Impulse Tracker samples and compression

Each sample has an 80-byte header that starts IMPS, found through the sample offset table. The sample data itself sits wherever the header’s SamplePointer says. Offsets on this page are in hexadecimal.

Sample header

Offset	Size	Type	Name	Description
00	4	ASCII	magic	`IMPS`.
04	12	text	filename	DOS 8.3 file name.
10	1	u8	null	Always 0.
11	1	u8	GvL	Global volume, 0 to 64.
12	1	u8	Flg	Sample flags.
13	1	u8	Vol	Default volume, 0 to 64.
14	26	text	name	Includes the terminating zero byte.
2E	1	u8	Cvt	Convert flags.
2F	1	u8	DfP	Default pan: bits 0 to 6 the pan, bit 7 set to use it. This is the opposite sense to the instrument’s DfP.
30	4	u32	Length	Length in samples, not bytes.
34	4	u32	Loop begin	In samples.
38	4	u32	Loop end	The sample after the loop’s last.
3C	4	u32	C5Speed	Samples per second at note C-5, 0 to 9,999,999.
40	4	u32	Sustain loop begin	In samples.
44	4	u32	Sustain loop end	The sample after the sustain loop’s last.
48	4	u32	SamplePointer	File offset of the sample data.
4C	1	u8	ViS	Vibrato speed, 0 to 64.
4D	1	u8	ViD	Vibrato depth, 0 to 64.
4E	1	u8	ViT	Vibrato waveform.
4F	1	u8	ViR	Vibrato rate, 0 to 64. The depth grows by the rate each cycle; the depth used is the running sum divided by 256.

A sample file (.its) adds cache information after the 80 bytes: the file size (4 bytes), date and time (2 bytes each) and a format byte (0 unchecked, 1 directory, 2 IT sample, 3 ST sample).

Sample flags

Bit	Value	Meaning when set
0	01	A sample is associated with this header.
1	02	16-bit samples (clear: 8-bit).
2	04	Stereo. Not supported in Impulse Tracker itself.
3	08	Compressed.
4	10	Loop on.
5	20	Sustain loop on.
6	40	Ping-pong loop (clear: forwards).
7	80	Ping-pong sustain loop (clear: forwards).

Convert flags

Bit	Value	Meaning when set
0	01	Samples are signed. Impulse Tracker 2.01 and earlier used unsigned; 2.02 and later use signed.
1	02	Motorola byte order for 16-bit samples.
2	04	Delta-encoded samples.
3	08	Byte delta (PTM loader).
4	10	TX-Wave 12-bit samples.
5	20	Prompt for left, right or all stereo.
6–7	C0	Reserved.

Bits other than bit 0 exist for loading other formats. Modern players act only on bit 0 and, for compressed samples, on delta encoding (bit 2).

Vibrato waveforms

Value	Waveform
0	Sine
1	Ramp down
2	Square
3	Random (the speed is ignored)

Sample data

Uncompressed data is Length values of 8 or 16 bits. Stereo data, written by other programs since Impulse Tracker has no stereo samples, holds the left channel first and then the right.

Compressed sample data

When bit 3 of the flags is set the data is split into blocks. Each block expands to at most 32,768 bytes (32,768 samples at 8 bits, 16,384 at 16 bits), and the last block may be shorter. The header’s Length gives the uncompressed length and says when to stop. Each block starts with a 16-bit size of the compressed data that follows, not counting those two bytes. The stream is read bit by bit, least significant bit first.

A block starts with a width of 9 bits (8-bit samples) or 17 bits (16-bit samples) and a running value of 0. Read width bits as v; depending on the width, v is either a change of width or a sample delta:

Width	Change of width
1 to 6	`v` equals the top bit alone. Read 3 more bits (8-bit) or 4 (16-bit) to get the new width.
7 up to one below the starting width	`v` lies from top bit − 4 to top bit + 3 (8-bit), or top bit − 8 to top bit + 7 (16-bit). The new width comes from `v` − (top bit + lower bound), where the lower bound is −4 or −8.
The starting width (9 or 17)	The top bit is set. The new width is (`v` without the top bit) + 1.

In the first two cases, the value read is increased by 1 to give a width, and by 1 more if the result is not less than the current width, so the width can never stay the same. In the third case the value is used as it stands. A width greater than the starting width means the block is damaged; the decoder gives up on that block.

Anything else is a sample: treat v as a signed number of width bits (sign-extend it), add it to the running value, and write the running value as a sample. The running value resets to 0 at each block.

Constant	8-bit	16-bit
Starting width	9	17
Bits for a change at widths 1 to 6	3	4
Lower bound for the second case	−4	−8
Upper bound for the second case	3	7

Samples compressed by version 2.15 and later are marked by bit 2 (delta) in the convert flags. For these, a second running value accumulates the first, and the second value is what gets written, so the data is delta-coded twice. Older files use one running value. Header values such as Cmwt do not distinguish the two; only the convert flag does. Convert flags apply per block. Stereo compressed data is the left channel in full, then the right.