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Impulse tracker

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Impulse tracker is a type of music tracker format that was created by Jeffery Lim in 1998. It was based off the scream tracker 3 format but added addition features such as instruments, 16-bit sample, stereo sample support and new note actions. Songs of this format are saved under the extension (.it)

Format[]

The impulse tracker is written in Little Endian byte order

The impulse tracker format is split into five main parts. these include

  • Header
    • Edit history
    • MIDI macros
    • Pattern names (OMPT only)
    • Channel names (OMPT only)
    • Plug-ins (OMPT only?)
    • Message
  • Instrument
    • Envelope/s
  • Sample Header
  • Pattern data
  • Sample Data
    • Author (OMPT only)

Header:[]

Header code (IMPM) 4 bytes at start
Name Length Discription
Song name 26 bytes

(including nulls)

Name of song
Pattern hilight 2 bytes Byte 1 is rows per beat

Byte 2 is rows per mesaure

(Only used in editing)

Order Number 1 short

(2 bytes)

Number of sequenced patterns in song
Instrument Number 1 short

(2 bytes)

Number of instruments in song
Sample Number 1 short

(2 bytes)

Number of samples in song
Pattern Number 1 short

(2 bytes)

Number of patterns in song
Created with tracker 1 short

(2 bytes)

ID of tracker that made this

(Impulse tracker is 1.0 to 2.17)

encoded as hexadecimal so 1.0

becomes 0x0100

Compatible with tracker 1 short

(2 bytes)

1.0 to 2.17

(encoded as hexadecimal)

Flags 1 short

(2 bytes)

  • Bit 0, Stereo if on; mono if off
  • Bit 1, Mixing (obsolete since 1.04)
  • Bit 2, Use instruments if on, use samples if off
  • Bit 3, Use linear slides if on, use Amiga slides if off
  • Bit 4, Use Old effects of on, use IT effects of off
  • Bit 5, Link G effect with E and F memory if on
  • Bit 6, MIDI pitch controlled if on
  • Bit 7, Request Embedded MIDI Macros if on

Bit 8-15 are reserved

(Bit 15 is extended filter range in Open MPT)

Special 1 short

(2 bytes)

  • Bit 0, Song message attached if on
  • Bit 1, Edit history embedded if on*
  • Bit 2, Highlight embedded if on
  • Bit 3, Embedded MIDI Macro

Bit 4-15 are reserved

*Some versions of Schism tracker set this off even if

there is embedded history.

Global volume 1 byte Global volume of song (0-128)
Mix volume 1 byte Mixing volume of song (0-128)
Initial speed 1 byte Starting tick speed of song
Initial tempo 1 byte Starting beats Per minute of song
Pan separation 1 byte Panning separation between channels (0-128)
Pitch wheel depth 1 byte Pitch wheel depth for MIDI controllers
Message Length 1 short

(2 bytes)

Length of attached message
Message offset 1 int

(4 bytes)

Offset of message in file
Reserved 4 bytes Open MPT writes "OMPT" here if it is an interpreted

Mod Plug file

Initial channel pan 64 bytes Each byte is a pan value (examples: 0 is left pan, 32 is center pan

and 64 is right pan) (0-64)

Initial channel volume 64 bytes Each byte is a channel volume (0-64)
Orders Length is order

number

0-199 are valid patterns,

254 is seperator and 255 is song end.

Instrument offsets Length is instrument

number * 4

Each offset is an integer offset to Instrument in file
Sample Header offsets Length is sample

number * 4

Each offset is an integer offset to sample header in file
Pattern offsets Length is pattern

number * 4

Each offset is an integer offset to pattern in file

Edit history:[]

Edit history para-pointer
Name Length Discription
Edit History number short

(2 bytes)

Number of contained edit histories
Edit histories 8 bytes per history Explained below
Edit history (8 bytes)
Name Length Discription
Fat date short

(2 bytes)

Date of edit in Microsoft Fat date format
Fat time short

(2 bytes)

Time of edit in Microsoft Fat date format
Run time int

(4 bytes)

Runtime in MS-DOS ticks

(1/18.2 of a second)

MIDI macros[]

There are three groups of MIDI macros an IT file can contain. These are only present if either bit 7 of flags or bit 3 of special flags are on.

  • 9 Global (9 * 32 bytes)
  • 16 Parametric and (16 * 32 bytes)
  • 128 Fixed (128 * 32 bytes)
Macro structure
Name Length Discription
Macro String 32 byte Holds 32 characters

MIDI macros are a total of 4896 bytes long.

