Sonic the Hedgehog CD

Sonic the Hedgehog CD (ソニック ザ ヘジホッグ CD), shortened to Sonic CD (ソニック CD) in re-releases, is a platform game in the Sonic the Hedgehog series, developed and published by Sega. It marked the first official appearance of both Metal Sonic and Amy Rose. It was released for the Sega Mega-CD in Japan on 23 September 1993, in Europe in October 1993 and for the Sega CD in North America on 19 November 1993. The game was ported to PC CD-ROM in 1996. The game was re-released for Xbox Live Arcade and iPod devices on Sega on 14 December 2011, and for the PlayStation Network on 20 December 2011. The Steam version was released 19 January 2012. All downloadable versions feature Miles "Tails" Prower as an unlockable character.

Storyline
The evil Dr. Robotnik steals Little Planet over Never Lake, where special stones known as the Time Stones are found. The Time Stones are powerful gems that are capable of controlling the passage of time. Robotnik had his eye on these stones all along, ever since the planet appeared on the last month of the year. Upon locating Little Planet, Dr. Robotnik and his robot cronies immediately chain Little Planet down on a mountain with the face of Robotnik carved into the mountain, then Robotnik and the badniks set about converting the world into a giant fortress. Sonic arrives on the scene to stop Robotnik from taking all the Time Stones that will allow him to manipulate time and conquer the world.

Using his unrivaled speed, Sonic must travel through the past, present and future of each Zone to ensure a good future and to find Dr. Robotnik's latest invention, Metal Sonic. At the end of the stage Palmtree Panic, Sonic meets the soon-to-be recurring character Amy Rose, who, much to his displeasure at first, has a massive crush on him. As he enters Collision Chaos, Amy follows him, and Metal Sonic swoops in and kidnaps her. Now Sonic must rescue Amy as well as defeat Robotnik. The doctor also makes an appearance at the end of every round. Once Sonic in Stardust Speedway, Sonic encounters Metal Sonic and the two raced against each other, with the doctor flying closely behind. Should the hedgehog outrace his metallic doppleganger, Metal Sonic gets struck by Dr. Robotnik's laser, leaving him out of the picture for the time being as Sonic is able to rescue Amy.

At the end of the game, depending on whether all of the Time Stones have been collected or if a good future has been ensured in every single zone, the game will end in either a good future or a bad future. In the good ending, Sonic and Amy land back on Sonic's world as Little Planet orbits away in freedom. In the bad ending, Sonic and Amy come home, but Robotnik escapes with a Time Stone. As he gloats of his victory, Sonic throws a rock at him, apparently bringing him down. The credits are identical in both endings, but the after-credits scenes are different. In the good ending, Little Planet's mythical seeds will land on the ground and many of its flowers will come into existence; in the bad ending, a time warp will ensue overhead, restoring the ruined mountain and its grip on the doomed Little Planet in an instant as the player is told to "Try again and save Little Planet forever."

Gameplay
The main innovation of this chapter in the Sonic series is the manner in which the player can travel to four different versions of each zone, each in one of three different time periods of the same location: Present, Past, Good Future and Bad Future. This is accomplished by speed posts scattered around the level, bearing the labels "Past", and "Future". After running through one of these posts, the player has to run at top speed for a few seconds without stopping, to travel into the respective time period. Because these signs are relative, there are no "Past" signs in the Past, and no "Future" signs in the Future; that is, warping to the past in the future returns the player to the "present" time and vice versa. Each stage has three "Acts" (although they are called "Zones" in this game, see below), the third of which always takes place in the future.

The different time zones have slightly different layouts and sprite placements, as well as significant changes in the level music, art and palette. In addition, the robots within a level fall into a state of disrepair as time passes; in the present, some machines have become worn down while in the future all of them have. This affects the speed and attacking ability of the robots; some of them become completely ineffective, while others do not significantly change.

The appearance of the future changes depending on the actions of the player in the past. Hidden within the past of every act, there is a robot generating machine. If this is destroyed within a zone or all seven Time Stones are already collected, all of Dr. Robotnik's robots will be destroyed in the past (as are some in the present). Should the player warp into the future, it is a "Good Future" in which there are no enemies and fewer hazards, and the landscape is a perfect balance of nature and technology. If the machine is not destroyed, the warp will lead the player into the "Bad Future" in which Dr. Robotnik's robots run rampant, there are more hazards (though due to wear on some of the enemies, not always as many as in the past) and heavy pollution has harmed the level (such as poisoned water or corroded structures).

