Prototype 510, sometimes known as CD Sonic the Hedgehog after the odd word ordering on the title screen, is an early prototype of Sonic CD. Its compile date is May 10th, 1993. The prototype showcases the game when it was roughly halfway complete; all the levels are functional, although only three can be accessed without level select. There's also quite a few differences from the final, ranging from early audio to additional power-ups.
The prototype was discovered by a group of fans called Sega Extreme in Summer 2000. The original disc has since been sold, and is now lost. The 510 prototype was the earliest known build of Sonic CD until 2019, when an even earlier one was discovered.
The game is always referred to as CD Sonic the Hedgehog, which is odd as the Japanese pronunciation does place the "CD" after the "Sonic the Hedgehog" part.
The title screen remains the same as it was in the v0.02 prototype, except a different song plays in this build.
A SEGA logo, complete with the classic "SEGA" choir, is present before the title screen is displayed. It was removed from later prototypes and the final version due to the BIOS including it.
Title cards have "CD SONIC THE HEDGEHOG" instead of "SONIC THE HEDGEHOG CD".
The HUD is the same as how it was in v0.02.
The lives counter is slightly higher than normal, and there are no zeros in the SCORE and RINGS counters.
Active time warps are not indicated on the HUD.
Amy looks different, having larger eyes and quills.
Each Zone has its own unique type of flower.
510 - v0.51
The v0.02 build used a red one with the words "You Say!," which was possibly a pun on the Japanese word "Yuusei." 510 - v0.51 both have a somewhat jollier Sonic. The final version re-drew Sonic's hand to be angled more toward his face.
The Exit selection in the Time Attack menu has a different graphic, depicting Little Planet.
Only the first three Zones are playable in normal gameplay. After clearing Tidal Tempest, the game sends the player to a scrolling screen with the words "COMIN' SOON" and then back to the title screen. The "COMIN' SOON" screen is still present in the final version, but is only accessible through the level select.
Getting a Game Over will also play the "COMIN' SOON" screen before going back to the title screen.
Code is in place for saving data, but only in Time Attack mode at this point.
Save data made by this build uses the filename "CDSONIC0000" instead of "SONICCD__xx", and is incompatible with later protos and the final.
Name entry is not yet possible in Time Attack mode.
The camera is locked for the first couple of seconds after the title card disappears.
You cannot control your momentum if you jump from a roll, which is normally the case in the Genesis games. This was changed in later protos and the final.
Coming within range of the goal does not cancel the time warp.
The time warp sequence cannot be skipped by pressing Start yet.
The Special Stage ring appears at the end of an Act regardless of how many rings you have collected, but it doesn't work properly yet, and entering it will not take you to the Special Stage.
Three additional powerups are present:
"S" Monitor: Sonic has increased speed and an invincibility shield. It has a red "S" symbol.
Blue Ring Monitor: Functions identically to the "S" Monitor. Has an image of a blue ring.
Clock Monitor: Has an image of a clock. This does not work correctly, but its role was to freeze time for a few seconds. It does however stop the spinning of rings and the movement of objects aside from Sonic.
Badniks are not disabled in Good Future variations.
Destroying both of the robot transporters in the Past does not change the future in Act 3 to a Good Future.
When you progress to the next Zone/Act, the next stage's title card won't show.
The game-ending "I'm outta here!" Easter egg does not work properly. The camera will follow Sonic as he jumps off the stage, and he'll die from a bottomless pit, losing a life and restarting the Act instead of causing an instant Game Over.
It is possible, using the second controller, to control a second, "virtual Sonic" in gameplay. Whether this is for debugging purposes or an early two-player prospect is unknown.
The music is largely identical to the final Japanese/European soundtrack, although fadeouts are missing.
The title screen/Time Attack music is almost completely different, with no resemblance to "Sonic - You Can Do Anything".
The Speed Shoes music is a different remix of "Sonic - You Can Do Anything":
The Special Stage theme is extended, lasting four-and-a-half minutes.
The Past stages all play the music from the Present, as their own tracks have not been implemented yet.
The Invincibility and Speed Shoes monitors do not play their respective music.
Pausing the game does not pause the music.
Several object sound effects are missing from the prototype.
The jump sound plays at a lower pitch.
The Super Peel-Out doesn't play any sound effects, but the Spin Dash does.
Destroying a Badnik plays a different sound rather than the traditional Badnik explosion sound.
