Multichannel module header: samples, order table and signature

A multichannel module starts with a 1,084-byte header: the song title, a header for each of the 31 samples, the song length, the restart byte, the order table and the signature, which gives the number of channels. The patterns follow straight after it. All numbers are big-endian, most significant byte first, and the header has no padding.

Layout

Offset	Size	Type	Name	Description
0	20	text	title	The song title.
20	930	31 × 30 bytes	samples	The sample headers, sample 1 first.
950	1	u8	song length	How many positions of the order table are played.
951	1	u8	restart	A restart position, or a value the player ignores.
952	128	u8 × 128	order	The pattern played at each position.
1080	4	ASCII	signature	The channel count, as in `8CHN` or `16CH`.

Title

The first 20 bytes are the song title, as single-byte text padded with zero bytes. A title of exactly 20 characters has no zero after it, and some files have no title at all.

Sample headers

From offset 20 come 31 sample headers of 30 bytes each. Sample numbers in the patterns count from 1, so the header of sample n is at 20 + (n − 1) × 30. A sample that is not used has a length of 0.

Offset	Size	Type	Name	Description
0	22	text	name	The sample name, padded with zero bytes. Often used for messages: credits, greetings and the like.
22	2	u16	length	The length in words of 2 bytes, so the data is 2 × length bytes. 0 means no sample. At most 65,535 words (128 KB).
24	1	u8	finetune	The low four bits are a signed finetune, −8 to +7 eighths of a semitone. The high four bits are unused.
25	1	u8	volume	The default volume, 0 to 64 (`40`).
26	2	u16	loop start	Where the loop starts, in words.
28	2	u16	loop length	The length of the loop in words. 1, or 0 in some files, means no loop.

00000014  70 69 61 6E 6F 00 00 00 00 00 00 00 00 00 00 00  piano...........
00000024  00 00 00 00 00 00 1F 40 0E 30 0F A0 0F A0        .......@.0....

A sample header built for this page

Bytes	Field	Value
`70 69 … 00`	name	“piano”, then 17 zero bytes to make 22
`1F 40`	length	8,000 words, 16,000 bytes
`0E`	finetune	−2
`30`	volume	48
`0F A0`	loop start	4,000 words, byte 8,000
`0F A0`	loop length	4,000 words: the loop is the second half of the sample

Finetune

The low four bits of byte 24 are a signed finetune in eighths of a semitone:

Low four bits	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
Finetune	0	+1	+2	+3	+4	+5	+6	+7	−8	−7	−6	−5	−4	−3	−2	−1

Within ProTracker’s three octaves the note’s period is looked up in ProTracker’s table for the sample’s finetune (see periods). For notes outside them, the explorer moves the period by the finetune directly: each eighth of a semitone divides the period by 2^1/96. The explorer reports a byte with any of the high four bits set.

Loops

Both loop fields count words. As ProTracker plays them:

If the loop length is 1 (or 0), there is no loop: the sample plays once, from its start to its length.
If the loop start is 0 and the loop length is more than 1, the sample plays its first length words, then repeats its first loop length words.
If the loop start is not 0, the sample plays from its start to the end of the loop, loop start + loop length words, then repeats the loop. Data after the end of the loop is never played.

A loop that ends after the end of its sample is a fault of the file, which the explorer reports.

Song length

The byte at offset 950 is the number of positions in the song, 1 to 128: positions 0 to song length − 1 of the order table are played, in order. The explorer reports a value of 0 or above 128.

Restart

The byte at offset 951 is, in NoiseTracker’s use, the position to go back to after the last one. PC trackers often write 0 here; ProTracker writes 127 (7F) and ignores the byte. The multichannel files examined for this site had 0. The explorer’s player, which ends the song at the end of the order list, does not use it.

Order table

The 128 bytes from offset 952 are the number of the pattern played at each position. Only the first song length entries are played.

The number of patterns stored in the file is one more than the highest number in all 128 entries, including those after the song length, as ProTracker counts. Some PC trackers counted only the positions in use and left other numbers in the rest, naming patterns they never stored. When the file is too short for the count from all 128 entries but fits the count from the played positions alone, the explorer takes the second count and says so. With patterns of several kilobytes each, counting one too many patterns would put the start of the sample data far off.

Signature

The four bytes at offset 1080 are ASCII and give the channel count, from 1 to 32 other than 4:

Signature	Bytes	Channels	Written by
`1CHN` to `9CHN`, not `4CHN`	`36 43 48 4E` for `6CHN`	the digit	PC trackers such as FastTracker
two digits and `CH`, up to `32CH`	`31 30 43 48` for `10CH`	the number	FastTracker 2 and other PC trackers
two digits and `CN`	`31 32 43 4E` for `12CN`	the number	a PC tracker; OpenMPT names none
`TDZ1` to `TDZ9`, not `TDZ4`	`54 44 5A 36` for `TDZ6`	the last digit	TakeTracker
`CD61`, `CD81`	`43 44 38 31` for `CD81`	6, 8 (the third character)	Octalyser (Atari STe and Falcon)
`OKTA`, `OCTA`	`4F 4B 54 41` for `OKTA`	8	Oktalyzer, as OpenMPT records it

The examples from the files examined for this site are 38 43 48 4E (8CHN), 31 30 43 48 (10CH) and 32 38 43 48 (28CH). A count of 0 or above 32 is not a known signature. Signatures with a count of 4 belong to the ProTracker module, and FLT8 and EXO8 to the StarTrekker module.

00000438  38 43 48 4E                                      8CHN

The signature at offset 1080 (438 in hexadecimal) of an 8-channel module

.MOD File Format – Header