In addition to the robot generating machine, hidden within the past of each level is a machine, which projects a hologram of Metal Sonic squashing one of that particular zone's animals underfoot. Destroying this machine causes animals to appear in the past and present levels. However the animals are always present in the Good Future, regardless of whether or not this machine was destroyed.

The third zone always takes place in the future and is mainly a short run up to the boss. Most boss battles are more elaborate than those in the other Sonic games and typically require fewer hits than the usual 6 or 8. These boss battles, however, require more effort to actually hit Robotnik; one battle takes place on a makeshift pinball table and requires the player to use flippers to get up to Robotnik. Two battles do not involve hitting Robotnik to damage him; one takes place on a giant treadmill where the objective is to wear out Robotnik's machine by running on it, and the other is a race against Metal Sonic. The appearance of the third zone depends on the player's actions in the other two; if the player has achieved a Good Future in the other two zones (or all the Time Stones are collected), this zone will be a Good Future as well. However, if only one or neither stage has been made into a Good Future, the third zone will be a Bad Future. If all the third zones have Good Futures, the player is able to see the good ending but if the player gets all of the Time Stones and all good futures, then a bonus ending is seen.

Sonic CD was the first Sonic game to use a backup save, using the internal Sega CD memory or a backup RAM cartridge. The game saves after the end of each third zone (after which, a new level begins) and records the best times of the player in the time attack mode.

The game itself has Sonic feeling generally more sluggish than normal. The spin dash he does is different to that of the position in Sonic the Hedgehog 2. A new move was added to complement the spin dash: the Super Peel-Out (aka Strike Dash or Figure 8). The Super Peel-Out, performed in a manner much the reverse of the spin dash, by pressing up and any trigger button, causes Sonic to rev in position until the button is released, at which point he speeds off. The difference between the Spin Dash and the Super Peel-Out is the Spin Dash damages enemies who get in its way, due to Sonic's curled attacking pose; the Super Peel-Out, whilst quicker to charge up than the spin dash, does no damage, instead leaving Sonic vulnerable to attack (however, this can be foiled by pressing down immediately after performing the Super Peel-Out, sending Sonic into a roll that is just as fast as the Super Peel-Out as well as making him invulnerable to enemy attack).

Also, while leaving the game idle for more than a few seconds makes Sonic tap his foot impatiently (as per usual); leaving the game idle for 3 minutes causes Sonic (i.e. a digitized voice clip) to say "I'm outta here!", followed by him jumping off screen, resulting in a "Game Over".

Rounds
What would typically be called a "Zone" in other Sonic games is called a "Round" in Sonic CD. Likewise, an "Act" now becomes a "Zone". In the level select screen, levels are listed according to round numbers. For example, what the level select refers to as "Round 1" is Palmtree Panic. "Round 2", however, is missing; the level select instead skips directly to "Round 3" (Collision Chaos), suggesting a level was cut during development. This is even featured in the PC version, where files for each level are separated into folders - there are folders named "R1" and "R3", but no "R2". The missing zone was confirmed by Christian Whitehead, who released the few art assets remaining, including the Antlion Badnik planned for "R2" and "R8". In the 2011 remake of this game, there was a planned R9 named Final Fever but it was ultimately scrapped. In total, there are seven rounds to complete, each in three different time zones, essentially making it four different levels per Round, for a total of 70 original level designs.