The time warp sound is the same as the one in v0.02, which is very grating on the ears.
The opening video background is a layered brick background, with "SONIC" inscribed on each brick. Also under the FMV are the Japanese symbols "開発画面", which translate to "development screen". The FMV is also much closer to the original animation by Studio Junio.
The scene showing Little Planet chained to the ground is played in reverse.
The video in the prototype has a speed of 50Hz.
The scene showing the mountain with Robotnik's face and the attached chain has not been added yet.
The opening song glitches throughout, with some bugs and breaks during play.
On the title screen, press Start on Controller 2 and then press Start on Controller 1. A black screen with a list of numbers and letters corresponding to various levels and events appears. Make a selection and press Start to enter it.
Note that you will be taken back to the title screen upon completion of the selected level or upon attempting time travel.
Press Start on Controller 2 at any time during gameplay to enable/disable Debug Mode.
Proper full level maps for each zone and timezone.
Separate prototype flower sprites with their corresponding timezones.
Palmtree Panic has been significantly updated since v0.02, featuring new graphics for different time-periods, alongside some bug-fixes.
The flower for this stage is more or less identical to the final. However, the flowers are unique color-wise to each time period.
Amy acts differently than in the final. She will still latch on to you if you run past her, but if you stand still, she also just stands there. (In the final, she latches on even if you stop.)
The first monitor is a blue ring, this was removed in the final.
The "clock" monitor was replaced with a ring monitor in the final, though it's in an area normally impossible to access.
There's also a "S" monitor that was moved to Act 3, this was also moved to the unused room.
There is a section of a tunnel that appears in the final game, but not in the prototype.
There is a section of a trunk in a waterfall, which does not appear in the final version, and there's also a section of a tunnel in the final game, which is in the prototype but inaccessible.
In the tunnel, Sonic is not wound to enter and cannot leave through the other opening. Therefore, Sonic must enter the tunnel at top speed to leave through the other opening. In a secret tunnel, Sonic walks horizontally to climb.
Sonic doesn't automatically start rolling in tunnels.
Act 2 has no breakable walls.
In the Bad Future of Act 3, if Sonic is launched by a spring at the beginning of the level, it gets out of focus and the same scenario sticks in the same position.
The palm trees, star-shaped bushes, and foreground pipes in both Futures of Act 3 are early designs and haven't been updated to match the Futures of the first two Acts yet.
The unused room in Act 3 which contains a ring, clock and S monitor is empty.
The EGG-HVC-D01 has a different design: the robot is entirely gray, the feet are flat with sharp edges, and it has pincers rather than the shields from the final version. In the Bad Future, parts of the robot are red, while in the Good Future they are lilac.
The flower for this stage doesn't animate, strangely.
The beginning of the first two Acts are different. Act 2, for instance, has more tubes to go down.
At the beginning of Act 1, Metal Sonic is able to electrify Sonic.
The yellow cups do not function properly yet. They'll catch Sonic, but he won't get sucked inside and will simply freeze there, playing the last animation he was in prior to touching it (jumping or running) until he is freed. The cups won't earn you any points yet, either.
The glass bumpers don't have a breaking animation yet and won't award any points for breaking them.
A strange group of three spinning teleporter-like objects that don't appear in the final can be placed with Debug Mode. When Sonic jumps into one, he freezes in place and they light up, then he teleports to another object in the group.
After landing the final hit on the boss, you have to wait for Eggman to leave the machine and fly away before you can advance.
The flower for this stage seems to go for a "bubble" aesthetic compared to the final's "wavy" one.
For some archaic reason, the unused drowning numbers in this build (which would be used in v0.51a), barring different coloring, are exactly the ones seen in some prerelease footage of Sonic the Hedgehog.
The water is a different color in all timezones, and the graphics are somewhat different.
Spike balls appear in fours.
The waterfall at the beginning of Act 2 is a murky brown and doesn't animate.
A ripple effect is added to the foreground. Normally, it's only present in the background.
There is no splashing effect when jumping in and out of water.
A number of objects exist that were taken out of later protos and the final, such as an elevator that's operated by standing on a switch.
Act 3 is always in the Good Future regardless of whether you destroyed both robot transporters, albeit with the Bad Future music.
You cannot pause the game for some reason.
You can only access these levels with the level select.