Pattern Names[]

Pattern names (PNAM)
Name Size Discription
Length int

(4 bytes)

Length of pattern names

(Name count is Length / 32)

Pattern name 32 bytes

per name

Name of pattern

Channel Names[]

Channel names (CNAM)
Name Size Discription
Length int

(4 bytes)

Length of pattern names

(Name count is Length / 20)

Channel name 32 bytes

per name

Name of channel

Plug-ins[]

todo

Message[]

Message
Name Size Discription
Message defined in message length The message string

Messages usually use CR for line endings but Chibi Tracker uses LF instead.

Instruments[]


Pre 2.0 instrument[]

Instrument code (IMPI) 4 bytes at start
Name Length Discription
DOS fie name 12 bytes

(including nulls)

Name of DOS file
Reserved 1 byte Reserved for future use
Flags 1 byte
  • bit 1 Use volume envelope if on
  • bit 2 Use loop envelope if on
  • bit 3 Use sustain loop envelope if on
Volume loop start 1 byte Number of loop start node of envelope
Volume loop end 1 byte Number of loop end node of envelope
Sustain loop start 1 byte Number of sustain loop start node of envelope`
Sustain loop end 1 byte Number of sustain loop end node of envelope
Reserved 2 bytes Reserved for future use
Fadeout Short

2 bytes

0-64 but the count is by 512
New note action 1 byte
  • 0 = note cut
  • 1 = continue
  • 2 = note off
  • 3 = note fade
Duplicate note check 1 byte 0 = false, 1 = true
Tracker version 2 bytes Only used in instrument files
Number of samples 1 byte Only used in instrument files
Reserved 1 byte Reserved for future use
Instrument name 26 bytes

(including nulls)

Name of instrument
Reserved 6 bytes Reserved for future use
Note Sample

Keyboard table

240 bytes

(120 2-byte pairs)

Byte 1 is note (0-119)

Byte 2 is sample

Pre 2.0 envelopes[]

Volume envelope
Volume envelopes 200 bytes The volume envelopes

(0-64)

FF indicates

end of envelope

Node points 25 * 2 bytes Byte 1 is tick

Byte 2 is magnitude

Post 2.0 instrument[]

Instrument code (IMPI) 4 bytes at start
Name Length Discription
DOS fie name 12 bytes

(including nulls)

Name of DOS file
Reserved 1 byte reserved for future use
New Note Action 1 byte
  • 0 = note cut
  • 1 = continue
  • 2 = note off
  • 3 = note fade
Duplicate check

type

1 byte
  • 0 = off
  • 1 = note
  • 2 = sample
  • 3 = instrument
Duplicate check

Action

1 byte
  • 0 = note cut
  • 1 = note off
  • 2 = note fade
Fade out (signed) short

(2 bytes)

0-128 but the count

is 1024

Pitch pan seperation 1 signed byte -32 to 32
Pitch pan centre 1 byte note to centre pitch

pan on (0-119)

Global volume 1 byte 0-128
Default pan 1 byte 0-64. if bit 128 is 1,

don't use.

Random volume

variation

1 byte percentage (0-100)
Random pan

variation

1 byte percentage (0-100)
Tracker version Short

(2 bytes)

Tracker version save as

(only used in instruments files)

Number of samples 1 byte Number of sample used

by this instrument

(only used in instrument files)

Reserved 1 byte Reserved for future use
Instrument name 26 bytes

(including nulls)

The name of the instrument
Initial filter cutoff 1 (signed) byte 0-127*
Initial filter resonance 1 (signed) byte 0-127**
MIDI channel 1 (signed) byte The MIDI channel to use for

this instrument (0-16)

MIDI program 1 byte The general MIDI instrument

to use for this instrument

(1-128)

MIDI bank 2 bytes 0-16384
Note Sample

Keyboard table

240 bytes

(120 2-byte pairs)

Byte 1 is note (0-119)

Byte 2 is sample

* Filter uses the formula 110*2^(0.25+ce/fe), where ce is cutoff * (256 + 256) and fe = 24*512 or 20*512 if using OpenMPT's extended filter range.) Each step is roughly one half of a semitone/half-step for the non-extended filter range. (I guess that the cutoff frequency doubles every 20 increments for the OpenMPT extended filters.)