 * Palmtree Panic: A tropical level with mountains and waterfalls in the background. The past features a more prehistoric looking Palmtree Panic. The bad future is completely mechanized with smog in the air and oil in the water. The good future is also mechanized, but bright, vivid, colorful, and clean, with potted plants and trees adorning the area as well.
 * Collision Chaos: An unusual mechanized forest with a pinball theme. In the past, Collision Chaos shows a rather surreal, orange tinted version of the level. The bad future is dark and creepy with gray machines. The good future shows a bright pink and blue futuristic paradise.
 * Tidal Tempest: An underwater area at the base of a volcano. In the past, it's an underground cavern, untouched by man or machine. The bad future shows a broken down, polluted, over-industrialized water plant. In the good future, Tidal Tempest is a fully operational turquoise aquarium harboring much plant life and fish. The water level appears to have risen over time: it's low in the past, higher in the present, and at its highest in either future.
 * Quartz Quadrant: Quartz Quadrant is a busy place with conveyor belts and platforms. The appearance of this level changes drastically throughout each time zone. It is a swamp in the past with hardly any technology, but a large quantity of quartz. It's an active mine and partially a swamp in the present. It is an overly mechanized mine with apparently no quartz in the bad future. It is an underground golden-colored city that is possibly made of quartz in the good future.
 * Wacky Workbench: A factory level located in a canyon. The past features an early construction of the Workbench. In the bad future, the level is ruined and rusty, while the good future shows an advanced pink/purple plant similar to that of a fictional toy factory.
 * Stardust Speedway: One of the fastest rounds in Sonic history. Stardust Speedway is a highway adorned with musical instruments above an enormous city. In the past, the land is old, ancient, resembling a Roman city, and vines adorn the highway as there is little to no technology to speak of. It also has Gothic-styled building in zone's center. In the bad future, Stardust Speedway has become a corrupted, polluted dystopian city underneath a large electrical storm with a completed statue of Robotnik in the zone's center. The good future looks like a giant futuristic amusement park, with bright pink and green colors dominating the landscape with a beautiful cathedral in the center of Zone 2.
 * Metallic Madness: Robotnik's base of operations on the Little Planet. The past shows the base still in construction with cranes adorning the skyline, while in the bad future, Metallic Madness is a dark, sinister completely broken-down base ruined from neglect. The good future still shows a mechanized factory, but it has become more in tune with nature, as though Robotnik was never there.

Special Stages
As in Sonic the Hedgehog, Special Stages can be accessed at the end of each zone if the player has collected, and is holding on to at least 50 rings. A giant ring will float above the finishing sign which Sonic can jump through to enter the special stage.

The special stage consists of a three-dimensional, flat surface. To complete a stage and collect the Time Stone reward, the player must seek and destroy six UFOs flying around the stage. The UFOs move around in an erratic fashion, which can make them hard to hit. If a UFO is destroyed, it gives a prize. A Ring Bonus for UFOs with yellow frames, and a temporary speed boost for ones with white frames. If the timer goes below 20 seconds, a special blue-and-red UFO appears in the center. Although this UFO doesn't count towards the actual UFO count (in other words, the number won't decrease), it awards the player an extra 30 seconds, allowing them more time.

In addition, there are many different types of stage environment that can make or break your game. Springs bounce Sonic upward, bumpers bounce Sonic back if he tries to stray off-course, fans make him hover for a short time, chopper tiles slow him down and cause Sonic to lose rings, and dash panels force him into different directions. If Sonic steps into the water portions of the stage, he will proceed slower, and ten seconds are lost from the clock. In the Special Stage Time Attack, the water does not cause a time penalty.

Unused Special Stage
An extra special stage can be accessed from the Sound Test menu by setting all the 3 selections to 07. According to a former Sega Europe QA Tester, an extra Time Stone was to be collectable here. When collected in addition to the previous Time Stones, "S" TV monitors would appear where the ring TVs were previously located. When broken, Sonic would morph into a gimped version of Super Sonic, which would simply grant invincibility and greater speed. Graphics would not change.

Beta Bonus Stage
In an early prototype version of Sonic CD, there was a bonus stage similar to the Special Stages in Sonic the Hedgehog (1991).

