ITTECH.txt

ITTECH.TXT defines the structure of the .IT file format, various notes on
reading files, handling effects, and other extremely useful information about
writing an IT player. Straight from Jeffrey Lim, it is the definitive reference
on how to implement the format successfully.

Following is a nearly verbatim copy of ITTECH.TXT as taken from Impulse Tracker
2.14p5. The only changes were to split it into sections.

## Impulse Header Layout

```txt
            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───┬───┬───┬───┬───────────────────────────────────────────────┐
    0000: │'I'│'M'│'P'│'M'│ Song Name, max 26 characters, includes NULL   │
          ├───┴───┴───┴───┴───────────────────────────────────────┬───────┤
    0010: │.......................................................│PHiligt│
          ├───────┬───────┬───────┬───────┬───────┬───────┬───────┼───────┤
    0020: │OrdNum │InsNum │SmpNum │PatNum │ Cwt/v │ Cmwt  │ Flags │Special│
          ├───┬───┼───┬───┼───┬───┼───────┼───────┴───────┼───────┴───────┤
    0030: │GV │MV │IS │IT │Sep│PWD│MsgLgth│Message Offset │   Reserved    │
          ├───┴───┴───┴───┴───┴───┴───────┴───────────────┴───────────────┤
    0040: │ Chnl Pan (64 bytes)...........................................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

          ├───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┤
    0080: │ Chnl Vol (64 bytes)...........................................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

          ├───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┤
    00C0: │ Orders, Length = OrdNum                                       │
          ├───────────────────────────────────────────────────────────────┤
    xxxx: │ 'Long' Offset of instruments, Length = InsNum*4 (1)           │
          ├───────────────────────────────────────────────────────────────┤
    xxxx: │ 'Long' Offset of samples headers, Length = SmpNum*4 (2)       │
          ├───────────────────────────────────────────────────────────────┤
    xxxx: │ 'Long' Offset of patterns, Length = PatNum*4 (3)              │
          └───────────────────────────────────────────────────────────────┘

          (1) Offset = 00C0h+OrdNum
          (2) Offset = 00C0h+OrdNum+InsNum*4
          (3) Offset = 00C0h+OrdNum+InsNum*4+SmpNum*4

            Note that if the (long) offset to a pattern = 0, then the
            pattern is assumed to be a 64 row empty pattern.

          PHiliht = Pattern row hilight information. Only relevant for pattern
                    editing situations.

          Cwt:      Created with tracker.
                     Impulse Tracker y.xx = 0yxxh
          Cmwt:     Compatible with tracker with version greater than value.
                     (ie. format version)
          OrdNum:   Number of orders in song.
          InsNum:   Number of instruments in song
          SmpNum:   Number of samples in song
          PatNum:   Number of patterns in song
          Flags:    Bit 0: On = Stereo, Off = Mono
                    Bit 1: Vol0MixOptimizations - If on, no mixing occurs if
                           the volume at mixing time is 0 (redundant v1.04+)
                    Bit 2: On = Use instruments, Off = Use samples.
                    Bit 3: On = Linear slides, Off = Amiga slides.
                    Bit 4: On = Old Effects, Off = IT Effects
                            Differences:
                           - Vibrato is updated EVERY frame in IT mode, whereas
                              it is updated every non-row frame in other formats.
                              Also, it is two times deeper with Old Effects ON
                           - Command Oxx will set the sample offset to the END
                             of a sample instead of ignoring the command under
                             old effects mode.
                           - (More to come, probably)
                    Bit 5: On = Link Effect G's memory with Effect E/F. Also
                                Gxx with an instrument present will cause the
                                envelopes to be retriggered. If you change a
                                sample on a row with Gxx, it'll adjust the
                                frequency of the current note according to:

                                  NewFrequency = OldFrequency * NewC5 / OldC5;
                    Bit 6: Use MIDI pitch controller, Pitch depth given by PWD
                    Bit 7: Request embedded MIDI configuration
                           (Coded this way to permit cross-version saving)

          Special:  Bit 0: On = song message attached.
                           Song message:
                            Stored at offset given by "Message Offset" field.
                            Length = MsgLgth.
                            NewLine = 0Dh (13 dec)
                            EndOfMsg = 0

                           Note: v1.04+ of IT may have song messages of up to
                                 8000 bytes included.
                    Bit 1: Reserved
                    Bit 2: Reserved
                    Bit 3: MIDI configuration embedded
                    Bit 4-15: Reserved

          GV:       Global volume. (0->128) All volumes are adjusted by this
          MV:       Mix volume (0->128) During mixing, this value controls
                    the magnitude of the wave being mixed.
          IS:       Initial Speed of song.
          IT:       Initial Tempo of song
          Sep:      Panning separation between channels (0->128, 128 is max sep.)
          PWD:      Pitch wheel depth for MIDI controllers
          Chnl Vol: Volume for each channel. Ranges from 0->64
          Chnl Pan: Each byte contains a panning value for a channel. Ranges from
                     0 (absolute left) to 64 (absolute right). 32 = central pan,
                     100 = Surround sound.
                     +128 = disabled channel (notes will not be played, but note
                                              that effects in muted channels are
                                              still processed)
          Orders:   This is the order in which the patterns are played.
                     Valid values are from 0->199.
