program variables/flags: 

       MODE = (PASS | RECORD | PLAYBACK)
       FRET = (ON | OFF)
       NOTE = [PITCH, VOLUME, DURATION]
       MEM_POINTER = memory address to get next note from or store to
       PREV_PITCH = the pitch of the last note recorded
       PREV_VOLUME = the volume of the last note recorded
       COUNTER = 0

main loop, executed every 100th of a second or so, 
called by timer interrupt: 

     local variables: pitch, volume

     check record button
     if it's depressed, set MODE = RECORD
     if MODE previously was RECORD 
        and record button is now not depressed,
	write current NOTE to memory followed by "stop" byte
        reset DURATION counter

     if MODE is now RECORD and was previously not RECORD,
        reset MEM_POINTER to beginning of memory area

     check playback button
     if it's depressed, set MODE = PLAYBACK
     if MODE is now PLAYBACK and previously wasn't,
	reset MEM_POINTER to beginning of memory area

     if neither record, nor playback buttons are depressed,
        set MODE = PASS
	check fret switch and set FRET to ON or OFF accordingly

     sample pitch and volume info from input
     if FRET = ON
	discretize pitch
	discretize volume

     if MODE = PASS

	NOTE.PITCH = pitch
	NOTE.VOLUME = volume

     if MODE = RECORD
	NOTE.PITCH = pitch
	NOTE.VOLUME = volume
	compare NOTE.PITCH to PREV_PITCH 
            and NOTE.VOLUME to PREV_VOLUME
	if both are the same, 
	   increment NOTE.DURATION

	if either are different, 
	   write NOTE to location pointed to MEM_POINTER
	   reset NOTE.DURATION to 1
	   increment MEM_POINTER

	PREV_PITCH = NOTE.PITCH
	PREV_VOLUME = NOTE.VOLUME

     if MODE = PLAYBACK
	increment COUNTER
	if COUNTER > NOTE.DURATION
	   get new NOTE from location pointed to by MEM_POINTER
	   reset COUNTER 
	   increment MEM_POINTER
	   if location MEM_POINTER is "stop" byte, 
	      reset MEM_POINTER to beginning of memory area

     send NOTE.PITCH and NOTE.VOLUME to output