Note: Words and double-words are stored Intel-style.

Offset: (decimal)    Contents:
-------              ---------
  0-7         ASCII text:  'GF1PATCH'
  8-10        ASCII text:  '100'  (pre 2.01 disks)
                           '110'  (2.01 disks)
  11          byte '00'
12-20         ASCII text   'ID#000002'
  21          byte '00'
  22          ASCII text:  copyright text
  82          byte  ;I call this Voices, always set to 1
  83          word  ;I call this Channels, always set to 14 decimal (0E hex)
  85          word  ;number of of samples, or waves
  87          word  ;master volume, usually set to 127 decimal (7F hex)
  89          dword ;unknown, not used with pre 2.01 patches, value is close
                    ;to the total length of the patch file.  May be used for
                    ;memory management purposes.
  131         ASCII text:  name of patch.
                    ;first letter is missing in pre 2.01 patches
                    ;apparently not used in 2.01 patches
  147         dword ;unknown, related to value at 89
                    ;set equal to 1 in pre 2.01 patches
  151         byte  ;unknown, but is always 1
  194         dword ;another value related to values at 89 and 147
                    ;not used in pre 2.01 patches
  198         word  ;number of samples, or waves

  239         Sample block, or wave block
                    ;there is a sample block for every sample in a patch

                    ;the following offsets are into the sample block

  0-5         ASCII text:  sample name
                    ;this is usually 'NoName'
   6          byte '00'
   7          byte  ;I call this the mode byte, use is unknown
   8          dword ;length of sample in bytes
   12         dword ;loop start in bytes offset from beginning of sample
   16         dword ;loop end in bytes offset from beginning of sample
   20         word  ;sampling frequency
   22         dword ;high frequency multiplied by 1000
   26         dword ;low frequency multiplied by 1000
   30         dword ;base frequency multiplied by 1000
   34         byte  ;detune, usually equal to zero
   35         byte  ;octave, usually equal to 2   
   36         byte  ;balance, usually 7, meaning center of stereo field
 37-48        12 bytes  ;envelope data, format unknown
                    ;I have observed the following:
                    ;first byte: higher = slower attack
                    ;second byte: higher = longer attack
                    ;fourth byte: level of release
                    ;seventh byte: lower=slower attack
                    ;eighth byte: lower = longer delay until attack
                    ;ninth byte: level of release
                    ;tenth byte: length of release
   49         byte  ;tremolo sweep
   50         byte  ;tremolo rate
   51         byte  ;termolo depth
   52         byte  ;vibrato sweep
   53         byte  ;vibrato rate 
   54         byte  ;vibrato depth
   55         byte  ;flags  7 6 5 4 3 2 1 0
                    ;I have observed the following:
                    ;7 6:  envelope scaling (I think)
                    ;5: 1=melodic, 0=percussion
                    ;4: 1=read sample backwards, 0=read sample forwards
                    ;2: 1=sample is looped, 0=sample plays through once
                    ;0: 1=sample is 16-bit, 0=sample is 8-bit
   56         byte  ;unknown, not used in pre 2.01 patches, usually '3C' hex
   59         byte  ;unknown, not used in pre 2.01 patches, usually '04'

   96         Sample
                    ;sample data is stored unsigned
                    ;16-bit samples are stored Intel style with the
                    ;least significant byte stored first.

                    ;If there is more than one sample in the patch, the next
                    ;sample block starts right after the end of the 
                    ;previous sample.