4-channel format:

Header:
songname        db      20 dup (0)
samples         db      31*size SampleRec dup (0)
songlen         db      0
restart         db      0
sequences       db      128 dup (0)
mk              dd      0
HeaderSize       =      $-Header

──────────────────────┐
Protracker Studio 16  │
──────────────────────┘
PS16-<----------------------------- Your description ------------------------->Z

struc   PS16Sample
length					dw			?
volume          dw      ?
repeat          dw      ?
replen          dw      ?
ends    PS16Sample

16-channel header format:
songname        db      75 dup (0)   ; ^Z terminated
songlen         db      0
numpatterns     db      0
commentofs      dd      0
sequences       db      128 dup (0)
samples         db      63*size PS16Sample dup (0)

16-channel pattern format:
patsize         dw      0
numlines        db      0           ( 1-254 )

Each track is compressed like:
      1     2     3     4     5     6     7     8     9    10    11    12
    ???   ???   ???   ???   ???   ???   ???   ???   ???   ???   ???   ???

     13    14    15    16    17    18    19    20    21    22    23    24
    856   808   762   720   678   640   604   570   538   508   480   453

     25    26    27    28    29    30    31    32    33    34    35    36
    428   404   381   360   339   320   302   285   269   254   240   226

     37    38    39    40    41    42    43    44    45    46    47    48
    214   202   190   180   170   160   151   143   135   127   120   113

     49    50    51    52    53    54    55    56    57    58    59    60
    ???   ???   ???   ???   ???   ???   ???   ???   ???   ???   ???   ???


Pattern Line - Byte                   (-1 End Track)

┌─Upper two bits of instrument
│ Period
│   ││
├┐┌─┴┴─┐
00111111

11111111
┌──┐┌──┐
Ins  Efx

11111111
────────
Data