Bosses
Like in the first game, an Act 3 (in this case, Zone 3) is the final level of the round, and where the player will face off with Robotnik. However, it takes place in a Bad Future, unless the player can change it to a Good Future.
 * 1) EGG-HVC-001: Robotnik's machine will fall into the boss arena, revealing a giant walker with Bumpers placed on its arms. His machine's feet are protected with spikes, making it difficult to attack from the front. The player will need to reach the machine's head, and then destroy the arms. Then, Robotnik isn't protected. Once his machine is defeated, Robotnik will fly away with a jet-pack strapped to his back. If the player obtains the invincibility at the beginning and quickly gets to Robotnik while it is still in effect, he can be defeated very easily.
 * 2) Collision Chaos boss: Robotnik's Eggmobile is strapped to a metal table where he will sit there laughing until Sonic can reach him and destroy his machine. The layout of the boss arena is a giant pinball-like maze where the player must hit the flippers and launch Sonic in the right direction. Then, the player can jump up and hit Robotnik's vehicle, and he will fly away in his Eggmobile.
 * 3) Tidal Tempest boss: As Sonic chases after Robotnik, hitting his machine about four times, Robotnik will flee into the water. Once Sonic approaches, Robotnik takes a bunch of air bubbles to surround him (proving to be a big mistake, because Sonic can grab air to survive underwater), and will shoot four electric pulses while passing through the middle to the other side. After destroying most of the shield, Robotnik is basically a free target. Then, the water will fall down, revealing dry land and a capsule with flower seeds.
 * 4) Quartz Quadrant boss: Robotnik appears in a tubular control area where a set of spikes guards the place from where Sonic came from, and a mine dropper that Robotnik controls. Sonic will need to run on the moving conveyor belt, but also avoid the mines Robotnik drops (Which will split into four tiny ones). After some big efforts, Robotnik's machine will crash and he will run away, giving Sonic room to complete the level.
 * 5) Wacky Workbench boss: The new machine designed by Robotnik is a giant rocket where he sits inside, piloting it. Once hit, Robotnik will get angry, and make four blocks fall down (on the third one, spikes will fall). Then, an electromagnetic shock will be made on the floor, causing the blocks to rise. Once Robotnik's rocket is destroyed, a massive explosion will occur, and Robotnik will run away.
 * 6) Metal Sonic: Once reaching Metal Sonic, Robotnik will appear in the Eggnobile and then uses a laser blaster, burning the track before the race. During the race, if Sonic falls too far behind, he will be destroyed by Robotnik. If Metal beats Sonic, the door will close behind Metal and Sonic will get zapped by Robotnik. If Sonic beats Metal, the door will close behind Sonic, Metal Sonic will be destroyed, and Robotnik can't breach the door. Sonic will have saved Amy Rose, and the level will end.
 * 7) Metallic Madness boss: Robotnik's final machine consists of a ball-shaped pod, with four giant rectangular blocks to protect his vehicle. He can use them to walk, shoot, and bounce. They will rotate around his pod, going at higher speeds each time he loses one. Each time Robotnik is hit, one of the blocks will fall off. Once the machine is destroyed, the factory starts exploding, and Amy Rose appears. To secure a good ending after this boss, Sonic either needs to collect all the Time Stones or create a good future in every since zone of the game.

Development
After the release of Sonic the Hedgehog, Lead Programmer Yuji Naka had grown dissatisfied with the rigid corporate policies at Sega, so he moved to the United States to work with the Sega Technical Institute. Incidentally, a large number of the original design team of Sonic also left for the U.S., to help instruct the American developers. With half of Sonic Team and two of its most important creators present, the Sega Technical Institute eventually got the job to develop Sonic the Hedgehog 2.

Meanwhile in Japan, Sonic the Hedgehog CD was handled by a separate development team, headed by Sonic creator Naoto Oshima. Initially, as revealed in interviews and magazine clippings, Sonic the Hedgehog CD, and Sonic the Hedgehog 2 for the Mega Drive/Genesis, Master System and Game Gear were all supposed to be the same game. However, during development, Sonic the Hedgehog CD evolved into a vastly different type of game. Eventually, the gameplay of Sonic the Hedgehog 2 would be favored for the future games, but this explains why the theme and handling of Sonic the Hedgehog CD are different, as well as the use of the original Sonic the Hedgehog's sprites for Sonic. The time posts also had pseudo-3D sprites similar to the Knuckles' Chaotix title screen.

Sonic the Hedgehog CD was released after Sonic the Hedgehog 2 but before Sonic the Hedgehog 3. Some fans believe the story of Sonic the Hedgehog CD either takes place before Sonic the Hedgehog 2 or after Sonic & Knuckles due to the misconception that Sonic the Hedgehog 3 takes place directly where Sonic the Hedgehog 2 left off, but in actuality there was some short time in between those events. However, due to considerable delays, it is apparent that Sonic the Hedgehog CD was supposed to launch at the same time, if not before, Sonic the Hedgehog 2.

Soundtrack Differences
The US and Japanese versions feature two different soundtracks, with the European release sharing the Japanese soundtrack. The Japanese soundtrack was composed by Naofumi Hataya, and featured songs by Keiko Utoku. The songs were entitled "Sonic - You Can Do Anything" and "Cosmic Eternity - Believe In Yourself".

The US version was delayed a few months to have a new soundtrack composed by Spencer Nilsen, who did other Sega CD soundtracks as well as some early Sega Saturn soundtracks. All the music (save for the Past tunes, which were in PCM audio rather than Red Book CD Audio) was replaced, and new themes were composed. The new theme was "Sonic Boom", performed by Pastiche (Sandy Cressman, Jenny Meltzer and Becky West). Both the opening and ending had similar lyrics but different instrumentation. This is credited as the "Special Edition for North America" soundtrack.