** Resonance used the formula 10^((-resonance*24.0)/(128.0f*20.0f)) but this usually uses a pre-calculated table.

Post 2.0 Envelopes[]

There are three envelope structures saved in the file. They are as follows:

  • Volume Envelope, (82 bytes total)
  • Panning envelope, (82 bytes total)
  • Pitch and filter envelope, (86 bytes total)
Envelope structure
Name Length Discription
Flags 1 byte
  • Bit 0, Envelope on/off
  • Bit 1, Loop on/off
  • Bit 2, Sustain loop on/of
  • (3-6 are reserved)
  • Bit 7, Pitch envelope used as filter*

(Bit 3 is envelope carry in OpenMPT)

Node count 1 byte Number of valid nodes in file.
Loop beginning 1 byte Beginning node of loop.
Loop end 1 byte Ending node of loop.
Sustain loop

beginning

1 byte Beginning node of sustain loop.
Sustain loop

end

1 byte Ending node of sustain loop.
Node points 3 bytes per node**

(25 nodes/75 bytes)

Byte 1 is Y position

(0-64 for volume and filter, -32 to 32 for panning and pitch) Short (bytes 2 and 3) used for tick position (0-9999)

Trailing byes* 7 bytes if version 2.0 to 2.14

4 bytes if 2.14p1 or above

Some trackers use this for extra hacks like OMPT

*Only used for the pitch envelope

**There is space for 25 total nodes, regardless of how many are used.

Samples[]

Sample Header (IMPS)
Name Length Discription
Dos filename 12 bytes

(including nulls)

Name of DOS file
Reserved byte 1 byte Reserved for future use
Global volume 1 byte 0-64
Flags 1 byte
  • Bit 0, Sample associated with header if on
  • Bit 1, 16-bit sample if on: 8-bit sample if off
  • Bit 2, Stereo if on; mono if off*
  • Bit 3, Compressed sample if on; PCM sample if off
  • Bit 4, Use loop if on
  • Bit 5, Use sustain loop if on
  • Bit 6, Ping-Pong loop if on; forwards only if off
  • Bit 7, Ping-Pong sustain loop if on forwards if off
Default volume 1 byte Default volume for sample (0-64)
Sample Name 26 bytes

(including nulls)

The name of the sample
Convert flags 1 byte
  • Bit 0, Samples are Signed if on; unsigned if off**
  • Bit 1, Big endian samples if on; little endian samples if off
  • Bit 2, Samples are delta encoded if on; samples are PCM if off***
  • Bit 3, Samples are byte delta encoded (for PTM loader)
  • Bit 4, Samples use 12-bit TX-Wave values
  • Bit 5, Prompt for Left/Right/All stereo

(Bits 6 and 7 are reserved. Bits 1 and 3 to 5 can usually be ignored as the were used for conversion of other formats)

Default pan 1 byte Bits 0-6 are panning value from 0-64.

Bit 7 must be on for panning to be applied.

Sample Length Int

(4 bytes)

Number of sample in file
Loop beginning Int

(4 bytes)

Beginning sample of loop
Loop end Int

(4 bytes)

Sample after the loop end
C5 (middle C)

speed

Int

(4 bytes)

Sample rate for Middle C

(0-9,999,999)

Sustain

loop beginning

Int

(4 bytes)

Beginning sample of sustain loop
Sustain

loop end

Int

(4 bytes)

Sample after the sustain loop end
Sample pointer Int

(4 bytes)

Pointer to sample data in the file

(Usually sample data is written after the patterns)

Vibrato speed 1 byte Changes frequency of vibrato (0-64)
Vibrato depth 1 byte How far the vibrato can go off tune

in each direction (0-64)

Vibrato Sweep 1 byte Delays the gain of the vibrato.

(The higher the number, the faster the vibrato gets to peak depth) 0-64

Vibrato waveform 1 byte
  • 0 is Sine
  • 1 is ramp down Sawtooth
  • 2 is Square and
  • 3 is random sine (possibly single purlin noise channel)

*Stereo samples was not supported in Impulse Tracker, but later trackers added stereo support.

**Samples in version 2.01 and earlier are unsigned, samples in 2.02 and later are signed.

***Compressed data is double delta encoded if this bit is on. IT 2.14p1 and later use this method and IT 2.14 and earlier always sets this off and delta encodes the data just once.

Sample Data[]

PCM data:[]

When not compressed: samples are stored as 1 byte (if 8-bit) or 2 bytes (if 16-bit) samples. The count of bytes is equal to the sample length * the bytes per sample. If the samples are stereo; all the left channel data is read first, then the right channel data is read.

Compressed data;[]

Each block of sample data when uncompressed is never more than 32 Kilobytes (0x8000 bytes) long.

