text reads "たのしさ∞ セガ・エンタープライゼス まぢん画" (Infinite fun. Sega Enterprises. - Mazin Picture). Drawn by Masato Nishimura, landscape designer for Sonic CD. Mazin is his childhood nickname, as explained here. Despite its creepiness, it's likely meant to be played for laughs given the fact that the Japanese boss theme is much more... cheery?, than the English one. FM NO.07

PCM NO.07

DA NO.07 The text "WELCOME TO SECRET SPECIAL STAGE" will appear, after which that stage will load (see below).

Unused Special Stage

As mentioned above, setting all three Sound Test selections to 07 will load this stage that features the enlarged head of Eggman as its background and has a rather difficult layout. It's possible that an extra Time Stone was to be collected here at one point, though it's probably just a little bonus thrown in for curious players, much like the hidden pictures.

Completing this Special Stage will display a secret credits screen.

Unused Sprites

Sprite Name Description Sonic being electrified An animation of Sonic being shocked by electricity. It may have been used in Wacky Workbench, which contains various level elements with the potential to electrify the player. It could also have been used in Collision Chaos as part of the Metal Sonic sequence in the 510 prototype. Used in the 2011 remake, when you get shocked in Wacky Workbench. Sonic falling An animation of Sonic falling into the distance. Purpose unknown. A similar animation is present in Sonic 1, also unused. Sonic waving his arms An animation of Sonic waving his arms with his palms outstretched. This could have been used in Tidal Tempest when walking through the water, to simulate an attempt at moving more quickly. Sonic looking backward Sonic looking over his shoulder; it's possible the pre-Stardust Speedway cutscene was meant to use it as Eggman is behind Sonic. Sonic sneezing An unused animation of Sonic sneezing. Purpose unknown. Curiously, he also appears sneezing in the game's intro. Sonic upside-down An animation of Sonic upside-down with his eyes wide and his mouth open. This suggests that this animation could have either been used for an alternate death animation or if Sonic was caught by an environmental hazard that dangles him in the air somehow. Interestingly, the first sprite was used in the Game Gear and Master System versions of Sonic 2 for the title card of Underground Zone. Sonic grabbing Sonic grabbing onto something and attempting to create traction. This implies that he may have been trying to stop something from pulling him into some sort of hazard. A modified version of this sprite appears in Sonic Crackers. Sonic on horizontal pole Similar to the other horizontal pole sprites from Wacky Workbench, this one has Sonic swinging his feet forward. Sonic shrugging An animation of Sonic shrugging his shoulders. These sprites are found in SPSS__.BIN, which contains Sonic's Special Stage-specific sprites. His spikes rise during each frame, in the final frames of the animation, the very bottoms of his feet disappear, and he has an annoyed frown on his face. Concept art for the game that features these sprites confirms that he is indeed supposed to have a disappointed look on his face, so it could've been a "failure to obtain the Time Stone" animation. Sonic hanging An animation of Sonic being pulled up by...something. Purpose unknown. These sprites are also found in SPSS__.BIN. Early screenshots show hot-air balloons, not UFOs, in the Special Stages, later revealed to be the v0.51 prototype; perhaps they caught Sonic and floated him around? Sonic falling back An animation of Sonic being knocked back. Purpose unknown. These sprites are also found in SPSS__.BIN. Sonic leaping An animation of Sonic leaping. Purpose unknown. These sprites are also found in SPSS__.BIN. Sonic with chest forward An animation of Sonic with his arms outstretched and his chest pushed out in front of him. Purpose unknown. Unused Amy sprites Sprites of Amy with hearts in her eyes. While they go unused here, they are used in the 2011 remake.

(Source: Sonic Retro)

Unused Objects

Sprite Name Description Oil Slick Oil slicks that would have appeared in Special Stages, just as they could be found normally in some prototypes, but are unused in the final game (and are still non-functional). Broken Switch A broken switch that may have been used in the Bad Future of a Zone to prevent access to certain routes. Regular switch sprite present for comparison. Moving Platforms Moves up and down while moving to the left. They come in two variations: a pair and a group of four, all of which do the same thing (though in the latter, the top row is flipped vertically). While technically used in the Futures of Quartz Quadrant, these variants for the Present and Past do not appear anywhere in their respective time zones. Conveyor Belt Platforms Conveyor belt platforms for the Past version of Quartz Quadrant. While these platforms exist in the Present and Futures, they do not appear anywhere in the Past at all, so these Past variants are never seen. Swinging Conveyor Belt A swinging conveyor belt platform for the Past version of Quartz Quadrant. Like the platforms above, this object exists in the Present and Futures, but this unique Past variant doesn't appear anywhere. Hand Appears to be an unused Special Stage item. Purpose unknown, but may have stopped the timer.