                     255 = "---", End of song marker
                     254 = "+++", Skip to next order
```

## Old Impulse Instrument Format (cmwt < 200h)

```txt
            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───┬───┬───┬───┬───────────────────────────────────────────────┐
    0000: │'I'│'M'│'P'│'I'│ DOS FileName (12345678.123)                   │
          ├───┼───┼───┼───┼───┬───┬───┬───┬───────┬───┬───┬───────┬───┬───┤
    0010: │00h│Flg│VLS│VLE│SLS│SLE│ x │ x │FadeOut│NNA│DNC│TrkVers│NoS│ x │
          ├───┴───┴───┴───┴───┴───┴───┴───┴───────┴───┴───┴───────┴───┴───┤
    0020: │ Instrument Name, max 26 bytes, includes NUL...................│
          ├───────────────────────────────────────┬───┬───┬───┬───┬───┬───┤
    0030: │.......................................│ x │ x │ x │ x │ x │ x │
          ├───────────────────────────────────────┴───┴───┴───┴───┴───┴───┤
    0040: │ Note-Sample/Keyboard Table, Length = 240 bytes................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

          ├───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┤
    0130: │ Volume envelope (200 bytes)...................................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

                                          ├───┴───┴───┴───┴───┴───┴───┴───┤
    01F8:                                 │ Node points (25x2 bytes)......│
                                          └───────────────────────────────┘

            Total length of old instrument header is 554 bytes.

          Flg:      Bit 0. On = Use volume envelope
                    Bit 1. On = Use volume loop
                    Bit 2. On = Use sustain volume loop
          VLS:      Volume loop start (node number)
          VLE:      Volume loop end (node number)
          SLS:      Sustain loop start (node number)
          SLE:      Sustain loop end (node number)
          FadeOut:  Ranges between 0 and 64, but the fadeout "Count" is 512.
                    Fade applied when:
                    1) Note fade NNA is selected and triggered (by another note)
                    2) Note off NNA is selected with no volume envelope
                       or volume envelope loop
                    3) Volume envelope end is reached

          DNC:      Duplicate note check (0 = Off, 1 = On)
          NNA:      New note action:
                            0 = Note cut
                            1 = Note continue
                            2 = Note off
                            3 = Note fade

          TrkVers:  Tracker version used to save the instrument. This is only
                    used in the instrument files.
          NoS:      Number of samples associated with instrument. This is only
                    used in the instrument files.

          Note-Sample/Keyboard Table.
           Each note of the instrument is first converted to a sample number
           and a note (C-0 -> B-9). These are stored as note/sample pairs
           (note first, range 0->119 for C-0 to B-9, sample ranges from
           1-99, 0=no sample)

           Volume envelope: Values from 0->64, 0FFh indicating end of envelope.
           (after which note fade applies)

          Node data: Tick THEN magnitude
```

## Impulse Instrument Format

```txt
            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───┬───┬───┬───┬───────────────────────────────────────────────┐
    0000: │'I'│'M'│'P'│'I'│ DOS FileName (12345678.123)                   │
          ├───┼───┼───┼───┼───────┬───┬───┬───┬───┬───┬───┬───────┬───┬───┤
    0010: │00h│NNA│DCT│DCA│FadeOut│PPS│PPC│GbV│DfP│RV │RP │TrkVers│NoS│ x │
          ├───┴───┴───┴───┴───────┴───┴───┴───┴───┴───┴───┴───────┴───┴───┤
    0020: │ Instrument Name, max 26 bytes, includes NUL...................│
          ├───────────────────────────────────────┬───┬───┬───┬───┬───────┤
    0030: │.......................................│IFC│IFR│MCh│MPr│MIDIBnk│
          ├───────────────────────────────────────┴───┴───┴───┴───┴───────┤
    0040: │ Note-Sample/Keyboard Table, Length = 240 bytes................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

          ├───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┤
    0130: │ Envelopes.....................................................│
          ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤

            IFC = Initial Filter cutoff
            IFR = Initial Filter resonance

            NNA = New Note Action
                    0 = Cut                 1 = Continue
                    2 = Note off            3 = Note fade

            DCT = Duplicate Check Type
                    0 = Off                 1 = Note
                    2 = Sample              3 = Instrument

            DCA: Duplicate Check Action
                    0 = Cut
                    1 = Note Off
                    2 = Note fade

          FadeOut:  Ranges between 0 and 128, but the fadeout "Count" is 1024
                    See the Last section on how this works.
                    Fade applied when:
                    1) Note fade NNA is selected and triggered (by another note)
                    2) Note off NNA is selected with no volume envelope
                       or volume envelope loop
                    3) Volume envelope end is reached

            PPS: Pitch-Pan separation, range -32 -> +32
            PPC: Pitch-Pan center: C-0 to B-9 represented as 0->119 inclusive

            GbV: Global Volume, 0->128
            DfP: Default Pan, 0->64, &128 => Don't use
            RV: Random volume variation (percentage)
            RP: Random panning variation (panning change - not implemented yet)

            MCh = MIDI Channel
            MPr = MIDI Program (Instrument)

          TrkVers:  Tracker version used to save the instrument. This is only
                    used in the instrument files.
          NoS:      Number of samples associated with instrument. This is only
                    used in the instrument files.

          Note-Sample/Keyboard Table.
           Each note of the instrument is first converted to a sample number
           and a note (C-0 -> B-9). These are stored as note/sample byte pairs
           (note first, range 0->119 for C-0 to B-9, sample ranges from
           1-99, 0=no sample)
```

#### Envelope layout

```txt
            Envelopes: 3 structures, first for volume (130h), second for
                       panning (182h), third for pitch (1D4h).

                       Each is structured as such:

            0   1   2   3   4   5   6.......
          ┌───┬───┬───┬───┬───┬───┬───────────────────────────────────┬───┐
    xxxx: │Flg│Num│LpB│LpE│SLB│SLE│ Node points, 25 sets, 75 bytes....│ x │
          ├───┼───┼───┼───┼───┼───┼───┬───┬───┬───┬───┬───┬───┬───┬───┼───┤

            Flg: Bit 0: Envelope on/off, 1 = on, 0 = off
                 Bit 1: Loop on/off, 1 = on, 0 = off
                 Bit 2: SusLoop on/off, 1 = on, 0 = off

            For Pitch envelope only:
                 Bit 7: Use pitch envelope as filter envelope instead.