The intro and ending FMV sequences were slightly re-edited to fit in time with the respective music. Since then, every re-release of the game in the US (up to this point) exclusively has this soundtrack, including both the PC version and the one in Sonic Gems Collection (the latter having the Japanese soundtrack in Japan but with the slightly altered programming of the US version's) as well as in the special features section of Sonic Mega Collection; apparently due to licensing issues, the Japanese soundtrack has never made it to the States. However, it has surfaced in Sonic Screensaver and Sonic Jam and as several remixes in some 8-bit Sonic games, making it seem to be a faulty argument.

This, rather infamously, caused the biggest JP/US version differentiation review clash ever when GameFan magazine, who gave the Japanese version 100%, gave the game a less-than-flattering score for the US version and it was made clear that the score had been based on the US version's soundtrack alone, as rather than any changes in the gameplay.

It is interesting to note, that the last development version of Sonic CD (after the Japanese release) which was run on American NTSC systems, contained the Japanese soundtrack completely intact, indicating that at one point in localization, the soundtrack wasn't considered for revision. However, when it came time to release, the soundtrack was completely replaced. This version had no other significant changes from its Japanese counterpart other than incorporating the "pause + button" trick to reset a stage. Also the Japanese "Past" themes were not taken from the North American version of Sonic CD, so the original Past themes are mainly all that remains of the original Japanese game's soundtrack.

In the 2011 re-release, both soundtracks were available as an option, with the Japanese version as the default. However, due to licensing issues, the vocals for Sonic - You Can Do Anything and Cosmic Eternity - Believe in Yourself had to be removed, although Sonic Boom (from the US soundtrack) was untouched. In addition, almost all of the music has a short loop to extend it indefinitely rather than fading out and playing at the beginning, which was an idea cut from development.

Ports
Sonic CD was ported to PC CD-ROM in 1996, marking Sonic's debut on the PC under the Sega PC brand. This version was released in Japan on 9 August 1996, in North America in 26 August 1996, and in Europe in 3 October 1996. Among the most noteworthy changes of this version was the fact that a longer version of the FMV animated intro sequence is available for this version. The Mega-CD version had a different, lower quality version of the intro and ending sequence. The Japanese version of the game had its manual translated from the US version, and all versions had the US soundtrack, with the "Past" tunes converted to normal CD tracks.

Due to the design of Microsoft DirectX in Windows NT-based machines (namely, Windows NT, Windows 2000, and Windows XP), the installation program for the game can crash. While this can be worked around (usually by using compatibility modes in Windows XP, which rarely works on Sonic CD's installer), the program will still not execute. Using a third-party patch written by a member of the Sonic community, Sonic CD will run on the latest versions of Windows.

While the DirectX version of Sonic CD for PC is the most common and the best-selling initial commercial game for Windows 95, it is not the first version of Sonic CD for PC. The original version of Sonic CD for PC was powered using Dino libraries, an Intel-developed precursor to DirectX. This version of Sonic CD was never individually sold at retail, it was only sold with Packard Bell computers as a pre-installed game, and sold as double-packs along with other PC Sonic games. Upon the release of DirectX 3, Sega ported the Dino dependencies to DirectX calls and released Sonic CD in its DirectX form.

Other differences in this version include some minor port issues - the title screen and Special Stages ran at roughly twice the speed they were intended for. In Collision Chaos, there is no trap door near the pinball boss that stops Sonic from going back, and thus the boss music plays with an odd transition. The Tidal Tempest boss no longer slows the game slightly when the bubbles generate at the beginning of the fight. There is now a loading screen that shows a picture of Little Planet from the DA Garden. That music test also includes past music, although Little Planet is still depicted in modern style, and some of the non-level music is mysteriously gone. Some sound effects in the sound test are also missing, so the number codes only use two listings rather than three. Some of the fading effects are missing. There is an option for 30fps and 60fps. The save menu eliminates the RAM cartridge option and is made more intuitive and less confusing. Lastly, there is also now a secret debug menu.

