Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!umich!terminator!usenet
From: potts@itl.itd.umich.edu (Paul Potts)
Newsgroups: comp.sys.mac.programmer
Subject: System 7.0 Sound Manager Functionality
Keywords: Sound, System 7.0
Message-ID: <1991Apr18.203859.26265@terminator.cc.umich.edu>
Date: 18 Apr 91 20:38:59 GMT
Sender: usenet@terminator.cc.umich.edu (usenet news)
Distribution: usa
Organization: Instructional Technology Laboratory, University of Michigan
Lines: 121

Hi all,

I've got a number of questions and concerns about the
Sound Manager. I'm currently working with System 7.0B6,
Think C with the revisions and header files from the
Beta 4 CD, a standalone version of Rez, and the .r files
from the Beta 4 CD as well.

My current project is a simulation of an audiometer, a
device used to test hearing. The device generates tones
at precise pitches and levels (750Hz at 60 dB, for
example). These pitches must be generated for an
arbitrary length of time, say, for as long as the
mouse is held down. Since this is a simulation, the
dB levels played through the headphones don't need to
be very accurate, but the pitches should be reasonably
so. The simulation is in Hypercard, but I have
experience writing XCMDs and intend to move the sound-
play functions to an XCMD.

For the prototype of this stack, I first used SoundEdit
to create samples of sine waves at various frequencies,
and saved them as tenth-of-a-second samples. I then hand-
edited the endpoints of the waves to eliminate, as much
as possible, any clicking, and play these sounds using
repeated calls to Hypercard's "play" function. The
results are surprisingly good. Some of the frequencies
generate clicks when played back-to-back like this, but
most of the lower ones don't. SoundEdit can only produce
these tones at volumes ranging from 0..100%, and I
would eventually like to have finer control than
that, but hey, the prototype works.

Now that it is time to turn the project from prototye
into a development version, I want to use the square-
wave synthesizer to generate these tones.

According to the documentation on the beta 1 CD
(until recently, all I had), it was possible to use
freqCmds and noteCmds (now called freqDurationCmds)
to play tones in two ways. The first was to pass a
value in param2 of the sound command in the range
1..127. The value was to be interpreted as a MIDI
note value. The second was to send the actual
frequency at which to play the note (as long as the
frequency was greater than 127).

This is the method I began attempting to use, and
wrote code to set up a sound channel and pass
the appropriate commands to it. I immediately began
to notice a problem. Values in the range 1..127 played
properly, but larger values didn't. Values between
127 and 255 seemed to map back to values in the range
1..127, producing strange tones. Values like 250, 500,
750, 1000, etc. - the exact tones I want my simulation
to reproduce - all produced the same extremely high
pitch.

After wrestling with my code for some time, and
determining that it was operating correctly, a colleague
and I dove into the system software code to follow the
frequency values, and see what happened to them. As we
suspected, the high bytes were getting stripped off, and
the values in the range 0..127 were being used to
calculate offsets from some base note such as middle C.
All this magic happens in one of the snth resources.

When I finally got access to the beta 4 CD/ROM and a
later draft of IM-6, I found that, in fact, the
documentation had been changed to reflect what was
actually going on. It seems that it is now only legal
to pass values in the range 1..127 to either freqCmds
or freqDurationCmds. Also, the SoundHeader data structure
has been slightly redefined. The field baseFrequency,
formerly a word, has been altered to two Byte fields.
The high byte is now used to select the encoding option,
and only a byte is left to select the base frequency.
This was altered to support different encoding options,
but perhaps a better method of extending the data
structure could have been found, one which didn't
require cutting the baseFrequency down to one byte.

Now I've got to look at other ways to play the sounds.
I have created a wave table out of a single sine wave,
but I can't use rateCmds with a waveTable, and have to 
use the MIDI values again. I can play the single wave
using the sampled sound synthesizer, but it can only be
played at note values, comes out too low. I will try
using rateCmd to play the wave at various frequencies,
but I'm not sanguine about my ability to use the base
frequency together with rateCmds to come up with appropriate
pitches.

Meanwhile, does anyone have ideas on a better way to do
this? I can't be the only one out there who needs to be
able to play sounds at arbitrary frequencies. Doesn't
anyone need to play notes at non-Western intervals?

I would attempt to do this myself using the older
Sound Driver calls, but they look very difficult, and
besides, they probably will not be well-supported in the
future.

If any Apple employees are reading, perhaps you have
a better perspective on the reasons that this important
functionality was removed from the Sound Manager. Since
this is my first project using the Sound Manager, I'm
not dead-certain that playing notes at arbitrary frequencies
ever worked (especially since the documentation I was
using was a draft). But it is something I would dearly
love to use.

Sorry to be so long-winded, and thanks for reading.

Please reply to me to potts@itl.itd.umich.edu.  If
I get interesting responses, I'll post them.

-Paul Potts-
Consultant, Office of Instructional Technology
Information Technology Division, University of Michigan
:x