From Sega Retro

X3I is a new instrument bank format developed by Oerg866 for use in his xm3smps/oerg music conversion tool. It is the successor to X3V. It can contain up to 65535 YM2612 (X3V can just hold 255) instruments formatted according to the SMPS specifications; additionally, it can store a description for each instrument.

File Structure

The internal structure of an X3I instrument bank is simple. The first three bytes spell "X3I", the next two bytes specify how many instruments there are in the bank (0 to 65535); after that, the actual instrument table begins, in the following format:

Voice Description length Description
25 bytes
1 byte
0 to 255 bytes

Note: The X3I format was created to eliminate the limit of just 255 instruments.

Opposed to YMX files, which store instruments in the format used by Sonic 2, X3I banks store instruments in the format used by all the other Sonic games.