            Num = Number of node points

            LpB = Loop beginning            SLB = Sustain loop beginning
            LpE = Loop end                  SLE = Sustain loop end

            Node point = 1 byte for y-value
                            (0->64 for vol, -32->+32 for panning or pitch)
                         1 word (2 bytes) for tick number (0->9999)

            Total length of an instrument is 547 bytes, but 554 bytes are
            written, just to simplify the loading of the old format. (Hence
            there are 7 'wasted' bytes per instrument)
```

## Impulse Sample Format

```txt
            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───┬───┬───┬───┬───────────────────────────────────────────────┐
    0000: │'I'│'M'│'P'│'S'│ DOS Filename (12345678.123)                   │
          ├───┼───┼───┼───┼───────────────────────────────────────────────┤
    0010: │00h│GvL│Flg│Vol│ Sample Name, max 26 bytes, includes NUL.......│
          ├───┴───┴───┴───┴───────────────────────────────────────┬───┬───┤
    0020: │.......................................................│Cvt│DfP│
          ├───────────────┬───────────────┬───────────────┬───────┴───┴───┤
    0030: │ Length        │ Loop Begin    │ Loop End      │ C5Speed       │
          ├───────────────┼───────────────┼───────────────┼───┬───┬───┬───┤
    0040: │ SusLoop Begin │ SusLoop End   │ SamplePointer │ViS│ViD│ViR│ViT│
          └───────────────┴───────────────┴───────────────┴───┴───┴───┴───┘

    The cache file has the following pieces of information added on:

            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───────────────┬───────┬───────┬───┬───────────────────────────┐
    0050: │ File Size     │ Date  │ Time  │Fmt│...........................│
          └───────────────┴───────┴───────┴───┴───────────────────────────┘

            Fmt. 0 = unchecked. 1 = directory, 2 = it sample, 3 = st sample

          GvL:      Global volume for instrument, ranges from 0->64
          Flg:      Bit 0. On = sample associated with header.
                    Bit 1. On = 16 bit, Off = 8 bit.
                    Bit 2. On = stereo, Off = mono. Stereo samples not supported yet
                    Bit 3. On = compressed samples.
                    Bit 4. On = Use loop
                    Bit 5. On = Use sustain loop
                    Bit 6. On = Ping Pong loop, Off = Forwards loop
                    Bit 7. On = Ping Pong Sustain loop, Off = Forwards Sustain loop
          Vol:      Default volume for instrument

          Length:   Length of sample in no. of samples NOT no. of bytes
          LoopBeg:  Start of loop (no of samples in, not bytes)
          Loop End: Sample no. AFTER end of loop
          C5Speed:  Number of bytes a second for C-5 (ranges from 0->9999999)
          SusLBeg:  Start of sustain loop
          SusLEnd:  Sample no. AFTER end of sustain loop

          SmpPoint: 'Long' Offset of sample in file.

          ViS:      Vibrato Speed, ranges from 0->64
          ViD:      Vibrato Depth, ranges from 0->64
          ViT:      Vibrato waveform type.
                            0=Sine wave
                            1=Ramp down
                            2=Square wave
                            3=Random (speed is irrelevant)
          ViR:      Vibrato Rate, rate at which vibrato is applied (0->64)

            The depth of the vibrato at any point is worked out in the following
            way:
              Every processing cycle, the following occurs:
                    1) Mov AX, [SomeVariableNameRelatingToVibrato]
                    2) Add AL, Rate
                    3) AdC AH, 0
                    4) AH contains the depth of the vibrato as a fine-linear slide.
                    5) Mov [SomeVariableNameRelatingToVibrato], AX  ; For the next
                                                                    ; cycle.

            For those that don't understand assembly, then the depth is
            basically the running-sum of the rate divided by 256.

            Sample vibrato uses a table 256-bytes long

       Convert - bits other than bit 0 are used internally for the loading
                 of alternative formats.
            Bit 0:
             Off: Samples are unsigned   } IT 2.01 and below use unsigned samples
              On: Samples are signed     } IT 2.02 and above use signed samples
            Bit 1:
             Off: Intel lo-hi byte order for 16-bit samples    } Safe to ignore
             On: Motorola hi-lo byte order for 16-bit samples  } these values...
            Bit 2:                                             }
             Off: Samples are stored as PCM values             }
              On: Samples are stored as Delta values           }
            Bit 3:                                             }
              On: Samples are stored as byte delta values      }
                  (for PTM loader)                             }
            Bit 4:                                             }
              On: Samples are stored as TX-Wave 12-bit values  }
            Bit 5:                                             }
              On: Left/Right/All Stereo prompt                 }
            Bit 6: Reserved
            Bit 7: Reserved

       DfP - Default Pan. Bits 0->6 = Pan value, Bit 7 ON to USE (opposite of inst)
```

## Impulse Pattern Format

```txt
            0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
          ┌───────┬───────┬───┬───┬───┬───┬───────────────────────────────┐
    0000: │Length │ Rows  │ x │ x │ x │ x │ Packed data................   │
          ├───┬───┼───┬───┼───┼───┼───┼───┼───┬───┬───┬───┬───┬───┬───┬───┤

          Length:   Length of packed pattern, not including the 8 byte header
                    Note that the pattern + the 8 byte header will ALWAYS
                    be less than 64k
          Rows:     Number of rows in this pattern (Ranges from 32->200)

          Patterns are unpacked by the following pseudocode... (this may look
          horrible, but in practise, it's just as convenient as the S3M
          pattern format for playback (but not for display))

          GetNextChannelMarker:
            Read byte into channelvariable.
            if(channelvariable = 0) then end of row
            Channel = (channelvariable-1) & 63              ; Channel is 0 based.
            if(channelvariable & 128) then read byte into maskvariable
              else maskvariable = previousmaskvariable for current channel

            if(maskvariable & 1), then read note. (byte value)
                    // Note ranges from 0->119 (C-0 -> B-9)
                    // 255 = note off, 254 = notecut
                    // Others = note fade (already programmed into IT's player
                    //                     but not available in the editor)

            if(maskvariable & 2), then read instrument (byte value)
                    // Instrument ranges from 1->99

            if(maskvariable & 4), then read volume/panning (byte value)
                    // Volume ranges from 0->64
                    // Panning ranges from 0->64, mapped onto 128->192
                    // Prepare for the following also:
                    //  65->74 = Fine volume up
                    //  75->84 = Fine volume down
                    //  85->94 = Volume slide up
                    //  95->104 = Volume slide down
                    //  105->114 = Pitch Slide down
                    //  115->124 = Pitch Slide up
                    //  193->202 = Portamento to
                    //  203->212 = Vibrato

            Effects 65 is equivalent to D0F, 66 is equivalent to D1F -> 74 = D9F
            Similarly for 75-84 (DFx), 85-94 (Dx0), 95->104 (D0x).