Each block always resets the bitrate to the sample bitrate + 1 (9 for 8-bit and 17 for 16-bit)

Each block is delta decoded once (if convert bit 2 is off) or twice (if convert bit 2 is on)

Compressed data block
Name Length Discription
Compressed data length Short

(2 bytes)

The length of the bitstream in bytes
Compressed data Above column bytes long The compressed bitstream of the block of data

(This bitstream is also in little endian format) So 257/-255 "0b100000001" and 65/-63 "0b1000001" wold be stored as byte1[00000001] then byte2[10000011]

Samples are decoded using 3 decompression types

Type A:[]

Used when length is between 1-6 bits wide. An unsigned value is read of length bits. If only the most signifiant (left most) bit is on and all others are off, 3 (if 8-bit) or 4 (if 16-bit) bits are read as an unsigned value to figure out new bit width. If not, convert value to signed and add to list for decoding.

Type B:[]

Used when length is between 7-bitrate (8 or 16) bits wide. An unsigned value is read of length bits. If value is equal or greater than leftmost bit + negative b value (-4 if 8-bit and -8 if 16-bit) or value is equal to or less than leftmost bit - b value (3 if 8-bit and 7 if 16-bit); subtract leftmost bit - negative b value from value and use it to figure out new bit width. If not, convert value to signed and add to list for decoding.

Type C:[]

Used when width is one bit greater than bit width (9 for 8-bit or 17 for 16-bit) An unsigned value is read of length bits. If the value is greater or equal to the left most bit, The value is limited to the lowest 8 bits (and value by 255 for example) and then become the new bit width. If not, get all bits but the leftmost bit, make a signed value out of it and add to list for decoding.

Calculating new bit width:[]

New bit width values from type A or type B compressions, need to be processed before becoming the new bit width. The value is first incremented by one, then incremented once more if the value is equal to the old bit width. After that the value is now the new bit width.

Delta decoding:[]

After all bytes from block are read, extracted and stored in a collection; the collection is delta decoded once or twice depending on convert bit 2 being on (twice) or off (once). The samples of this block are now decompressed. This is done by adding the current value onto the previous value and if double delta decoded, adding the previous value onto the earlier value.

Stereo:[]

If a stereo sample; all the right channel data is decompressed after and completely separate from the left channel data.

Patterns[]

Pattern (note, the whole pattern struct is never more than 0xffff (64 kilobytes) long
Name Length Discription
Pattern length short

(2 bytes)

Length of packed data

- the first 8 bytes

Row count (signed) short

(2 byte)

Row count. IT allows between 32-200

rows but OpenMPT can have more.

Reserved 4 bytes Reserved bytes
packed data pattern length

bytes long

The packed data

Unpacking the data:[]

for each row
    read channelMask byte
    while channelMask > 0
        channel = (channelMask - 1) & 63
        if channelMask bit 7 is 1
            read maskVariable byte
        else
            use last maskVariable from this channel
        endif
        
        // note
        if maskVariable bit 0 is on
            get note byte
        else if maskVariable bit 4 is on
            use last note byte
        endif
        
        // instrument
        if maskVariable bit 1 is on
            get instrument byte
        else if maskVariable bit 5 is on
            use last instrument byte  
        endif
        
        // volume
        if maskVariable bit 2 is on
            get volume byte
        else if maskVariable bit 6 is on
            use last volume byte  
            
        // effect and effect value
        if maskVariable bit 3 is on
            get effect byte
            get effect value byte
        else if maskVariable bit 7 is on
            use last effect byte
            use last effect value byte
        endif
        
        read new channelMask byte
    end while
end for

Note:[]

Note ranges between 0 (tracker C0 or MIDI C-1) to 119 (Tracker B9 or MIDI B8). It is advised to increment any value between 0 and 119 by one when reading to distinguish between a note continue event and no note.

254 = note cut (^^)

255 = note off (==)

anything else is fade out (~~)

Volume:[]

  • 0-64 Volume (v)
  • 65-74 Fine volume up (a)
  • 75-84 Fine volume down (b)
  • 85-94 Volume slide up (c)
  • 95-104 Volume slide down (d)
  • 105-114 Pitch slide down (e)
  • 115-124 Pitch slide up (f)
  • 128-192 Pan 0-64 (p)
  • 193-202 Portamento to note (g)
  • 203-212 Vibrato (h)

Effect:[]

  • A Set Tempo
  • B Jump to order pattern
  • C Break to row
  • D Volume slide
  • E Portamento up
  • F Portamento down
  • G Portamento to note
  • H Vibrato
  • I tremor
  • J Arpeggio

Currently incomplete...

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