In addition to the PC port, Sonic CD is part of the Nintendo GameCube (and, in Japan and Europe, PlayStation 2) compilation Sonic Gems Collection. This version is, in turn, ported from the PC version with some enhancements regarding the game's frame rate and action speed (with no frame rate slow-down). This is likely because to emulate the Mega-CD original, Sega would have to emulate both the Mega Drive/Genesis processors in addition to the new processors for the Mega-CD; which might have been difficult, if not impossible for GameCube and PlayStation 2 hardware to do at full speed. As a result of the PC port, Sonic CD on Sonic Gems Collection features audio converted from 11 kHz WAV for all its sound effects (which is noticeably lower quality than the other games included), as well as the high-color versions of the intro and ending videos; however, it also is missing certain effects present in both the Mega-CD and PC version, such as transparent water in Tidal Tempest Zone, or fades of any sort. There are other mistakes as well, such as the background of Stardust Speedway's Bad Future containing static lightning in reverse, and the sound effects not registering properly for the Stardust Speedway boss. The soundtrack in this version depends on the region, though European versions of the game still contain the American soundtrack. The Stardust Speedway boss can play either the bad or good future just like the American and PC version, and the Little Planet theme still places under the Time Attack menu. Lastly, the Debug Mode was not properly ported over by the programmers, and is incomplete.

Remake and a Special Port
A port of the game was released in December 2011 to Xbox 360, PlayStation 3, Android, Windows Phone, and iPhone. The default music is from the Japanese release, but the player can switch the music from the Japanese soundtrack to the U.S. soundtrack. On 14 December 2011, Sonic CD was released for the Xbox Live Arcade, iOS, and Android. The PC and Windows Phone 7 versions were released in early 2012. The PlayStation Network port was released on 20 December. There are achievements for both Xbox Live Arcade and PC, PlayStation 3 Trophies, special iOS features and more. Tails is a playable character, unlockable after the player beats the game with Sonic. This version includes options to choose between using the Sonic CD Spin Dash or the Sonic 2/3 Spin Dash instead, and also widescreen TV support as it is running on a new engine (named the 'Retro Engine' by Christian "Taxman" Whitehead, the director of this port).