(Source: Sonic Retro)

Unused Monitors

There are some monitors that aren't used in the final game but are used in some of the prototypes. The player can access the monitors in all major releases of the game from the Debug Mode. They can also access some of these in Palmtree Panic Act 3 by beating the boss, using debug to place themselves far below the coordinates of the boss fight, and then exiting debug.

Sprite Name Description Clock Monitor This monitor is supposed to stop time for a few seconds, but it doesn't work correctly. It can stop the animations of rings and monitors as well as rotating palettes, but that's it. Blue Ring Monitor This monitor has no effect. In the v0.02 and 510 prototypes, it had the same effect as the "S" monitor, while in the v0.51 and 712 prototypes it acted as an invisible shield that can stack with a regular shield. Its icon would be repurposed for the Combine Ring powerup in Knuckles' Chaotix and redrawn for the similar Hyper Ring powerup in Sonic Mania. "S" Monitor Breaking this monitor gives Sonic invincibility, power sneakers and a shield all at the same time. The icon is used in the 2013 remakes of Sonic 1 and Sonic 2 for the S monitor, although the monitor works like in Sonic 3.

Unused Sounds

Time Warp

This sound effect is odd in that it's a CD audio track on the disc. When the game disc is inserted into a CD player, one can hear this sound on track #2. The v0.51 and 712 prototypes used this sound for time travel.

Other Sounds

Sound ID Sound Description FM NO.21 Not sure what it is. Sounds like it would come from a harmonica? FM NO.22 is a duplicate of this sound. FM NO.26 Sounds like a Spin Dash or a Super Peel-out being jammed. FM NO.48 A different version of the sound heard when pressing a button. FM NO.49 The warning bell usually heard underwater in other Sonic games, only high-pitched. The 2011 rerelease uses this sound, albeit at a lower pitch. FM NO.50 Sounds like something being shot. FM NO.54 An electrical sound of sorts. FM NO.55 A loud, harsh buzzing sound. FM NO.60 Not sure, but it sounds like something rising or powering up. FM NO.61 This sounds like FM NO.55 , but it has a resonating sound underneath it. FM NO.63 An echo-y spike-like sound. FM NO.66 A very high-pitched "ding" sound. Later used in Sonic Mania for the Hyper Ring powerup. FM NO.67 A higher-pitched version of Sonic losing his rings. FM NO.68 This one sounds like FM NO.63 , only without an echo. FM NO.69 This one sounds similar to the door sound at the end of the Metal Sonic race. FM NO.75 This sounds like variant of FM NO.68 . Perhaps it could have played alongside the sound. FM NO.76 Another variant of FM NO.68 . FM NO.77 Yet another variant of FM NO.68 . FM NO.78 You guessed it. Another FM NO.68 variant. PCM NO.02 Sonic saying "Alright!". PCM NO.05 Sonic saying "Yeah!". This was used as the extra life sound in the v0.51 and 712 prototypes, in the final game he says "Yes!" instead.

"Comin' Soon" Screen

This screen, which features a scrolling banner saying "COMIN' SOON" while playing the invincibility theme, was used as far back as the v0.02 prototype when the only two levels in the prototype have been completed and used for subsequent prototypes after clearing certain Rounds. It manages to survive all the way into the final game and can be viewed by selecting "COMMING" in the Level Select.

"CX" Room

Near the end of Quartz Quadrant Zone 1 Past, there is a small, inaccessible room with glitched collision and the letters "CX" in the background made from shadowed yet used tiles. It could be possible it's a signature from one of the game designers, as these letters also appear in the Sonic CD team's best times screen (though with an extra "X" added), as seen above. Who the initials belong to is unknown.

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.

(Source: Sonic Retro)

In addition, there are a few other differences in the American version compared to the original Japanese and European games:

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.

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.

Leftover code and data