AMOS Music Bank format

AMOS music banks are one of the standard AMOS file formats.

All multi-byte integers are in big-endian format.

The format is divided into sections. The format typically starts with a Standard AMOS bank header, which may be truncated or damaged depending on how it was ripped:

Offset	Size	Description
0	4 bytes	AmBk ID (0x416D426B)
4	2 bytes	bank number (should be 3)
6	2 bytes	flags
8	4 bytes	bank length, only in bits 27-0
12	8 bytes	bank type (should be "Music ")

Whatever the format of the standard AMOS bank header, it is immediately followed by the music bank's main header:

Offset	Size	Description
0	4 bytes	offset from start of this header to instrument data section
4	4 bytes	offset from start of this header to to song data section
8	4 bytes	offset from start of this header to to pattern data section
12	4 bytes	always 0

These offsets point to other sections of the file, which might not be consecutive or even in the order given.

Instrument data section

Offset	Size	Description
0	2 bytes	number of instruments (n)
2	n * 32 bytes	each instrument's data, in this format: Offset Size Description 0 4 bytes offset from start of instrument data section to the sample data for this instrument. 4 4 bytes offset from start of instrument data section to the repeating section of this instrument's sample data If the sample doesn't repeat, this will point to either 4 or 2 bytes of zeroes 8 2 bytes In a repeating sample, this is the offset of the repeating part (in 4-byte longwords, from the start of the sample data for that sample) In a non-repeating sample, it is the length of the sample (in 2-byte words). 10 2 bytes length (in 2-byte words) of repeating section of sample data If the sample doesn't repeat, this will either be 2 or 1. 12 2 bytes default volume of sample, from 0x00 (min) to 0x40 (max) It may also accidentally include a Protracker finetune value in the upper 8 bits, which the AMOS player ignores 14 2 bytes sample length (in 2-byte words) 16 16 bytes sample name (ASCII text, padded with spaces or null bytes)
2 + n * 32	? bytes	raw sample data

WARNING: samples lengths can often be incorrect. Do not rely on the "sample length" field here. To find the true length of a sample, it is necessary to take all sample offsets and sort them into order. Each sample's length is that of its offset subtracted from the offset of the next sample. For the final sample, its offset should be subtracted from the overall length of the instruments data block (which will be the offset of the instruments block subtracted from the offset of the section following it, or the overall length of the file if no other section follows it).

Song data section

The song data section starts with a list of offsets to songs:

Offset	Size	Description
0	2 bytes	number of songs (n)
2	n * 4 bytes	offsets from start of song data section to each song

Each song has this format:

Offset	Size	Description
0	2 bytes	default tempo, 1-100, default is 17. not used by player!
2	2 bytes	offset from start of this song to audio channel 0 playlist
4	2 bytes	offset from start of this song to audio channel 1 playlist
6	2 bytes	offset from start of this song to audio channel 2 playlist
8	2 bytes	offset from start of this song to audio channel 3 playlist
10	2 bytes	seems to be unused, should be zero
12	16 bytes	name of song (ASCII text, padded with spaces or null bytes)

A playlist is a stream of 2-byte words, giving the list of patterns to play in order for that channel.

Valid pattern numbers start from 0 and go up to however many patterns there are. The list is terminated with 0xFFFE, sometimes 0xFFFF.

Each channel has its own set of patterns, so pattern 0 on channel 0 is not the same as pattern 0 on channel 1.

Pattern data section

The pattern data section starts with a list of patterns

Offset	Size	Description
0	2 bytes	number of patterns (n)
0x02	n * 8 bytes	pattern offsets: Offset Size Description 0 2 bytes offset from start of pattern data section to this pattern for channel 0 2 2 bytes offset from start of pattern data section to this pattern for channel 1 4 2 bytes offset from start of pattern data section to this pattern for channel 2 6 2 bytes offset from start of pattern data section to this pattern for channel 3

A pattern is a stream of 2-byte word commands and notes.

Commands have bit 15 set ($8000-$FFFF). Notes do not have bit 15 set ($0000-$7FFF).

Commands come before notes and are executed immediately. When a note appears, processing the pattern finishes for that vertical blank, and processing will re-commence once the delay counter elapses (see the delay command below).

Unlike SoundTracker, which allows one command per note, there can be any number of commands per note, although generally it's not useful, as repeating the same command again and again would simply overwrite its value.

Notes

For a note, all that matters is bits 12 to 0 ($000-$FFF). This, like Soundtracker, is an Amiga hardware note period (audXper).

When a note is triggered, the current note volume is set to the default volume for the instrument being played.

A note cannot be played before the first "set instrument" command, as notes do not implicitly have an instrument associated with them.

Commands

Each command has a parameter, although many commands don't need and therefore ignore their parameter. The command value is the bits 14 to 8 of each word, the parameter value is bits 7 to 0.

All "effect" commands which have a lasting effect, between notes are assumed to continue running forever until they are changed to another effect, or stopped with the "stop effect" command. A channel can only run one effect. So if AMOS did a "shift down" command, a "delay" of 14 and then a "stop effect", the equivalent in SoundTracker would be to repeat the portamento down command 14 times, once in each position, and positions after that would simply have the empty command (000).