Trivia

 * The names of all seven stages are alliterations.
 * This is the first Sonic game available for the PC.
 * In the Sonic Gems Collection version of Sonic CD, the water in the game is clear. This is because it is a direct port of the PC version, which utilized a certain graphics card for the water which wasn't emulated.
 * The past soundtracks can't be played in the D.A. Garden, but they can be played in the "Play Music" option in the PC and Sonic Gems Collection versions of the game. This is because the past music isn't considered redbook audio, and played off the Mega Drive hardware rather than the CD unit (which is also why it loops properly).
 * One of Sonic's animal friends only appears in this game - a fish who is not credited as it only appeared briefly in Tidal Tempest, being tormented by the holographic Metal Sonic. Once the projector is defeated, the fish will follow Sonic, as would that of an average Flicky.
 * Sonic CD is the first in the Sonic game series to feature animated cutscenes, which were produced by Toei Animation, with cooperation from Studio Junio.
 * The animated short that introduces the Mega-CD version of Sonic CD is substantially shorter than the intro in the PC and GameCube ports, as well as the Sega Saturn Sonic Jam video, but animates slower. Bscap023.jpg
 * The uncut intro shows more of Sonic running through the landscape and over a lake. Strangely, a very small cut (about two seconds of footage) of the mountainside that Dr. Robotnik uses to tether the little planet down, which has a massive carving of Robotnik's face on it, is missing from the uncut intros but present in the Mega-CD version. There are also a short segment missing from the ending FMV of the Mega-CD version that were restored in Sonic Jam and Sonic Mega Collection. This infamous segment is viewed in its original form at the Pencil Test.
 * When accessing the debug mode of Sonic CD, a secret picture of Tails appears. Tails also appears in the Tornado, at the D.A. Garden/Play Music.
 * The cover of the Japanese boxart contains a message "To live a life of power, you must have faith that what you believe is right, even if others tell you you're wrong. The first thing you must do to live a life of power is to find courage. You must be ready to reach beyond the boundaries of Time itself. And to do that, all you need is the will to take that first step..."
 * In addition to the change of soundtrack, the U.S. version also replaced Amy Rose's name in the manual with that of Princess Sally to garner support for the television show, despite the fact that the Sally character was already established as a completely different looking and separate character. Oddly enough, the earlier European manual had a nearly identical storyline, except the character was identified as "Amy". The PC port used a storyline closer to the European release. In both manuals, she was kidnapped before the in-game event in Collision Chaos, spoiling the surprise.
 * This was the first Sonic game to include centiseconds in the time as opposed to just minutes and seconds (others would follow such as Sega 32X's Knuckles' Chaotix, as well as all of the Competition Race stages in Sonic the Hedgehog 3).
 * Loading the game CD into a regular CD player will enable one to hear the game's music.
 * In the Japanese and European Mega-CD versions, the bad ending had the text "TRY AGAIN AND SAVE LITTLE PLANET FOREVER". In the North American release, the second line was removed, resulting in simply "TRY AGAIN". This change is retained in the PC port (which was based on that release) and the 2011 digital version, the latter of which may be due to the then-upcoming Sonic the Hedgehog 4: Episode II. The North American version had other minor changes besides soundtrack, such as allowing the player to restart the level in their current time period at the cost of a life.
 * Issue #25 of the Archie comic book features a cover story loosely based on Sonic CD.
 * Both soundtracks of the game share the same past theme (which is a remix of the JP version) due to how the past themes were encoded into the game itself using PCM format, instead of the rest of the themes which utilize the Mega CD's Redbook audio capabilities.
 * Sonic the Comic adapted the story of Sonic CD in its "The Sonic Terminator" arc, from Sonic the Comic #24 to #28.
 * In Sonic Adventure, when Amy is remembering "the good ol' days", Sonic is seen rescuing Amy from Metal Sonic in her daydream fantasy. This is probably her remembering scenes from Sonic CD. However, in the flashback, Amy doesn't retain her old school design and Sonic will have any upgrades he has acquired by that point. Most likely, the programmers did not bother to create a new character model of Amy's old design for use in one cutscene.
 * This is the second Sonic game with voices, the first being SegaSonic the Hedgehog. When Sonic does one of two actions, such as when remaining idle for three minutes, Sonic will shout "I'm outta here" and jump off the screen, resulting in a game over, or when getting an extra life, Sonic will shout "Yes!". In the U.S. version, when the words "GAME OVER" appear on the screen, evil laughing can be heard, presumably Robotnik. In Palmtree Panic, if Amy grabs Sonic, she will giggle, and when she is grabbed by Metal Sonic in Collision Chaos, she yipes.
 * Most of the sprites for Sonic appear to come from Sonic the Hedgehog, with the exception of some, such as the Super Peel Out, spring jumps, and the 3D images from the beginning of Palmtree Panic, Wacky Workbench, and Metallic Madness. This is probably the extent of the sprite collection only for Sonic CD.
 * The same goes for Tails in the 2011 version, but the sprites were from Sonic the Hedgehog 2. However, in addition to his skin being paler in this game, there are new animations for our two-tailed friend, besides the same animations used for Sonic with a kitsune twist. When Tails starts flying his animation is taken directly from Sonic the Hedgehog 2; but when Tails tires himself out from flying, one can see that his panting animation is taken from the unused Sonic Crackers ROM with an added arm. Tails' swimming animation however was simply a redone sprite from Sonic the Hedgehog 3 replacing the head with that of Sonic the Hedgehog 2, same with the panting animation.
 * This is the first Sonic game in which Sonic runs with his arms held out backwards (either by using the Super Peel Out or moving as a fast enough speed).
