Sonic the Hedgehog CD (ソニック・ザ・ヘッジホッグ ＣＤ Sonikku za Hejjihoggu Shīdī?), more commonly known as Sonic CD (ソニック ＣＤ Sonikku Shīdī?), is a 2D platformer game in the Sonic the Hedgehog series, developed by Sonic Team 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 23 November 1993. The game was ported to PC CD-ROM in 1996.
- 1 Plot
- 2 Gameplay
- 3 Characters
- 4 Rounds
- 5 Development
- 6 Soundtrack differences
- 7 Re-releases
- 8 Cheat codes
- 9 Reception
- 10 Trivia
- 11 Videos
- 12 References
- 13 External links
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 Round 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 dismay 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 in the third Zone of every Round. Once 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 doppelganger, 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 and the game tells the player "You're Too Cool!"; 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."
Sonic the Hedgehog CD is a side-scrolling 2D platforming video game which plays similar to past Sonic games. The playable character here is Sonic, whose most basic actions involve running and jumping. There are a total of seven levels (known as round) in the game, each divided in three Zones.
The main goal of Sonic the Hedgehog CD is to beat a series of Zones in less than ten minutes. Additionally, the player can also find Rings, which work as Sonic's most basic defense against obstacles. If Sonic is hit while having at least one Ring in hand, he will survive at the cost of losing all his Rings. However, if he is hit without any Rings, he will lose a life, or get a Game Over if he does not have any lives left.
Throughout the first two Zones of a round, the player can find Time Warp signs, whose main purpose is to make their users enter the Time Warp. If Sonic touches a Time Warp sign and then runs at a determined speed during a period of time, he will enter the Time Warp and end up in the right timeline; touching a "Past" Time Warp sign will send Sonic to the past, while a "Future" one will send him to the future. However, in the past, only the "Future" Time Warp signs are seen, which will send Sonic back to the present. The same is applied to the future.
The player will start at the present in the first two Zones of a round, while Zone 3 will always take place in the future. By default, the future will be depicted as a chaotic place ruled by Dr. Robotnik that has ruined Little Planet's flora and fauna due to the excessive pollution. However, if the player travels to the past and finds a robot transporter, they will be able to destroy it and create a "good future" in the Zone. Getting a good future in the first two Zones of a round will make Zone 3 of it take place in the good future.
|Button formation||Character movement|
|left/right + down||Spin Attack|
|/ /||Spin Jump|
|up + / /||Super Peel Out|
|down + / /||Spin Dash|
|START button||Pause the game / Skip the Time Warp animation|
- Air Bubble
- Little Planet flowers (first appearance)
- Metal Sonic Projector (first appearance)
- Robot transporter (first appearance)
- Super Ring (first appearance)
- Time Bonus (first appearance)
- Time Stone (first appearance)
- UFO (first appearance)
Gimmicks and obstacles
- Amy Rose (first appearance)
- Dr. Robotnik
- Metal Sonic (first appearance)
- Miles "Tails" Prower (cameo)
- EGG-HVC-001 (Palmtree Panic) (only appearance)
- Collision Chaos boss (Collision Chaos) (only appearance)
- Tidal Tempest boss (Tidal Tempest) (only appearance)
- Quartz Quadrant boss (Quartz Quadrant) (only appearance)
- Wacky Workbench boss (Wacky Workbench) (only appearance)
- Metal Sonic (Stardust Speedway) (first appearance)
- Metallic Madness boss (Metallic Madness) (only appearance)
- 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 an established casino. In the past, Collision Chaos shows a rather surreal, orange tinted forest. 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 buildings in the 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.
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 does not count towards the actual UFO count (in other words, the number wo not 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.
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 Ohshima. Initially, as revealed in interviews and magazine articles, 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 Sonic the Hedgehog's sprites for Sonic.
The North American 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 vocals by Keiko Utoku. The opening and ending songs were entitled "Sonic - You Can Do Anything" and "Cosmic Eternity - Believe In Yourself".
The North American version was delayed a few months to have a new soundtrack composed by Spencer Nilsen, who worked on other Sega CD soundtracks as well as some early Sega Saturn soundtracks. All the background music and jingles were replaced, save for the "Past" tunes. Those tracks were sampled and played back using the Ricoh RF5C164 in the Sega CD, rather than the streaming Red Book CD audio used for the remainder of the soundtrack. 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. The North American soundtrack received an album release the following year as Sonic the Hedgehog Boom.
The 1996 PC release featured the North American soundtrack in all regions, including Japan and Europe. The Sonic Gems Collection version, which was based upon the PC port, featured the North American soundtrack in the North American and European releases, and the Japanese soundtrack in the Japanese release. The enhanced 2011 version of the game featured the Japanese version of the soundtrack by default, with the option to switch to the North American soundtrack. However, the songs "Sonic - You Can Do Anything" and "Cosmic Eternity - Believe in Yourself" lack vocals, due to licensing issues.