            (Fine) Volume up/down all share the same memory (NOT shared with Dxx
            in the effect column tho).

            Pitch slide up/down affect E/F/(G)'s memory - a Pitch slide
            up/down of x is equivalent to a normal slide by x*4

            Portamento to (Gx) affects the memory for Gxx and has the equivalent
            slide given by this table:

            SlideTable      DB      1, 4, 8, 16, 32, 64, 96, 128, 255

            Vibrato uses the same 'memory' as Hxx/Uxx.

            if(maskvariable & 8), then read command (byte value) and commandvalue
                    // Valid ranges from 0->31 (0=no effect, 1=A, 2=B, 3=C, etc.)

            if(maskvariable & 16), then note = lastnote for channel
            if(maskvariable & 32), then instrument = lastinstrument for channel
            if(maskvariable & 64), then volume/pan = lastvolume/pan for channel
            if(maskvariable & 128), then {
                    command = lastcommand for channel and
                    commandvalue = lastcommandvalue for channel
            }
            Goto GetNextChannelMarker
```

## Mathematics

```txt
    Abbreviations:
     FV = Final Volume (Ranges from 0 to 128). In versions 1.04+, mixed output
          devices are reduced further to a range from 0 to 64 due to lack of
          memory.
     Vol = Volume at which note is to be played. (Ranges from 0 to 64)
     SV = Sample Volume (Ranges from 0 to 64)
     IV = Instrument Volume (Ranges from 0 to 128)
     CV = Channel Volume (Ranges from 0 to 64)
     GV = Global Volume (Ranges from 0 to 128)
     VEV = Volume Envelope Value (Ranges from 0 to 64)

    In Sample mode, the following calculation is done:
     FV = Vol * SV * CV * GV / 262144               ; Note that 262144 = 2^18
                                                    ; So bit shifting can be done.

    In Instrument mode the following procedure is used:

     1) Update volume envelope value. Check for loops / end of envelope.
     2) If end of volume envelope (ie. position >= 200 or VEV = 0FFh), then turn
            on note fade.
     3) If notefade is on, then NoteFadeComponent (NFC) = NFC - FadeOut
            ; NFC should be initialised to 1024 when a note is played.
     4) FV = Vol * SV * IV * CV * GV * VEV * NFC / 2^41

    Linear slides work like this:
     Final frequency = Original frequency * 2^(SlideValue/768)

    (I used a lookup table for the multipliers here)

    For command Exx, SlideValue = -4*EffectValue
    For command EEx, SlideValue = -EffectValue
    For command Fxx, SlideValue = 4*EffectValue
    For command FEx, SlideValue = EffectValue

    Note that sample vibrato always uses Linear slides.

    Notes about effects (as compared to other module formats)

    C               This is now in *HEX*. (Used to be in decimal in ST3)
    E/F/G/H/U       You need to check whether the song uses Amiga/Linear slides.
    H/U             Vibrato in Impulse Tracker is two times finer than in
                    any other tracker and is updated EVERY tick.
                    If "Old Effects" is *ON*, then the vibrato is played in the
                    normal manner (every non-row tick and normal depth)
    E/F/G           These commands ALL share the same memory.
    Oxx             Offsets to samples are to the 'xx00th' SAMPLE. (ie. for
                    16 bit samples, the offset is xx00h*2)
                    Oxx past the sample end will be ignored, unless "Old Effects"
                    is ON, in which case the Oxx will play from the end of the
                    sample.
    Yxy             This uses a table 4 times larger (hence 4 times slower) than
                    vibrato or tremelo. If the waveform is set to random, then
                    the 'speed' part of the command is interpreted as a delay.

    If you read through this document and there are ANY points which you have
    troubles with (and have to try out), then let me know - because someone
    else will have the same questions - and I'd like to make this DOC as easy
    to understand as possible.

    For Panning....
     Here's the rough procedure used:

      NotePan = ChannelPan
      if InstrumentPan=On then NotePan = InstrumentPan
      NotePan = NotePan+(InstrumentNote-PPCenter)*PPSeparation/8

    Pitch Envelopes
     Each value on the envelope equates to half a semitone. This is interpolated
     64 times for smooth pitch sliding. Positive values indicate a pitch variation
     UP of x semitones, negative values indicate a pitch variation down.
```

## General Info

```txt
    The player in Impulse Tracker 'allocates' channels to notes whenever they
    are *PLAYED*. In sample mode, the allocation is simple:
                   Virtual Channel (number) = 'Host' channel (number)

    In instrument mode, the following procedure is used:

        Check if channel is already playing ---Yes--> set 'background' flag on.
                    |                                 'Trigger' NNA. If NNA=cut,
                    No                                then use this virtual
                    |                                 channel.
                    |                                          |
                    |<------------------ else -----------------/
                    |
                    v
        Search and find the first non-active virtual channel.
                    |
        Non-active channel found? ----Yes----> Use this for playback.
                    |
                    No
                    |
                    v
       Search through and find the channel of lowest volume that is in the     #
       'background' (ie. no longer controlled directly)                        #
                    |                                                          #
       Background channel found? ----Yes----> Use this for playback.           #
                    |                                                          #
                    No                                                         #
                    |                                                          #
                    v                                                          #
       Return error - the note is *NOT* allocated a channel, and hence is not  #
       played.                                                                 #

       This is actually quite a simple process... just that it's another of
       those 'hassles' to have to write...

