Impulse Tracker instruments

An instrument maps notes to samples and adds envelopes and new note actions. Each one starts IMPI and is found through the instrument offset table. There are two layouts, chosen by Cmwt in the header: below 0200h the old layout, otherwise the new layout. Both are 554 bytes when stored. Offsets on this page are in hexadecimal.

Old instrument (Cmwt below 200h)

Offset	Size	Type	Name	Description
00	4	ASCII	magic	`IMPI`.
04	12	text	filename	DOS 8.3 file name.
10	1	u8	null	Always 0.
11	1	u8	Flg	Envelope flags: bit 0 use the volume envelope, bit 1 volume loop, bit 2 sustain loop.
12	1	u8	VLS	Volume loop start, a node number.
13	1	u8	VLE	Volume loop end.
14	1	u8	SLS	Sustain loop start.
15	1	u8	SLE	Sustain loop end.
16	2	–	unused
18	2	u16	FadeOut	0 to 64. The fade count is 512.
1A	1	u8	NNA	New note action.
1B	1	u8	DNC	Duplicate note check: 0 off, 1 on.
1C	2	u16	TrkVers	Tracker version; meaningful only in instrument files.
1E	1	u8	NoS	Number of samples; meaningful only in instrument files.
1F	1	–	unused
20	26	text	name	Includes the terminating zero byte.
3A	6	–	unused
40	240	keyboard	note-sample table	120 pairs of note and sample.
130	200	u8 × 200	volume envelope	The precalculated envelope, 0 to 64 per tick; `FF` marks its end.
1F8	50	u8 × 2 × 25	nodes	25 nodes of two bytes: tick, then magnitude.

Fade-out applies after a note fade, after a note-off with no envelope or a looping envelope, and after the volume envelope ends.

New instrument (Cmwt 200h or later)

Offset	Size	Type	Name	Description
00	4	ASCII	magic	`IMPI`.
04	12	text	filename	DOS 8.3 file name.
10	1	u8	null	Always 0.
11	1	u8	NNA	New note action.
12	1	u8	DCT	Duplicate check type.
13	1	u8	DCA	Duplicate check action.
14	2	u16	FadeOut	0 to 128. The fade count is 1,024.
16	1	s8	PPS	Pitch-pan separation, −32 to +32.
17	1	u8	PPC	Pitch-pan centre, a note 0 to 119.
18	1	u8	GbV	Global volume, 0 to 128.
19	1	u8	DfP	Default pan 0 to 64; bit 7 set means do not use it.
1A	1	u8	RV	Random volume variation, percent.
1B	1	u8	RP	Random pan variation; not implemented in Impulse Tracker.
1C	2	u16	TrkVers	Tracker version.
1E	1	u8	NoS	Number of samples.
1F	1	–	unused
20	26	text	name	Includes the terminating zero byte.
3A	1	u8	IFC	Initial filter cutoff.
3B	1	u8	IFR	Initial filter resonance.
3C	1	u8	MCh	MIDI channel.
3D	1	u8	MPr	MIDI program.
3E	2	u16	MIDIBnk	MIDI bank.
40	240	keyboard	note-sample table	120 pairs of note and sample.
130	82	envelope	volume envelope
182	82	envelope	panning envelope
1D4	82	envelope	pitch envelope	Or a filter envelope, by its flag.

The data is 547 bytes, and 554 are stored so that the old layout still loads. ModPlug and OpenMPT spaced instruments differently in some versions: alpha 5 to beta 1 used 560 bytes and beta 2 and later 557, so use the offset table rather than a fixed size.

New note action

Value	Meaning
0	Note cut
1	Note continue
2	Note off
3	Note fade

Duplicate check type

Value	Meaning
0	Off
1	Note
2	Sample
3	Instrument

Duplicate check action

Value	Meaning
0	Note cut
1	Note off
2	Note fade

Keyboard table

120 pairs of bytes. The pair at each index is what that key plays.

Byte	Value
Note	0 to 119, C-0 to B-9.
Sample	1 to 99; 0 means no sample.

Envelope (new layout)

Each of the three envelopes is 82 bytes.

Offset	Size	Name	Description
+0	1	Flg	Flags (below).
+1	1	Num	Number of node points used.
+2	1	LpB	Loop start node.
+3	1	LpE	Loop end node.
+4	1	SLB	Sustain loop start node.
+5	1	SLE	Sustain loop end node.
+6	75	Nodes	25 nodes of three bytes: a value, then a 16-bit tick from 0 to 9,999.
+81	1	pad	Unused.

Flag bit	Meaning when set
0	Envelope on.
1	Loop on.
2	Sustain loop on.
3	Envelope carry. Not in ITTECH.TXT; documented by OpenMPT.
7	Pitch envelope only: use it as a filter envelope.

Envelope	Node values
Volume	0 to 64.
Panning	−32 to +32.
Pitch	−32 to +32: each step is half a semitone, interpolated 64 times per step. Positive raises the pitch.

.IT File Format – Instruments