 * The American theme song, "Sonic Boom", is a playable track in Super Smash Bros. Brawl on the Green Hill Zone stage.
 * Originally, the Japanese theme, "Sonic - You Can Do Anything", was going to be put in.
 * If the sound test is unlocked and codes are inputted, several hidden pictures can be unlocked:
 * On the Sega CD version, you can access the sound test menu by pressing down 3 times, Left once, right once, and pressing "A". Then, when you hear the ring collecting sound, press start.
 * A screen with Tails next to a car, saying "See you next game!" (NOTE: also activates debug mode. Sound test menu FM40 PCM12 DA11), which is probably a note of the upcoming Sonic Drift video game.
 * A human-like Sonic screen, with what appears to be a Batman-like Sonic suit. (FM42 PCM04 DA21)
 * A screen with Sonic, Metal Sonic, and Dr. Robotnik in a band called "The Fastest DJ". (FM42 PCM03 DA01)
 * A drawing of Sonic with big, grey eyes with Japanese text above him. The text reads: "ゆーあーくーる", which can be translated as "You are cool" in English. (FM44 PCM11 DA09)
 * A creepy looking hidden picture of multiple Sonics bent over and giving the thumbs up with a heavily distorted, Dali-esque face with a message in white Japanese text. The text reads: "たのしさ∞ - セガ・エンタープライゼス - まぢん画"; which can be translated as: "Fun is infinite with Sega Enterprises. Majin." "Majin" can mean a number of different things in English (demon, devil, genie) and it can be spelled "madjin", "madin" or "madzin" when romanized. The overall look and atmosphere of the hidden screen make it look like the message came from a "demon". The accompanying music, the Dr. Robotnik battle song from the North American soundtrack, further adds to the creepiness factor. (FM46, PCM12, DA25) However, further investigation shows that "majin" seems to, in fact, simply be a clever nickname of Masato Nishimura, who inserted まぢん into a number of other games such as Shenmue. In short, his given name can be read as "Majin".
 * The picture would also appear if attempting to play an early pirated copy of the game.
 * In the 2011 remake, upon entering the code in the Sound Test, "PCM:32, DA: 8", a new hidden picture will be unlocked exclusively to this version, depicting Sonic in a desert stage known as Desert Dazzle. This went unused during the development of the remake due to time constraints, but the design of the stage looks similar to the scrapped Dust Hill Zone of Sonic the Hedgehog 2. The initials "C.W." is of the remake's developer, Christian Whitehead who is also known as "The Taxman".
 * If Desert Dazzle were added, it would have likely taken the place of the "R2" folder that is missing in the original Mega CD version, taking place between Palmtree Panic and Collision Chaos.
 * During the ending credits, when Sonic is in Collision Chaos, he is seen spin dashing on some boxes, much like the Bonus Stage in Knuckles' Chaotix.
 * In debug mode, there are sprites of Sonic sneezing. Sonic also sneezes in the intro.
 * In debug mode, an unused monitor containing a silver ring can be added during gameplay. When the monitor is broken, the power up will give Sonic 50 rings and will make the Lamp Post sound effect.
 * In the animated cutscenes, Sonic's shoes have the gold buckles on the sides, but the buckles don't appear during gameplay.
 * In the Japanese and European versions of the Invincibility music, if one listens closely, a countdown can be heard near the end; starting from five to one. After one, the power-up ends.
 * Sonic CD is the first game in the series in which animals do not pop out of any destroyed Badniks. Instead, seeds that sprout flowers will pop out and slowly flutter to the ground. It has been suggested that this is the case because Dr. Robotnik wanted to replace the planet's plantlife with machinery in this case. (1)
 * The concept of something happening if the player is idle is similar to that of Knuckles' Chaotix, as in said game, Metal Sonic will appear and attack the player. This could be because both games are made by much of the same staff.
 * The opening theme for U.S. version, Sonic Boom, is given as the title for the E3 2011 demo for Sonic Generations. It is also an unlockable song.
 * This game is represented in Sonic Generations. Metal Sonic reappears as the Rival Boss of the Classic Era in both the home and portable versions.
 * This is the first Sonic game (and the only pre-Sonic Adventure game, along with the 8-Bit version of Sonic the Hedgehog 2, and Sonic Chaos) to have a separate theme for the Speed Shoes as opposed to speeding up the level's music.
 * The first achievement that can be obtained in the 2011 remake, "88 Miles per Hour", is a clear reference to the Back to the Future trilogy, as the DeLorean needed to hit 88 MPH to go through time. It is also believed that Sonic does, in fact, need to be traveling 88 MPH in order to back in time.
 * Sonic the Hedgehog 4: Episode II has a connection to CD in that Little Planet returns to Sonic's world (seen in the screenshots) as it's being completely mechanized for Dr. Eggman's plan in the game. Also, Metal Sonic returns directly from this appearance as a boss and the secondary villain in the game.
 * Ken Balough has also stated that Sonic CD has been said to take place between Sonic the Hedgehog and Sonic the Hedgehog 2, but there is currently no official spot on the timeline other than it takes place sometime before Sonic the Hedgehog 4.
 * The achievement "King of the Rings" could be a reference to a fairly old Sonic song with the same name.
 * 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.
 * This is the first Sonic game to have a continue feature at the menu screen.
 * Nearly all of the names of the games Badniks are simply Japanese for the animal they were based off of.
 * Because of the Bad Futures' art direction and the U.S. soundtrack (most noticeably the Bad Future tracks, the boss theme and the Game Over theme), this has been dubbed one of the darkest Sonic games ever made, next to Shadow the Hedgehog and Sonic '06, despite its age rating.
 * 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.
 * In the 2011 re-release, there was originally going to be a R9 Boss (should there actually be a R2) where it is only accessible after the player has collected all of the Time Zones but was ultimately scrapped.