       ### Note: This is by far the simplest implementation of congestion
                 resolution. IT 2.03 and above have a greatly enhanced
                 method which more selectively removes the most insignificant
                 channel. Obviously, there is no best way to do this - I
                 encourage you to experiment and find new algorithms for
                 yourself.
```

## Internal Tables

```txt
    FineSineData       Label   Byte
            DB       0,  2,  3,  5,  6,  8,  9, 11, 12, 14, 16, 17, 19, 20, 22, 23
            DB      24, 26, 27, 29, 30, 32, 33, 34, 36, 37, 38, 39, 41, 42, 43, 44
            DB      45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57, 58, 59
            DB      59, 60, 60, 61, 61, 62, 62, 62, 63, 63, 63, 64, 64, 64, 64, 64
            DB      64, 64, 64, 64, 64, 64, 63, 63, 63, 62, 62, 62, 61, 61, 60, 60
            DB      59, 59, 58, 57, 56, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46
            DB      45, 44, 43, 42, 41, 39, 38, 37, 36, 34, 33, 32, 30, 29, 27, 26
            DB      24, 23, 22, 20, 19, 17, 16, 14, 12, 11,  9,  8,  6,  5,  3,  2
            DB       0, -2, -3, -5, -6, -8, -9,-11,-12,-14,-16,-17,-19,-20,-22,-23
            DB     -24,-26,-27,-29,-30,-32,-33,-34,-36,-37,-38,-39,-41,-42,-43,-44
            DB     -45,-46,-47,-48,-49,-50,-51,-52,-53,-54,-55,-56,-56,-57,-58,-59
            DB     -59,-60,-60,-61,-61,-62,-62,-62,-63,-63,-63,-64,-64,-64,-64,-64
            DB     -64,-64,-64,-64,-64,-64,-63,-63,-63,-62,-62,-62,-61,-61,-60,-60
            DB     -59,-59,-58,-57,-56,-56,-55,-54,-53,-52,-51,-50,-49,-48,-47,-46
            DB     -45,-44,-43,-42,-41,-39,-38,-37,-36,-34,-33,-32,-30,-29,-27,-26
            DB     -24,-23,-22,-20,-19,-17,-16,-14,-12,-11, -9, -8, -6, -5, -3, -2

    FineRampDownData   Label   Byte
            DB      64, 63, 63, 62, 62, 61, 61, 60, 60, 59, 59, 58, 58, 57, 57, 56
            DB      56, 55, 55, 54, 54, 53, 53, 52, 52, 51, 51, 50, 50, 49, 49, 48
            DB      48, 47, 47, 46, 46, 45, 45, 44, 44, 43, 43, 42, 42, 41, 41, 40
            DB      40, 39, 39, 38, 38, 37, 37, 36, 36, 35, 35, 34, 34, 33, 33, 32
            DB      32, 31, 31, 30, 30, 29, 29, 28, 28, 27, 27, 26, 26, 25, 25, 24
            DB      24, 23, 23, 22, 22, 21, 21, 20, 20, 19, 19, 18, 18, 17, 17, 16
            DB      16, 15, 15, 14, 14, 13, 13, 12, 12, 11, 11, 10, 10,  9,  9,  8
            DB       8,  7,  7,  6,  6,  5,  5,  4,  4,  3,  3,  2,  2,  1,  1,  0
            DB       0, -1, -1, -2, -2, -3, -3, -4, -4, -5, -5, -6, -6, -7, -7, -8
            DB      -8, -9, -9,-10,-10,-11,-11,-12,-12,-13,-13,-14,-14,-15,-15,-16
            DB     -16,-17,-17,-18,-18,-19,-19,-20,-20,-21,-21,-22,-22,-23,-23,-24
            DB     -24,-25,-25,-26,-26,-27,-27,-28,-28,-29,-29,-30,-30,-31,-31,-32
            DB     -32,-33,-33,-34,-34,-35,-35,-36,-36,-37,-37,-38,-38,-39,-39,-40
            DB     -40,-41,-41,-42,-42,-43,-43,-44,-44,-45,-45,-46,-46,-47,-47,-48
            DB     -48,-49,-49,-50,-50,-51,-51,-52,-52,-53,-53,-54,-54,-55,-55,-56
            DB     -56,-57,-57,-58,-58,-59,-59,-60,-60,-61,-61,-62,-62,-63,-63,-64

    FineSquareWave     Label   Byte
            DB      128 Dup (64), 128 Dup (0)

    EmptyPattern            Label
            DW      64, 64, 0, 0
            DB      64 Dup (0)

    ;───────────────────────────────────────────────────────────────────────────────

    PitchTable              Label   DWord   ; Values are 16.16 bit
       DW      2048, 0,   2170, 0,   2299, 0,   2435, 0,   2580, 0,   2734, 0 ; C-0
       DW      2896, 0,   3069, 0,   3251, 0,   3444, 0,   3649, 0,   3866, 0 ;>B-0

       DW      4096, 0,   4340, 0,   4598, 0,   4871, 0,   5161, 0,   5468, 0 ; C-1
       DW      5793, 0,   6137, 0,   6502, 0,   6889, 0,   7298, 0,   7732, 0 ;>B-1

       DW      8192, 0,   8679, 0,   9195, 0,   9742, 0,   10321, 0,  10935, 0
       DW      11585, 0,  12274, 0,  13004, 0,  13777, 0,  14596, 0,  15464, 0

       DW      16384, 0,  17358, 0,  18390, 0,  19484, 0,  20643, 0,  21870, 0
       DW      23170, 0,  24548, 0,  26008, 0,  27554, 0,  29193, 0,  30929, 0

       DW      32768, 0,  34716, 0,  36781, 0,  38968, 0,  41285, 0,  43740, 0
       DW      46341, 0,  49097, 0,  52016, 0,  55109, 0,  58386, 0,  61858, 0

       DW      0, 1,      3897, 1,   8026, 1,   12400, 1,  17034, 1,  21944, 1
