This is the Sonic Community Hacking Guide for Sonic the Hedgehog CD. Sonic CD works differently from any other Sonic game. All of the level data (level layout, compressed art, Sonic sprites, etc) is stored in its respective MMD file. MMD files are contained within the ISO.

Thanks to this system, there is an astounding repetitiveness in each MMD. Each MMD has its own Sonic sprite, even though Sonic looks the same throughout the whole game. Each MMD has its own checkpoint art, even though the checkpoints never change art. This means that there are 70 versions of one common piece of art. If you want to edit that art fully, you must comb through each level MMD and substitute the regular art with the replaced art.

The level MMDs have the filename format RXYZ__.MMD:

• X is the zone. R1 is zone 1.
• Y is the act. R12 is zone 1, act 2.
• Z is the time period. There are 4 letters that can be put here; A, B, C, or D. A is the present period. B is the past period. C is the good future period. D is the bad future period. So R12D__.MMD would contain act 2 of zone 1, in the bad future period.