Under the Surface of Classic Sonic

Due to the demo programs in the Japanese and PAL versions (not USA) of Sonic CD being built off of a prototype, they retain the logic for enabling debug mode by pressing start on player 2's controller. You can do this so that you don't need to enter the code in the sound test.

There exists a collision bug where the check used to prevent Sonic from clipping on top of a surface he's too far below from isn't done if he's moving horizontally in the air. This can easily be seen if you put a solid object on a top solid surface and hit it from the bottom.

...and Sonic CD...

There exists a collision bug where the check used to prevent Sonic from clipping on top of a surface he's too far below from isn't done if he's moving horizontally in the air. This can easily be seen if you put a solid object on a top solid surface and hit it from the bottom.

This is what the missing check does, which is present when mostly falling downwards and less horizontally.

There exists a collision bug where the check used to prevent Sonic from clipping on top of a surface he's too far below from isn't done if he's moving horizontally in the air. This can easily be seen if you put a solid object on a top solid surface and hit it from the bottom.

Here's a silly difference between the 510 and 0.28A (512) prototypes of Sonic CD. Special stage rings always appear in 510, but never in 0.28A. 0.28A added a ring count check, but only checked the top byte. As a result, they can only appear if the player has at least 12800 rings.

Loops were planned on being implemented in Chaotix, even being present in prototypes... yeah, I can see why they were removed (footage provided by @mdtravis.bsky.social)

There's a ROM hack of Chaotix called Knuckles' Clackers by GenesisDoes that reimplements loops, and even uses the masking sprite... but it still illustrates the problematic nature of loops with the tethering mechanic.

Loops were planned on being implemented in Chaotix, even being present in prototypes... yeah, I can see why they were removed (footage provided by @mdtravis.bsky.social)

There's a hidden tunnel that can be found right towards the end of the 3D loop section at the beginning of Palmtree Panic act 1 that I'm not sure is even accessible via normal means. Anyone have any ideas?

Curiously, there's a couple of hidden springs around here as well, but they cannot be touched, because they are completely obscured by the ramp.

There's a hidden tunnel that can be found right towards the end of the 3D loop section at the beginning of Palmtree Panic act 1 that I'm not sure is even accessible via normal means. Anyone have any ideas?

No, you can't time travel in this area. In the present, the camera will refuse to go back once you've gone past it, and if you are in the past or future, time travel gets disabled.

For some reason, the demo for Palmtree Panic act 1 was rerecorded for the USA release of Sonic CD. Here's a comparison showing the differences. Left: Japan/Europe, Right: USA

The Tidal Tempest act 3 demo was also rerecorded. What I find peculiar is how whoever recorded the USA demo slowly walked to the boss while the Japanese demo caught up. Were they trying to eyeball the Japanese demo? I dunno... Left: Japan/Europe, Right: USA

In the PAL version of Sonic CD, the physics of Sonic were modified to more closely resemble the NTSC physics while at a lower frame rate. As such, separate demo controller data had to be recorded for the special stage demos. Special Stage 1 Left: NTSC, Right: PAL

Special Stage 6 Left: NTSC, Right: PAL

A few ways to play and see it: 1. Modify the disc image to set the byte at 0x2A75353 from 1 to 0 (712 prototype only) 2. Modify RAM in an emulator to set address 0xFF1581 to 0 (works in final) 3. Use PAR code FF1581:00 or Game Genie code AAL9-8AEB in an emulator (works in final)

Note that you may find some parts of the level corrupted. This is because part of the level data is overwritten with the demo controller data, with the demo avoiding showing those corrupted parts.

Thanks to c4/seafour4 on the Hidden Palace Discord server, a new prototype signpost sprite was discovered in the Palmtree Panic act 1 demo (DEMO11A.MMD) in the 712 prototype of Sonic CD, which was previously only seen in a magazine.

Thanks to c4/seafour4 on the Hidden Palace Discord server, a new prototype signpost sprite was discovered in the Palmtree Panic act 1 demo (DEMO11A.MMD) in the 712 prototype of Sonic CD, which was previously only seen in a magazine.

Thanks to c4/seafour4 on the Hidden Palace Discord server, a new prototype signpost sprite was discovered in the Palmtree Panic act 1 demo (DEMO11A.MMD) in the 712 prototype of Sonic CD, which was previously only seen in a magazine.