       DW      27146, 1,  32657, 1,  38496, 1,  44682, 1,  51236, 1,  58179, 1

       DW      0, 2,      7794, 2,   16051, 2,  24800, 2,  34068, 2,  43888, 2
       DW      54292, 2,  65314, 2,  11456, 3,  23828, 3,  36936, 3,  50823, 3

       DW      0, 4,      15588, 4,  32103, 4,  49600, 4,  2601, 5,   22240, 5
       DW      43048, 5,  65092, 5,  22912, 6,  47656, 6,  8336, 7,   36110, 7

       DW      0, 8,      31176, 8,  64205, 8,  33663, 9,  5201, 10,  44481, 10
       DW      20559, 11, 64648, 11, 45823, 12, 29776, 13, 16671, 14, 6684, 15

       DW      0, 16,     62352, 16, 62875, 17, 1790,  19, 10403, 20, 23425, 21
       DW      41118, 22, 63761, 23, 26111, 25, 59552, 26, 33342, 28, 13368, 30

    FineLinearSlideUpTable  Label ; Values are 16.16 bit
            DW      0, 1,     59, 1,    118, 1,   178, 1,   237, 1    ; 0->4
            DW      296, 1,   356, 1,   415, 1,   475, 1,   535, 1    ; 5->9
            DW      594, 1,   654, 1,   714, 1,   773, 1,   833, 1    ; 10->14
            DW      893, 1                                            ; 15

    LinearSlideUpTable      Label ; Value = 2^(Val/192), Values are 16.16 bit
            DW      0,     1, 237,   1, 475,   1, 714,   1, 953,  1  ; 0->4
            DW      1194,  1, 1435,  1, 1677,  1, 1920,  1, 2164, 1  ; 5->9
            DW      2409,  1, 2655,  1, 2902,  1, 3149,  1, 3397, 1  ; 10->14
            DW      3647,  1, 3897,  1, 4148,  1, 4400,  1, 4653, 1  ; 15->19
            DW      4907,  1, 5157,  1, 5417,  1, 5674,  1, 5932, 1  ; 20->24
            DW      6190,  1, 6449,  1, 6710,  1, 6971,  1, 7233, 1  ; 25->29
            DW      7496,  1, 7761,  1, 8026,  1, 8292,  1, 8559, 1  ; 30->34
            DW      8027,  1, 9096,  1, 9366,  1, 9636,  1, 9908, 1  ; 35->39
            DW      10181, 1, 10455, 1, 10730, 1, 11006, 1, 11283,1  ; 40->44
            DW      11560, 1, 11839, 1, 12119, 1, 12400, 1, 12682,1  ; 45->49
            DW      12965, 1, 13249, 1, 13533, 1, 13819, 1, 14106,1  ; 50->54
            DW      14394, 1, 14684, 1, 14974, 1, 15265, 1, 15557,1  ; 55->59
            DW      15850, 1, 16145, 1, 16440, 1, 16737, 1, 17034,1  ; 60->64
            DW      17333, 1, 17633, 1, 17933, 1, 18235, 1, 18538,1  ; 65->69
            DW      18842, 1, 19147, 1, 19454, 1, 19761, 1, 20070,1  ; 70->74
            DW      20379, 1, 20690, 1, 21002, 1, 21315, 1, 21629,1  ; 75->79
            DW      21944, 1, 22260, 1, 22578, 1, 22897, 1, 23216,1  ; 80->84
            DW      23537, 1, 23860, 1, 24183, 1, 24507, 1, 24833,1  ; 85->89
            DW      25160, 1, 25488, 1, 25817, 1, 26148, 1, 26479,1  ; 90->94
            DW      26812, 1, 27146, 1, 27481, 1, 27818, 1, 28155,1  ; 95->99
            DW      28494, 1, 28834, 1, 29175, 1, 29518, 1, 29862,1  ; 100->104
            DW      30207, 1, 30553, 1, 30900, 1, 31248, 1, 31599,1  ; 105->109
            DW      31951, 1, 32303, 1, 32657, 1, 33012, 1, 33369,1  ; 110->114
            DW      33726, 1, 34085, 1, 34446, 1, 34807, 1, 35170,1  ; 115->119
            DW      35534, 1, 35900, 1, 36267, 1, 36635, 1, 37004,1  ; 120->124
            DW      37375, 1, 37747, 1, 38121, 1, 38496, 1, 38872,1  ; 125->129
            DW      39250, 1, 39629, 1, 40009, 1, 40391, 1, 40774,1  ; 130->134
            DW      41158, 1, 41544, 1, 41932, 1, 42320, 1, 42710,1  ; 135->139
            DW      43102, 1, 43495, 1, 43889, 1, 44285, 1, 44682,1  ; 140->144
            DW      45081, 1, 45481, 1, 45882, 1, 46285, 1, 46690,1  ; 145->149
            DW      47095, 1, 47503, 1, 47917, 1, 48322, 1, 48734,1  ; 150->154
            DW      49147, 1, 49562, 1, 49978, 1, 50396, 1, 50815,1  ; 155->159
            DW      51236, 1, 51658, 1, 52082, 1, 52507, 1, 52934,1  ; 160->164
            DW      53363, 1, 53793, 1, 54224, 1, 54658, 1, 55092,1  ; 165->169
            DW      55529, 1, 55966, 1, 56406, 1, 56847, 1, 57289,1  ; 170->174
            DW      57734, 1, 58179, 1, 58627, 1, 59076, 1, 59527,1  ; 175->179
            DW      59979, 1, 60433, 1, 60889, 1, 61346, 1, 61805,1  ; 180->184
            DW      62265, 1, 62727, 1, 63191, 1, 63657, 1, 64124,1  ; 185->189
            DW      64593, 1, 65064, 1, 0,     2, 474,   2, 950,  2  ; 190->194
            DW      1427,  2, 1906,  2, 2387,  2, 2870,  2, 3355, 2  ; 195->199
