The main innovation of this chapter in the Sonic series is the manner in which the player can travel to three different versions of each zone, each a different time period of the same location. 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 teleports 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 level, 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. Should the player warp into the future, it is a "Good Future" in which there are no enemies and fewer hazards. 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.
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.
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, whereas in the Sonic the Hedgehog 2 checkpoints are used to enter special stages. It is because of this that there are speculations that Sonic CD began development before Sonic 2. A giant ring will float above the finishing sign which Sonic can jump through to enter the special stage. They consist of a three-dimensional, flat surface. To complete a stage and collect the Time Stone reward, the player must seek and destroy six purple UFOs flying around the stage. If a UFO is destroyed, it gives a prize of either a super ring (have gold markings and give progressively larger bonuses starting with 20 rings when destroyed in series) or speed sneakers (have grey markings and temporarily boost speed). Collecting the seven time stones, only possible in the special stage, automatically guarantees that the player will reach the good ending even if one of the previously completed zones did not have a Good Future, and that all futures of upcoming zones will be good as well.
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 level design and atmosphere is significantly different in Sonic CD than other Sonic games. Sonic CD's levels are comparatively more colorful and psychadelic, giving it a slightly surreal, dream-like quality compared to other Sonic games.
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 purple 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. Springs bounce Sonic upward, bumpers bounce Sonic back if he tries to stray off-course, fans make Sonic hover for a short time, chopper tiles slow Sonic down and cause him to lose rings, and dash panels force him into different directions. Stepping into the water portions of the stage will cause Sonic to proceed slower and quickly lose time. In the Special Stage Time Attack, the water does not cause a time penalty.
Meanwhile in Japan, Sonic CD (or at this point, "CD Sonic" as it was first known ) was handled by a separate development team, headed by Sonic creator Naoto Ohshima. Initially, as revealed in interviews and magazine clippings , Sonic CD, and Sonic the Hedgehog 2 for the Mega Drive/Genesis, Master System and Game Gear were supposed to be the same game. However, during development, Sonic CD evolved into a vastly different type of game. Eventually, the gameplay of Sonic the Hedgehog 2 would be favoured for the future games, but this explains why the theme and handling of Sonic CD are different, as well as the use of most of the original Sonic the Hedgehog's sprites for the latter title. The time posts also had sprites similar to Knuckles' Chaotix. However, some vestiges of the original tie-in remain.
Sonic CD was released after Sonic the Hedgehog 2 but before Sonic the Hedgehog 3. Some fans believe the story of Sonic CD either takes place before Sonic the Hedgehog 2 or after Sonic & Knuckles due to the fact that Sonic the Hedgehog 3 takes place where Sonic the Hedgehog 2 left off. However, due to considerable delays and a few easter eggs, it is apparent that Sonic CD was supposed to launch at the same time, if not before, Sonic the Hedgehog 2, though technically "Sonic CD" happened both before and after every game because Sonic travels through time.
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; the Japanese soundtrack has never made it to the States. However, it has surfaced in Sonic Screensaver and Sonic Jam and has several remixes in some 8-bit Sonic games.
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.
If the Sega CD version of Sonic CD is played on a standard CD player, one can listen to all the "Present" and "Future" stage music with each "track" having a different song. The "Past" stage music is PCM audio and can only be played through the game's sound test. However, the PC port includes the past soundtracks in the Red Book standard as well nearer to the end of the CD.
One of the last North American development versions of Sonic CD contained the Japanese soundtrack completely intact. Ultimately, the soundtrack was completely replaced.
Notably, a remastered version of "Sonic Boom" was used as an unlockable music piece for use on a Sonic the Hedgehog-themed stage in Super Smash Bros. Brawl. This revision of the song is based on the opening movie version, but features slightly improved instrumentation as well as having been reconfigured to loop.
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
The PC port of Sonic CD is only playable on the older operating systems like Windows 95, Windows 98 and Windows Me. The DirectX version of game can still be played on Windows XP or Windows Vista via a fan-created patch.
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 ported from the PC version with some enhancements regarding the game's frame rate and action speed (with no frame rate slow-down), however it also suffered from minor graphical errors such as scrambled backgrounds and the lack of colour in the water from Tidal Tempest. The soundtrack in this version depends on the region, though European versions of the game contain the American soundtrack (unlike previous European releases which featured the Japanese soundtrack).
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