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return_of_samus:technical_information:music_format

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Music Format

This page aims to explain how Metroid II's music works on the inside.

Bank 4 is dedicated to all the music and sound data and code. The game almost always leaves this bank in the swappable bank area of the system's memory map in order to focus on interpreting the music data.

Game Boy Audio Specs

Just for quick reference, the Game Boy utilizes a stereo PSG composed of four monophonic channels. Their details are listed below.

# Details
1 Pulse-wave - Can generate pulse waves at duty cycles of 12.5%, 25%, 50%, or 75%. Also has a frequency sweep.
2 Pulse-wave - Same as above, except lacking the sweep.
3 4-bit Wavetable - Can generate customizable waveforms from a table of 32.
4 White Noise - Can generate only white noise. Used as percussion in music.

Song List

At 0x11F30 in the ROM starts the list where the game finds the next music track to play. There are 32 2-byte pointers to each of them. Right after this list ends is another list, this time 32 1-byte flags determining how each song in the last list utilizes the stereo panning.

STEREO FLAGS
DB - Pulse channel 2 is panned to the right.
DE - Pulse channel 1 is panned to the left, and pulse channel 2 is panned to the right.
FF - Song is totally center-panned.
Song Flag
01 - Baby Metroid FF
02 - VS. Queen Metroid FF
03 - Chozo Ruins FF
04 - Main Caves FF
05 - Sub Caves 1 FF
06 - Sub Caves 2 FF
07 - Sub Caves 3 FF
08 - Final Caves FF
09 - Metroid Hive (no intro) FF
0A - Item Acquired DB
0B - Queen Metroid Hallway FF
0C - VS. Metroid FF
0D - Sub Caves 4 FF
0E - Earthquake DE
0F - Metroid Defeated DE
10 - Nothing FF
11 - Title FF
12 - Samus Appearance DE
13 - Ending FF
14 - Chozo Ruins (dupe of 03) FF
15 - Main Caves (not intro) FF
16 - Sub Caves 1 (no intro) FF
17 - Sub Caves 2 (no intro) FF
18 - Sub Caves 3 (no intro) FF
19 - Final Caves (dupe of 08) FF
1A - Metroid Hive (dupe of 09) FF
1B - Item Acquired (dupe of 0A) DB
1C - Queen Metroid Hallway (dupe of 0B) FF
1D - VS. Metroid (dupe of 0B) FF
1E - Sub Caves 4 (no intro) FF
1F - Metroid HiveFF
20 - Missile Expansion DE

Song Header

Each song is defined by a header of 0x0B bytes.

SONG HEADER
Note Offset - 1 byte - A signed value determining how much to add to or subtract from the song's pitch.
Tempo Offset - 2 bytes - Points to the tempo array to be used in the song.
Channel Instruction Pointers - 8 bytes (2 for each channel) - Points to the instruction list for each sound channel to play. Can be set to 0000 to completely ignore the channel, and can even point to the same list, as some songs set both pulse channels to the same pointer.

Instruction Lists

These contains all the pointers to each instruction pattern.

INSTRUCTION LIST COMMANDS
0000 - Stops playing sound through the channel for the rest of the song's playing.
00F0 - Loops the channel starting from the specified offset following the command.

Instructions

Each offset pointed to by the instruction lists is just a sequence of bytes the game interprets. What they do is detailed below.

00

Ends the instruction pattern, passing it on to the next pointer in the instruction list.

01

A rest note.

03

Echoes the preceding note at a volume of 0x46.

05

Echoes the preceding note at a volume of 0x66.

Ax

Changes the note length, x being the requested index in the currently-loaded tempo array.

F1

Initializes the instrument in the pulse and wave channels. Pulse - Envelope, sweep, and duty cycle are represented by 3 bytes in that order. Wave - First 2 bytes point to the waveform to be loaded, last one defines volume.

F2

Loads the tempo array from the specified 2-byte pointer.

F3

Transposes the song.

F4

Repeats a section of instructions the number of times specified by the byte right after.

F5

Encloses the section mentioned above.

Note Values

Just about every other byte is a note value. They range from 0x02 to 0x9E, and should all be even values. They are listed below.

Note 0 1 2 3 4 5 6
C 0x02 0x1A 0x32 0x4A 0x62 0x7A 0x92
C# 0x04 0x1C 0x34 0x4C 0x64 0x7C 0x94
D 0x06 0x1E 0x36 0x4E 0x66 0x7E 0x96
D# 0x08 0x20 0x38 0x50 0x68 0x80 0x98
E 0x0A 0x22 0x3A 0x52 0x6A 0x82 0x9A
F 0x0C 0x24 0x3C 0x54 0x6C 0x84 0x9C
F# 0x0E 0x26 0x3E 0x56 0x6E 0x86 0x9E
G 0x10 0x28 0x40 0x58 0x70 0x88
G# 0x12 0x2A 0x42 0x5A 0x72 0x8A
A 0x14 0x2C 0x44 0x5C 0x74 0x8C
A# 0x16 0x2E 0x46 0x5E 0x76 0x8E
B 0x18 0x30 0x48 0x60 0x78 0x90

In determining what pitch the values play at in the song, the following formula is used.

base note value + (0x18 * note level) + noteoffset - if noteoffset is an odd number
(In the case of a negative noteoffset like 0xFE, replace noteoffset with (256 - noteoffset))

Tempo

From 0x1009E-0x10106 are nine arrays of 0xD bytes. They are loaded as the song tempo in the header or by instruction F2, and each individual byte is loaded by an Ax instruction.

Offset Values seemingly geared to 4/8/16-beat Values seemingly geared to 3/6/12-beat Unknown beat
0x1009E 01, 01, 02, 04, 08, 10 [448BPM] 03, 06, 0C 01, 03, 01, 20
0x100AB 01, 02, 04, 08, 10, 20 [224BPM] 06, 0C, 18 02, 05, 01, 40
0x100B8 02, 03, 06, 0C, 18, 30 [150BPM] 09, 12, 24 04, 08, 01, 60
0x100C5 02, 04, 08, 10, 20, 40 [112BPM] 0C, 18, 30 05, 0A, 01, 80
0x100D2 03, 05, 0A, 14, 28, 50 [90BPM] 0F, 1E, 3C 07, 0E, 01, A0
0x100DF 03, 06, 0C, 18, 30, 60 [75BPM] 12, 24, 48 08, 10, 02, C0
0x100EC 03, 07, 0E, 1C, 38, 70 [64BPM] 15, 2A, 54 09, 12, 02, E0
0x100F9 04, 08, 10, 20, 40, 80 [56BPM] 18, 30, 60 0A, 14, 02, FF
0x10106 04, 09, 12, 24, 48, 90 [50BPM] 1B, 36, 6C 0C, 1A, 02, FF
return_of_samus/technical_information/music_format.1584575725.txt.gz · Last modified: 2020/03/18 23:55 by kkzero