Wednesday, August 28, 1991
					01:48 AM PDT


These notes are intended to help with deciphering my source to the
MIRAGEFREQ program.  The key to this program is that the Commodore SID
chip has good pitch resolution (16 bit) and is repeatable.  

The BASIC source was converted to text by a program I wrote for the
occasion (BASIC source is stored tokenized on the C-64/128).  While I
was at it, I also added in a feature to convert the special print
control characters to text contained in brackets; for example, the
following print statement:

	100 print"[CLR][C/DN][C/DN][BLUE]"

would clear the screen (and home the cursor), move the cursor down
twice, and then change the text color to blue.  

Hopefully that helps enough to make some sense of it.  

The BASIC program does essentially the following in a loop:

	- Query the Mirage for the sampling rate, etc.
	- Format and display the data, adjust the sampling tone
	- Check for and handle any user input.


The following should prove helpful:  

	1-50	'main' -calls init and eventually the main loop
	100-190 Initialization: setting up the interface, loading
		the .ml code, etc.
	200-290 Main loop.  Updates the screen and the tone.
	300-395 Gets and handles user input
	400-405 Screen header print routines
	410-425 Routine to get pitch bend range
	450-465 Routine to get the MIDI channel
	500-550 Handles calling the machine language routine
	600-699 Help
	700-710 Calculates the internal filter frequency.  This 
		is because the tables in the Advanced Sampler's Guide
		don't list all of the notes.  I think I basically got
		lucky playing with some log functions and 'derived' the
		formula shown.
	750-790 Calculates the note name and accuracy in cents.  This
		is a pretty useful function, so I'll describe it.  The
		relationship between any two adjacent notes is a factor
		of the twelfth root of two.  This routine 'normalizes'
		the frequency by forcing it into an octave between two
		C's (ch and cl for Chigh and Clow).  The frequency for a
		C is 440 times (the twelfth root of two, cubed).  Taking
		12root2 (tr) to the third power produces the ratio of a
		minor third or three half steps.  This, times 440 (A)
		gives us a C.  This is not necessary, but I needed some
		basis, and C seemed like a good place.

		Now for the key to the whole thing.  Given a frequency,
		divide it by the frequency of the C below it.  Take the
		log of the result, and divide this by the log of the
		12root2.  Voila!  The result is the note number (0=C,
		11=B) and the fractional part.  Round to the nearest
		integer, and subtract from the original.  The integer is
		the note, the difference times 100 is the accuracy in
		cents.  If you don't care to go through the
		'normalizing' step (I wrote that part before I knew how
		to make this work), you simply need to do a modulo 12 on
		the result.  Make sure your modulo function handles
		negative numbers correctly.


The file MOREINFO.TXT is the output of the MOREINFO program.  There is
no standard way of reading text files on the C-64 so in general a
program is supplied.

The assembly language text is for the Merlin assembler.  Merlin spaces
the text on the screen, requiring only one space for each field - I
assume this saves a significant amount of space - precious on a C-64/128.
I tabbed some of the source lines over to make it more readable, but the
fields still don't line up.  Have fun.

Well, I hope that helps.   Feel free to drop me a line with any
questions.

					-jk

					Internet: johnny@ptp.hp.com
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