Sonic Classic Collection/Technical information

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This short article is in need of work. You can help Sonic Retro by adding to it.

TODO:

• Move emulation information from the main page
• More information about Game.pak
• Document any modifications made to the Mega Drive ROMs
• Full list of tracks from the .mods files?

Music and Sound effects

Rather than emulating the Mega Drive audio hardware, all of the music and sound effects are pre-recorded and stored in .mods files. These can be unpacked using this QuickBMS script.

The file format consists of a 32-bit integer containing the number of sound effects or tracks, followed by an array of 32-bit integers, each containing a pointer to a structure containing information about the particular sound. If a pointer is identical to the following pointer, then that pointer is for an empty sound, and should be ignored. The structure is described below:

Offset	Type	Description
0	4 Bytes	Length of the sound in bytes, including this header.
4	4 Bytes	Number of bytes from the end to be looped.
8	2 Bytes	Sample rate.
A	6 Bytes	Unknown.

The audio data follows immediately after this header, using the IMA ADPCM codec described here.

UI graphics

The graphics for the user interface are stored in the screensGame_eu.dat file. This can be unpacked using this QuickBMS script. The file begins with the following header:

Offset	Type	Description
0	4 Bytes	Number of image layouts.
4	4 Bytes	Pointer to image layout data, plus 0x25DF000.
8	4 Bytes	Number of image names
C	4 Bytes	Pointer to image name pointers, plus 0x23D1000.
10	4 Bytes	Number of compressed image names
14	4 Bytes	Pointer to compressed image name pointers, plus 0x23D1000.
18	4 Bytes	Number of compressed images
1C	4 Bytes	Pointer to compressed image index, plus 0x23D1000.

Each image layout consists of 192 32-bit words containing pointers to the 16-bit BGR555 graphics data for each row in the image, plus 0x25DF000.

Some images are instead compressed using the LZ11 algorithm described here. These contain raw 256x192 images using 16-bit BGR555 graphics, and are referred to by the index, which is an array of the following structure:

Offset	Type	Description
0	4 Bytes	Size of the compressed file.
4	4 Bytes	Pointer to the compressed file, plus 0x25DF000.

Names are stored as an array of pointers to the null terminated strings containing the name of the corresponding image, plus 0x23D1000. The array only contains the names for the English images - the list should be repeated for each of the remaining languages.