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From: ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University)
Newsgroups: comp.sys.mac.programmer
Subject: Re: System 7.0 Sound Manager Functionality
Message-ID: <1991Apr26.152439.3551@waikato.ac.nz>
Date: 26 Apr 91 03:24:39 GMT
References: <1991Apr18.203859.26265@terminator.cc.umich.edu>
Distribution: usa
Organization: University of Waikato, Hamilton, New Zealand
Lines: 48

I recently implemented an XCMD in HyperCard 2.0 that puts up a window
on which you can play notes. You hold down the mouse button anywhere
within the window, and it works out a pitch based on the position of
the pointer. While holding the button down, you can move the pointer
within the window, and the pitch changes accordingly. It even changes
smoothly, if you don't move the pointer too fast.

The way I did this is as follows.

* As soon as the button is pressed, I find my sound data and allocate
a sound channel.

* I feed a soundCmd command into the channel to specify the sound to be
played, followed by an ampCmd command to set the amplitude to zero, and
finally a freqCmd to start an indefinite-duration sound playing, at
the pitch of the sound's base note.

* I then use a getRateCmd to find the current relative sampling rate.
Note that some versions of the sound documentation indicate that getRateCmd
returns the current rate in param2; this is INCORRECT. Instead, you must
pass the address of a variable of type Fixed in param2, and this will
be set to the current rate.
  By the way, I send all my commands in this application with SndDoImmediate,
as I don't need any queueing, but this is probably unimportant.

+ I determine the current mouse position, and work out a new sample rate,
if I remember correctly as follows (noteNumber may have a fractional part,
and may be negative after the second line):

  	noteNumber := (mousePos.h - leftWindowEdge) / windowWidth * scale;
	noteNumber := noteNumber - baseNoteOfSound;
	newRate := originalRate * 2 ** (noteNumber / 12)

* I then set the new rate with a rateCmd, followed by an ampCmd to
change the amplitude to 255 (maximum), so the user actually hears the
sound playing.

* If the mouse position should change before the button is released,
I loop again from the step marked "+".

* When the user releases the mouse button, I send a quietCmd to
cut the sound, and deallocate the channel.

Lawrence D'Oliveiro                       fone: +64-71-562-889
Computer Services Dept                     fax: +64-71-384-066
University of Waikato            electric mail: ldo@waikato.ac.nz
Hamilton, New Zealand    37^ 47' 26" S, 175^ 19' 7" E, GMT+12:00
You are in a twisty little maze of Word 4.0 dialog boxes, all different.