AKAO frames are most complicated frames in FF7 sound system. ("AKAO" is frame magic, probably developed by Minoru Akao, Square Enix sound programmer :) )
Frame is similar to MIDI sequence - it's custom tracker format for playing sequence sound, well tuned specially for PSX.
This frames are in all FF7 game modules: Field, Battle, Worldmap and in minigames.
All files with exension *.SND are AKAO.
MINI/ASERI2.SND - Battle Arena theme
MINI/SENSUI.SND - used in Submarine minigame
ENEMY6/OVER2.SND - game over sequence
ENEMY6/FAN2.SND - battle win "fanfare" sequence
MOVIE/OVER2.SND - same game over sequence, don't know, why to duplicate data
Other AKAO frames are hard-wired in other files.
AKAO frame structure
Header (size: 16 bytes)
static const uint8_t magic; // "AKAO" C-string aka frame *MAGIC* uint16_t id; // frame ID, used for playing sequence uint16_t length; // frame length - sizeof(header) uint8_t unknown; // some numbers, can't find their usage
Channel info (size: 4 bytes + 2 bytes * <channels count>)
First there is 32-bit number (offset 0x10), which represents bitmask of used channels in this frame, after this frame there is <channels count> offsets to channel opcode data counting from current offset.
Channel Commands [AKAO Opcodes]
Most complicated part.
For every channel in AKAO frame there is set of commands to perform. This is similar to Field opcodes. Here I'll call this sound commands "opcodes". Every opcode has it's own number of arguments (from no-arguments, to 3 arguments).
Example (home-created AKAO frame):
41 4b 41 4f - AKAO string
34 12 - frame ID: 0x1234
16 00 - frame length 0x16 in hex or 22 in decimal
04 00 96 12 18 22 46 28 - unknown data
01 00 00 00 - this indicates, that used only one channel
00 00 - offset to first channel opcodes: in our example 0x00 means that next to this offset is opcodes for first channel
e8 a8 66 - sets tempo, parameter 0x66a8
ea 00 50 - sets reverb depth
a8 55 - load sample 0x55 from INSTR.ALL to channel
aa 40 - sets channel volume
c2 - turns on reverb effect
a1 0c - sets volume pan
c8 - sets loop point
66 - 0x66 % 11 = 3 (3 means to take 3rd number from play length table), 0x66 / 11 = 9 (9 means to take pitch from loaded instrument record index)
ca - returns to saved loop point with opcode c8
This example plays Chocobo "Whoo-Hoo" (instrument number 0x55) repeatedly.