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Re: How is the IIgs beep generated?

vladitx wrote:

On Dec 23, 11:56 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:


vladitx wrote:


Surely you have tried PWM at 15.7 kHz (65 cycles), where no modulation
occurs, just slightly inaccurate PWM. Is the carrier that noticeable?

>

When I was younger, it would have driven me from the room!
Today, I probably wouldn't notice it.  ;-)

>

I did quite of PWM on the PC's speaker with carriers in the audible
range, and can't remember being that repulsive. Maybe the magic of the
"useful" 6-bit sound helped (for me), compared to the usual bleeps.


Perhaps the PC speaker and mounting was not very responsive to
such high frequencies, serving as a _de facto_ lowpass filter.

I've never measured the Apple II speaker response, but when I
was testing PWM DACs, both my wife and my daughters complained
mightily from the next room (!) when I tested the 11kHz versions.  ;-)


By the way, the 15.7 kHz freq may be already heard from the CRT, so
the listeners should be trained to ignore it to a certain extent. :)


Good point--though many people find that quite annoying if it
is loud.


I've never built a software DAC running at 15.7kHz, partly because
using it would require resampling all the sound files out there, most
of which are at 11.05kHz.  It would be pretty easy to try, and the
extra 19 cycles would certainly be useful.  ;-)



There are command-line tools to do batch resampling for you, like this
one:

http://sox.sourceforge.net/


Right--it's easy to do with the right tools, in fact SOUND.EDITOR will
resample on the Apple II.  But I wanted a program that could directly
play and manipulate the huge store of 11kHz sampled sounds, rather
than requiring users to do extra steps.


One downside is that the maximum variation in pulse width would still
have to be 32 cycles, since 64 is unattainable in a 65-cycle loop,
and any other choice results in unequal steps.  The effect would be
a reduction in maximum volume by 46/65.



If you still decide to give it a try, keep us posted. This sacrifice
is interesting to me at least.


I'm going to try it out.  Particularly for Crate.Synth, since the
AppleCrate II audio system has good lowpass filtering, and 15kHz
shouldn't be a problem for it.  In fact, 11kHz might also be OK...  ;-)

I'm away from the AppleCrate II for a few months, but I can do some
preliminary work on the assumption that the filtering will be adequate.


BTW, there would not be significant PWM inaccuracy, since the
stretched cycle would always occur at the same point in the loop,
causing only a slight nonlinearity in transfer function, but no
beat frequency.



Exactly what I was implying. ;-)


We're on the same page.  ;-)

-michael

NadaNet and AppleCrate II: parallel computing for Apple II computers!
Home page: http://home.comcast.net/~mjmahon

"The wastebasket is our most important design
tool--and it's seriously underused."