This difference in soundtracks, rather infamously, caused the biggest JP/NA version differentiation review clash ever when gaming magazine GameFan, who gave the Japanese version 100%, gave the game a less-than-flattering score for the U.S. version and it was made clear that the score had been based on the U.S. version's soundtrack alone, rather than any changes in the gameplay.
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 was not 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.
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 such as the title screen and Special Stages running 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 and 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, 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.
On 14 December 2011, Sonic the Hedgehog CD was remastered 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 December 20. Notable additions include Tails being an unlockable playable character, the option to toggle between either the Japanese or U.S. soundtracks and the ability for the Spin Dash to be performed in its original incarnation (Original) or from Sonic the Hedgehog 2/Sonic the Hedgehog 3 (Genesis) Spin Dash. It features widescreen TV support as it is running on a new engine (named the "Retro Engine" by Christian Whitehead, the director of the re-release).
The PS3 version of Sonic the Hedgehog CD, although incompatible with the PS3's successor, the PlayStation 4, became the launch title for the open beta of PlayStation Now on the PS4, becoming the first Sonic game to be released on PlayStation Now. The Xbox 360 version was made compatible with its successor the Xbox One when backwards compatibility was introduced for said console.
In 2018, the 2011 version of Sonic the Hedgehog CD was included as a "classic" game in the Sega Forever service for mobile devices, making it currently the third Sonic title to get added to the list, along with the original Sonic the Hedgehog and Sonic the Hedgehog 2.
Palmtree Panic's Music was changed to "Palmtree Party" in this 2011 re-master; however, the title of the Round was not changed.
- Stage Select:
- Mega-CD: At the title screen, press UP, DOWN, DOWN, LEFT, RIGHT, B.
- PC: At the title screen, press UP, DOWN, DOWN, LEFT, RIGHT, SPACE.
- View staff's Time Attack records:
- Mega-CD: At the title screen, press RIGHT, RIGHT, UP, UP, DOWN, C.
- PC: At the title screen, press RIGHT, RIGHT, UP, UP, DOWN, SPACE.
- Move title screen clouds:
- Mega-CD: At the title screen, hold A and press UP, DOWN, DOWN, DOWN, DOWN, UP. The clouds can then be controlled using the directional pad on the second controller.
- Sound Test:
- Mega-CD: At the title screen, press DOWN, DOWN, DOWN, LEFT, RIGHT, A.
- PC: At the title screen, press DOWN, DOWN, DOWN, LEFT, RIGHT, SPACE.
- Edit Mode:
- Mega CD: At the sound test, select the following then press START: FM40, PCM12, DA11. An image of Tails will also be displayed.
- PC: At the sound test, select the following then press SPACE: PCM#12, DA#11. An image of Tails will also be displayed.
- Secret Special Stage:
- Mega CD: At the sound test, select the following then press START: FM#07, PCM#07, DA#07.
- PC: At the sound test, select the following then press SPACE: PCM#07, DA#07.
- Hidden Images:
- At the sound test, select the following then press START. For the PC version, select the PCM and DA numbers and press SPACE:
|"You are cool"
The Palmtree Panic Good Future theme plays here.
|Parody of Batman.
The final boss theme plays here.
The Metallic Madness Present theme plays here.
|"Infinite fun. Sega Enterprises"
- Mazin Picture
The boss theme plays here.
|Tails and his favorite car, the Lotus Seven. Also enables Edit Mode.
The end credits theme plays here.
|Computer and Video Games||85/100|
|Electronic Gaming Monthly||34/40 (SCD)|
|GameFan||400/400 (MCD) |
|Electronic Games||92% (SCD)|
|Sega Force Mega||85% (MCD)|
Sonic CD received critical acclaim, with a consensus that it was one of the best games for the platform. The game was praised for its innovative time-travel based gameplay, presentation and music. It became the platform's best-seller. The Android version later sold more than 100,000 paid downloads.
Mega placed Sonic CD at #3 in their list of the Top 10 Mega-CD Games of All Time. The game was awarded Best Sega CD Game of 1993 by Electronic Gaming Monthly. In May 2009, GamePro listed Sonic CD as one of the Top 20 Best Platformers: 1989 to 2009, ranking the game in 12th place. GamesRadar listed Sonic CD as the 68th best game of all time.
- All seven of the game's Rounds have alliterative names.
- 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 was not emulated.
- The "Past" background music tracks, which are in PCM format, cannot be played in the D.A. Garden. They can, however, be played in the "Play Music" option in the PC and Sonic Gems Collection versions of the game, but the image of the Little Planet remains the same as the "Present" version.
- Sonic CD is the first in the Sonic game series to feature animated cutscenes.
- 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.