            DW      3841,  2, 4327,  2, 4818,  2, 5310,  2, 5803, 2  ; 200->204
            DW      6298,  2, 6795,  2, 7294,  2, 7794,  2, 8296, 2  ; 205->209
            DW      8800,  2, 9306,  2, 9814,  2, 10323, 2, 10835,2  ; 210->214
            DW      11348, 2, 11863, 2, 12380, 2, 12899, 2, 13419,2  ; 215->219
            DW      13942, 2, 14467, 2, 14993, 2, 15521, 2, 16051,2  ; 220->224
            DW      16583, 2, 17117, 2, 17653, 2, 18191, 2, 18731,2  ; 225->229
            DW      19273, 2, 19817, 2, 20362, 2, 20910, 2, 21460,2  ; 230->234
            DW      22011, 2, 22565, 2, 23121, 2, 23678, 2, 24238,2  ; 235->239
            DW      24800, 2, 25363, 2, 25929, 2, 25497, 2, 27067,2  ; 240->244
            DW      27639, 2, 28213, 2, 28789, 2, 29367, 2, 29947,2  ; 245->249
            DW      30530, 2, 31114, 2, 31701, 2, 32289, 2, 32880, 2 ; 250->254
            DW      33473, 2, 34068, 2                               ; 255->256

    FineLinearSlideDownTable Label ; Values are 0.16 bit
            DW      65535, 65477, 65418, 65359, 65300, 65241, 65182, 65359 ; 0->7
            DW      65065, 65006, 64947, 64888, 64830, 64772, 64713, 64645 ; 8->15

    LinearSlideDownTable    Label ; Values are 0.16 bit
            DW      65535, 65300, 65065, 64830, 64596, 64364, 64132, 63901 ; 0->7
            DW      63670, 63441, 63212, 62984, 62757, 62531, 62306, 62081 ; 8->15
            DW      61858, 61635, 61413, 61191, 60971, 60751, 60532, 60314 ; 16->23
            DW      60097, 59880, 59664, 59449, 59235, 59022, 58809, 58597 ; 24->31
            DW      58386, 58176, 57966, 57757, 57549, 57341, 57135, 56929 ; 32->39
            DW      56724, 56519, 56316, 56113, 55911, 55709, 55508, 55308 ; 40->47
            DW      55109, 54910, 54713, 54515, 54319, 54123, 53928, 53734 ; 48->55
            DW      53540, 53347, 53155, 52963, 52773, 52582, 52393, 52204 ; 56->63
            DW      52016, 51829, 51642, 51456, 51270, 51085, 50901, 50718 ; 64->71
            DW      50535, 50353, 50172, 49991, 49811, 49631, 49452, 49274 ; 72->79
            DW      49097, 48920, 48743, 48568, 48393, 48128, 48044, 47871 ; 80->87
            DW      47699, 47527, 47356, 47185, 47015, 46846, 46677, 46509 ; 88->95
            DW      46341, 46174, 46008, 45842, 45677, 45512, 45348, 45185 ; 96->103
            DW      45022, 44859, 44698, 44537, 44376, 44216, 44057, 43898 ;104->111
            DW      43740, 43582, 43425, 43269, 43113, 42958, 42803, 42649 ;112->119
            DW      42495, 42342, 42189, 42037, 41886, 41735, 41584, 41434 ;120->127
            DW      41285, 41136, 40988, 40840, 40639, 40566, 40400, 40253 ;128->135
            DW      40110, 39965, 39821, 39678, 39535, 39392, 39250, 39109 ;136->143
            DW      38968, 38828, 38688, 38548, 38409, 38271, 38133, 37996 ;144->151
            DW      37859, 37722, 37586, 37451, 37316, 37181, 37047, 36914 ;152->159
            DW      36781, 36648, 36516, 36385, 36254, 36123, 35993, 35863 ;160->167
            DW      35734, 35605, 35477, 35349, 35221, 35095, 34968, 34842 ;168->175
            DW      34716, 34591, 34467, 34343, 34219, 34095, 33973, 33850 ;176->183
            DW      33728, 33607, 33486, 33365, 33245, 33125, 33005, 32887 ;184->191
            DW      32768, 32650, 32532, 32415, 32298, 32182, 32066, 31950 ;192->199
            DW      31835, 31720, 31606, 31492, 31379, 31266, 31153, 31041 ;200->207
            DW      30929, 30817, 30706, 30596, 30485, 30376, 30226, 30157 ;208->215
            DW      30048, 29940, 29832, 29725, 29618, 29511, 29405, 29299 ;216->223
            DW      29193, 29088, 28983, 28879, 28774, 28671, 28567, 28464 ;224->231
            DW      28362, 28260, 28158, 28056, 27955, 27855, 27754, 27654 ;232->239
            DW      27554, 27455, 27356, 27258, 27159, 27062, 26964, 26867 ;240->247
            DW      26770, 26674, 26577, 26482, 26386, 26291, 26196, 26102 ;248->255
            DW      26008                                                  ; 256
```

## Effect Info

```txt
    Here's about all the info I can think of for effects. "Process" variables are
    variables used internally by effects to control the direction of playback..
    This section has not been completed yet.

    First, here is the rough flow chart for processing information, it's not fully
    detailed, but all of the important steps are outlined.