The flower for this stage does not look very good with the palettes assigned to it, barring the Bad Future variation.
There are no rings.
A monitor showing Sonic's face can be found in the background at the very beginning of Act 1 Good Future. This doesn't appear in the final. In both this build's Bad Future and the final version's Bad Future, there is an Eggman monitor in the same position. This Sonic monitor was restored in the 2011 rerelease of Sonic CD, appearing in Act 3 instead.
The outdoor background in the Good Future has a different palette. The sky is black, the mountains are darker, and the foliage is less refined.
All objects use the same sprites across all timezones, appear slightly different from the final, and have incorrect palettes.
The conveyor belts were intended to be controlled by standing on buttons, but they don't do anything.
Breakable walls cannot be destroyed.
The collision is wonky in some places.
Act 2 is devoid of objects in the Past and both Futures, and some of the tubes don't work.
Act 3 is also missing objects, and there is a string of different item monitors before the boss room.
The spikes that appear behind you during the boss fight have very basic sprites in both Futures.
The flower for this stage is the exact same as Quartz Quadrant's, just with different palettes.
There are no rings.
In the Present, the background has a green color scheme, while the final has more of a blue color.
The Past has a blue color scheme for the foreground. In the final, the foreground is bright green.
The Bad Future's background has a purple color scheme.
All three Acts are missing objects across all timezones.
Tubes and tunnels do not work.
The electrical conduits cannot hurt you.
Sonic does not curl into a ball when he bounces off the floor.
The angel statue in Act 1 Past is a solid object and doesn't give you rings.
There is no boss in Act 3. The prison capsule is present, however.
The flower for this stage also doesn't look great. It uses rotating colors in its variations, making it look unusual.
Act 1 has a crazy mixture of colors moving up and down in the Present.
Parts of the foreground in the Past are blue instead of yellow.
The Good Future has a blue color scheme, and the background is blue and black.
The sky in the Bad Future is constantly flashing red.
All three Acts have no objects or rings.
Acts 2 and 3 has a slightly higher starting position.
In Act 3, Metal Sonic does not appear to initiate the race.
Amy does not appear at the end of Act 3. Instead, a capsule with a glitched palette can be found above where Amy would be.
Another flower that doesn't really seem great with the palettes it's given.
There are no rings.
Badniks have incorrect palettes.
The background in Act 1 Past has a purple tint.
The first two Acts have several bottomless pits that were taken out in the final version.
There are several objects in this Zone that were taken out of the final:
A trapdoor similar to the ones in Scrap Brain Zone from Sonic 1.
Two shifting platforms with spikes by their sides found in tight corridors.
A set of platforms that behave like scales. They shift according to Sonic's weight.
A type of crusher that has a hole underneath that comes in a row of four. They push Sonic into the ground instead of crushing him, similarly to the crushers in Wacky Workbench.
Collapsing platforms have different graphics.
The starting positions in Acts 2 and 3 are much higher up.
The conveyor belts and spinning wheels in Act 2 don't work. There are also small variants of the spinning wheels.
Turntables that transfer you behind iron curtains in Act 2 are missing.
Many objects are missing in the second half of Act 2.
The shrink/growth lasers (placeable with Debug Mode) have a delay before firing each laser. In the final, they are constantly firing.
Act 2 is cut off in the "Mini Sonic" maze, as the Goal is at the same coordinates as this area.
Act 3 has a different layout and glitched bumpers.
The iron curtain in Act 3 is covering the turntable and spiked wall when it shouldn't.
The background in the final boss arena is different in both Futures.
Amy does not appear after defeating the final boss, and no ending sequence has been implemented yet.
The Special Stage is not yet playable, though a demo of it can be viewed by waiting on the title screen.
During the demo, the text "The programmer has a nap. Holdout! Programmer!" will flash on-screen repeatedly.
There are no UFOs to destroy yet.
The layout is an earlier version of Special Stage 2.
There are oil slicks present that were taken out in the final, becoming unused graphics. Through hacking, it turns out they are non-functional.
There is an unused object that appears to move around in 3D space. It has no proper graphics implemented. Perhaps it's a prototype of the UFO or just a test object? There's also a set of unused sprites that would have been used for different distances away from the object.
In addition to that, there are also two unused Special Stage maps. There is a reference to a third one, but the data was scrapped, leaving it to use Special Stage 1's stamps and palette, and Special Stage 3's map.