- 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 is also a short segment missing from the ending FMV of the Mega-CD version that was restored in Sonic Jam and Sonic Gems Collection. This segment is viewed in its original form at the Pencil Test.
- When accessing the Edit Mode of Sonic CD, a secret picture of Tails appears. Tails also appears in the Tornado, at the D.A. Garden/Play Music.
- The front cover of the Japanese version contains the 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..."
- The prologue in the North American instruction manual is almost identical to its European counterpart. However, in the North American Sega CD version, Amy Rose is named Princess Sally. This was done to tie in with the television show, in which Sally was the lead heroine. The manual describes the character as a young hedgehog, whereas in the television show Princess Sally is a chipmunk. This alteration is not present in the PC release, where she is named Amy.
- 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 the soundtrack, such as allowing the player to restart the level in their current time period at the cost of a life.
- Sonic the Hedgehog #25 and Sonic the Hedgehog #290 of the Archie comic book both feature adaptations of Sonic the Hedgehog CD's story.
- Sonic the Comic adapted the story of Sonic the Hedgehog CD in its "The Sonic Terminator" arc, from Sonic the Comic #24 to #28.
- 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 outer 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 and Stardust Speedway, if Amy hugs Sonic, she will giggle, and when she is grabbed by Metal Sonic in Collision Chaos, she yipes.
- Many players have believed that Sonic shouts "I'm outta here!" when he jumps off the screen and causes a game over. It was not until years later until Masato Nishimura (a landscape designer for the game and the one who provided the voice clip for Sonic) corrected this stating that Sonic actually says "I'm outer here!".
- Most of the sprites for Sonic 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.
- Most of Tails' sprites were reused from Sonic the Hedgehog 2, but with a paler color palette. As with Sonic, Tails also received several new sprites. 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 was a modified sprite from Sonic the Hedgehog 3 replacing the head with the one in Sonic the Hedgehog 2, same with the panting animation.
- Despite using sprites from Sonic the Hedgehog, Sonic does not bob up and down at all when he walks: his head stays perfectly steady.
- 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 at a fast enough speed).
- 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 Sand Shower 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".
- 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 Edit Mode, there are sprites of Sonic sneezing.
- In Edit 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 lamppost sound effect.
- 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.
- 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.
- The first achievement that can be obtained in the 2011 remake, "88 Miles per Hour", is a reference to the Back to the Future film series, as the DeLorean needed to hit 88 MPH to travel through time.
- Sonic the Hedgehog 4: Episode II has a strong connection to this game, as Little Planet returns to Sonic's world and is Dr. Eggman's plan is once again to completely mechanize it. Also, Metal Sonic returns directly from this appearance as a boss and is the secondary villain in the game.
- According to Yasushi Yamaguchi in a 1993 article on the Japanese magazine BEEP Mega Drive, the story of Sonic the Hedgehog CD takes place between the original Sonic the Hedgehog and Sonic the Hedgehog 2.
- This is the first Sonic game to utilize a game save feature.
- When Amy is encountered in Palmtree Panic, she will only hug Sonic if he is facing the opposite direction. Amy will bounce off Sonic if she tries to hug him while he has a shield.
- LordDullahan (8 July 2013). Sonic the Hedgehog 4 Episode I and II are Now Available on OUYA! Sonic CD to follow.. SEGA Blog. Sega. Archived from the original on 12 July 2013.
- "1999 Video Game Buyer's Guide". Electronic Gaming Monthly: 141. 1999.
- GameFan, volume 1, issue 12 (November 1993), page 22
- GameFan, volume 2, issue 2 (January 1994), page 18
- Electronic Games, issue 15 (December 1993), page 140
- Peeples, Jeremy (27 June 2004). Sonic CD. Sega-16. Retrieved on 19 April 2014.
- Chris; Mark (January 1994). "Sonic the Hedgehog CD". Sega Force Mega 2 (7): 102–4.
- Electronic Gaming Monthly's Buyer's Guide. 1994.
- Official Gallup UK Mega-CD sales chart, February 1994, published in Mega issue 17
- Mega magazine issue 26, page 74, Maverick Magazines, November 1994
- McKinley Noble (6 May 2009). The 20 Best Platformers: 1989 to 2009, Feature Story from. GamePro. Archived from the original on 2009-11-19. Retrieved on 2011-11-23.
- The 100 best games of all time, Xbox 360 Features. GamesRadar (2011-04-01). Retrieved on 2011-11-23.
- Masato Nishimura on Twitter (Japanese). Twitter (9 July 2017). Retrieved on 6 December 2018.
- Sonic CD – Developer Interview Collection; originally featured in BEEP and Marukatsu MD magazines. Archived from the original on 24 March 2020. Retrieved on 2 June 2020.
- Sonic the Hedgehog CD at Wikipedia, the free encyclopedia.