           ┌─────────────────────────────────────────────────────────┐
           │ Set note volume to volume set for each channel          │
           │ Set note frequency to frequency set for each channel    │
           └────────────┬────────────────────────────────────────────┘
                        │
           ┌────────────┴────────────┐
           │ Decrease tick counter   │        Yes
           │  Is tick counter 0 ?    ├─────────────────────────┐
           └────────────┬────────────┘                         │
                        │                                      │
                    No  │                ┌─────────────────────┴──────────────────┐
           ┌────────────┴────────────┐   │ Tick counter = Tick counter set        │
           │ Update effects for each │   │                  (the current 'speed') │
           │  channel as required.   │   │      Decrease Row counter.             │
           │                         │   │        Is row counter 0?               │
           └───┬─────────────────────┘   └────────────┬──────────┬────────────────┘
               │                                  No  │          │
               │                ┌─────────────────────┘          │ Yes
               │                │                                │
               │  ┌─────────────┴──────────────┐ ┌───────────────┴────────────────┐
               │  │ Call update-effects for    │ │  Row counter = 1               │
               │  │ each channel.              │ │                                │
               │  └──────────────┬─────────────┘ │ Increase ProcessRow            │
               │                 │               │ Is ProcessRow > NumberOfRows?  │
               ├─────────────────┘               └────────┬──────────────────┬────┘
               │                                      Yes │                  │ No
               │         ┌────────────────────────────────┴──────────────┐   │
               │         │  ProcessRow = BreakRow                        │   │
               │         │  BreakRow = 0                                 │   │
               │         │  Increase ProcessOrder                        │   │
               │         │  while Order[ProcessOrder] = 0xFEh,           │   │
               │         │                         increase ProcessOrder │   │
               │         │  if Order[ProcessOrder] = 0xFFh,              │   │
               │         │                         ProcessOrder = 0      │   │
               │         │  CurrentPattern = Order[ProcessOrder]         │   │
               │         └─────────────────────┬─────────────────────────┘   │
               │                               │                             │
               │                               ├─────────────────────────────┘
               │                               │
               │         ┌─────────────────────┴──────────────────────────┐
               │         │ CurrentRow = ProcessRow                        │
               │         │ Update Pattern Variables (includes jumping to  │
               │         │  the appropriate row if requried and getting   │
               │         │  the NumberOfRows for the pattern)             │
               │         └─────────────────────┬──────────────────────────┘
               │                               │
               ├───────────────────────────────┘
               │
           ┌───┴───────────────┐       Yes        ┌───────────────────────────────┐
           │ Instrument mode?  ├──────────────────┤ Update Envelopes as required  │
           └───┬───────────────┘                  │ Update fadeout as required    │
               │                                  │ Calculate final volume if req │
               │ No (Sample mode)                 │ Calculate final pan if req    │
               │                                  │ Process sample vibrato if req │
           ┌───┴─────────────────────────────────┐└───────────────┬───────────────┘
           │ Calculate final volume if required  │                │
           │ Calculate final pan if requried     │                │
           │ Process sample vibrato if required  │                │
           └───┬─────────────────────────────────┘                │
               │                                                  │
               │                                                  │
               └─────────────────────────┬────────────────────────┘
                                         │
                        ┌────────────────┴──────────────────┐
                        │ Output sound!!!                   │
                        └───────────────────────────────────┘

    Axx     Set Tempo

            if (xx != 0) {
                    Maxtick = xx;
                    Currenttick = xx;
            }

    Bxx     Jump to Order

            ProcessOrder = xx - 1;
            ProcessRow = 0xFFFE; // indicates new pattern internally for IT...

    Cxx     Break to Row

            BreakRow = xx;
            ProcessRow = 0xFFFE;

    Dxx     Volume slide down

            if (xx == 0) then xx = last xx for (Dxx/Kxx/Lxx) for this channel.

            Order of testing: Dx0, D0x, DxF, DFx

    Dx0     Set effect update for channel enabled if channel is ON.
            If x = F, then slide up volume by 15 straight away also (for S3M compat)
            Every update, add x to the volume, check and clip values > 64 to 64
    D0x     Set effect update for channel enabled if channel is ON.
            If x = F, then slide down volume by 15 straight away also (for S3M)
            Every update, subtract x from the volume, check and clip values < 0 to 0
    DxF     Add x to volume straight away. Check and clip values > 64 to 64
    DFx     Subtract x from volume straight away. Check and clip values < 0 to 0

    Hxy     Vibrato

            if (x != 0) {
                    speed = 4*x;
            }
            if (y != 0) {
                    depth = y * 4;
                    if(OldEffects) depth <<= 1;
            }
            Set effect update for channel enabled if channel is ON.
            Goto InitVibrato (explained later)

    Ixy     Tremor, ontime x, offtime y

            if (x != 0) {
                    ontime = x;
                    if (oldeffects) ontime++;
            }
            if (y != 0) {
                    offtime = y;
                    if (oldeffects) offtime++;
            }

    Nxx     Channel volume slide down

            if (xx == 0) then xx = last Nxx for this channel.

            Order of testing: Nx0, N0x, NxF, NFx

    Nx0     Set effect update for channel enabled.
            Every update, add x to the volume, check and clip values > 64 to 64
    N0x     Set effect update for channel enabled.
            Every update, subtract x from the volume, check and clip values < 0 to 0
    NxF     Add x to volume straight away. Check and clip values > 64 to 64
    NFx     Subtract x from volume straight away. Check and clip values < 0 to 0

    Uxy     Fine Vibrato

            if (x != 0) {
                    speed = 4*x;
            }
            if (y != 0) {
                    depth = y;
                    if(OldEffects) depth <<= 1;
            }
            Set effect update for channel enabled if channel is ON.
            Goto InitVibrato (explained later)

    Wxx     Global volume slide down

            if (xx == 0) then xx = last Wxx for this channel.

            Order of testing: Wx0, W0x, WxF, WFx

    Wx0     Set effect update for channel enabled.
            Every update, add x to the volume, check and clip values > 128 to 128
    W0x     Set effect update for channel enabled.
            Every update, subtract x from the volume, check and clip values < 0 to 0
    WxF     Add x to volume straight away. Check and clip values > 128 to 128
    WFx     Subtract x from volume straight away. Check and clip values < 0 to 0

    .. sorry this is incomplete..
```