Robbie and S. K. Goodman are discussing the problem of illicit copying of
MIDI files on computer discs.  Here's a thought:

A number of commercial distributors of MIDI files on disc use various
methods for encrypting 'tagging' data within the MIDI file itself,
disguised as MIDI data itself.  One possibility is a binary coding, by
shifting the velocity of notes by various amounts.  Each file sold is
uniquely encoded.  The idea is not to prevent copying, but to be able to
trace an illicit file back to the original purchaser (who could then, I
suppose, be severely chastised!).

Jack Perron

 [ There's a problem with music roll data, though: the MIDI-velocity is
 [ irrelevant!  The file staill has perfectly valid if the tag coding is
 [ erased or deleted by setting all the velocities to the same value.
 [ Other than that, this method does sound effective for synth files
 [ *which depend upon* MIDI-velocity information.  -- Robbie