.MOD File Format – Header

Layout

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.

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.

00000014  62 61 73 73 00 00 00 00 00 00 00 00 00 00 00 00  bass............
00000024  00 00 00 00 00 00 04 00 00 40 00 00 04 00        .........@....

The name is “bass”; the length 04 00 is 1,024 words, 2,048 bytes; finetune 0, volume 64; the loop starts at 0 and is 1,024 words long, so the whole sample repeats.

Finetune

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

The note’s period is looked up in ProTracker’s table for the sample’s finetune (see periods). The explorer reports a byte with any of the high four bits set.

Loops

Both loop fields count words. As the explorer plays them, following ProTracker:

1. If the loop length is 1 (or 0), there is no loop: the sample plays once, from its start to its length.
2. 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.
3. 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; ProTracker writes 127 (7F) there and ignores it. What StarTrekker wrote here is not recorded in the sources used for this page. The explorer’s player does not use the byte.

Order table

The 128 bytes from offset 952 give, for each position, the number of the first stored pattern of the pair to play. The entries are even: 0 means the pair of stored patterns 0 and 1, 2 the pair 2 and 3, and so on, so the 8-channel pattern played is the entry divided by two. OpenMPT’s loader puts it plainly: “FLT8 has only even order items, so divide by two”. Only the first song length entries are played; the explorer reports an odd entry among them.

The number of stored patterns is worked out as for a ProTracker module, from the highest number in all 128 entries, and rounded up to whole pairs:

pairs  = (highest entry ÷ 2, rounded down) + 1
stored = 2 × pairs    4-channel patterns of 1,024 bytes

000003B6  05 7F 00 02 04 02 06 00 00 00 …

The song is 5 positions long and plays the 8-channel patterns 0, 1, 2, 1 and 3. The highest entry is 6, so 4 pairs, 8 stored patterns, are in the file, and the sample data starts at 1084 + 8 × 1,024 = 9,276.

Signature

The four bytes at offset 1080 are FLT8 or EXO8:

FLT4 and EXO4, StarTrekker’s 4-channel signatures, mark files laid out as ProTracker modules. Other 8-channel signatures, such as 8CHN, CD81 and OKTA, store 8 cells in each row and belong to the multichannel format.

00000438  46 4C 54 38                                      FLT8