There is no implicit delay of one position between commands or notes, as there is in SoundTracker. Instead, a "delay" (0x10) command must be executed.

Other than the "set tempo" and "low pass filter on/off" commands, the effect of every command is limited to the channel it runs on.

The commands are as follows:

Command value	Description of effect
$80xx	end of pattern: advance to the next pattern in the playlist Also stops any currently running effects like the "stop effect" command
$81xx	old slide up: not supported by player. identical to slide up?
$82xx	old slide down: not supported by player. identical to slide down?
$83xx	'set volume: sets the volume of the note currently playing This is equivalent to the SoundTracker "C" command. Uses the commmand value, valid range 0 to 63 (therefore $8300 - $833F) Surprisingly, the valid maximum Amiga volume 64 ($40) isn't supported by the player and will be rounded down to 63
$84xx	stop effect: stops any currently running effect
$85xx	repeat: set or jump to the repeat mark If the parameter is zero, this is the "repeat mark". If the parameter is non-zero, jump back to the repeat mark for the number of times given by the parameter This is equivalent to the Protracker "E50" and "E6x" commands
$86xx	low-pass filter on: equivalent to the SoundTracker "E01" command
$87xx	low-pass filter off: equivalent to the SoundTracker "E00" command
$88xx	set tempo: sets the tempo of replay The parameter is between 1 and 100 (therefore $8801 - $8864) AMOS has a different tempo system from SoundTracker. SoundTracker's speed is a number between 1 and 255, and that is the number of vertical blanks between advancing one position in the pattern. AMOS keeps a counter and adds the tempo value to it once per vertical blank. When the counter is greater or equal to 100 (or 120 for NTSC), AMOS advances the position and reduces the counter by 100 (or 120 for NTSC) This gives an average SoundTracker speed of (100/tempo) at 50Hz or (120/tempo) at 60Hz, which gives a (tempo/2) Hz change in position, regardless of frame rate. This is quite nice, but as timing is still in 50 or 60Hz multiples, it's possible that instruments may sound fractionally out of time -- for example, tempo=17 has a cycle of 8 positions advancing every 6 vertical blanks and the 9th position advancing after 5 vertical blanks. To convert from SoundTracker speeds to AMOS tempos, the converter uses the formula: amos_tempo = 100 / soundtracker_speed That's not an accurate conversion, and conversion back is even more fun. For simple conversion, I recommend the following roughly accurate speeds: AMOS tempo SoundTracker speed 1 100 (F64) * 2 50 (F32) * 3 33 (F21) * 4 25 (F16) 5 20 (F14) 6 17 (F11) 7 14 (F0E) 8 12 (F0C) 9 11 (F0B) 10 10 (F0A) 11 9 (F09) 12 8 (F08) 13-15 7 (F07) 16-18 6 (F06) 19-22 5 (F05) 23-28 4 (F04) 29-40 3 (F03) 41-66 2 (F02) 67-100 1 (F01) Note: the first three speeds are not possible with Protracker, which uses F20 and above to set the CIA tempo.
$89xx	set instrument: sets the instrument that the next note command will use Parameter is from 0 (first instrument) to the last instrument
$8Axx	arpeggio: arpeggiates the current instrument The parameter is broken into the upper 4 bits and lower 4 bits. Equivalent to the SoundTracker "0" command. This effect will continue until a new effect, or change of pattern
$8Bxx	tone portamento: causes the currently playing note to slide towards the pitch of the next note Equivalent to the SoundTracker "3" command. This effect will continue until a new effect, or change of pattern
$8Cxx	vibrato: varies the currently playing note's pitch according to a sine wave The upper 4 bits of the parameter give the speed of the vibrato, the lower 4 bits give the depth. Equivalent to the SoundTracker "4" command This effect will continue until a new effect, or change of pattern
$8Dxx	volume slide: Raises or lowers the volume of the currently playing note The upper 4 bits of the parameter will raise the volume by that amount every vertical blank. If zero, the lower four bits are considered. They lower the volume by that amount every vertical blank. Equivalent to SoundTracker "A" command
$8Exx	portamento up: raises the pitch of the currently playing note Decreases the note period by the amount given by the parameter every vertical blank, to the lowest period of 113 Equivalent to SoundTracker "1" command This effect will continue until a new effect, or change of pattern
$8Fxx	portamento down: lowers the pitch of the currently playing note Increases the note period by the amount given by the parameter every vertical blank, to the highest period of 856 Equivalent to SoundTracker "2" command This effect will continue until a new effect, or change of pattern
$90xx	delay: states that after the current list of commands and final note has been played The channel will wait for the number of positions given by the parameter, before executing any more commands and notes Each note played should have a corresponding delay command.
$91xx	position jump: changes the position in the song The position is taken from the parameter (position 0 = start of song) and then "end of pattern" command fires Equivalent to SoundTracker "B" command