Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!bridge2!gpz
From: gpz@bridge2.ESD.3Com.COM (G. Paul Ziemba)
Newsgroups: sci.electronics
Subject: Re: PC speaker to stereo (update)
Message-ID: <1473@bridge2.ESD.3Com.COM>
Date: 11 Apr 90 01:07:55 GMT
References: <6062@jarthur.Claremont.EDU>
Distribution: na
Organization: 3Com Corp., Mt. View, CA
Lines: 67

rspangle@jarthur.Claremont.EDU (Randy Spangler) writes:

[...]

>Problem: I still get a high whine (mostly >2KHz) even when the speaker isn't
>doing anything.

[...]

>How do I fix this?


I'll bet that the speaker is driven by the output of some
TTL chip (maybe via a capacitor to block the DC component).
If this is not the case (i.e., there is some special
non-digital oscillator making the beep), the following
explanation is not correct and I'd be very surprised :-) :-)

In the real world, digital logic devices are actually analog
circuits that generate variable output voltages even when
they are generating a constant "logic level". That is, even
if a gate is putting out logic "1", its output voltage may
vary + or - depending on variations in the power supply
voltage, the voltage at other gate inputs on the same chip,
and the load on any of the chip outputs.

In your PC, the load current on the power supply is a very
complex function with high frequency components. The
variations are caused by chip enables/disables, memory
accesses, etc. These load changes translate into variations
in the power supply voltage at your speaker driver chip (not
to mention everywhere else in the computer) and show up as
noise on the output.

In general, the variations in output voltage are small, so
that digital inputs in the computer ignore them. However,
your setup involves a linear (we hope) amplifier and your
(relatively sensitive) ears, so the noise becomes
noticeable.

To remove the noise, you need to remove the small amplitude
variations but preserve the basic square wave. This process
is called "limiting", and is used in FM receivers to remove
amplitude variations in radio signals.  A limiter is just a
very high-gain amplifier that gets driven far into
saturation by its input signal. It is usually followed by a
filter to remove higher frequencies so that the output is
close to a sine wave, but in your case that's probably not
needed. You might try:

1. A TTL or CMOS buffer (inverting or non-inverting) with a power
   supply separate from the computer's.

2. An op-amp operating open-loop as a comparator with a reference
   voltage between Voh and Vol (again, with a separate power supply).

Of course, these should be inserted _before_ any series
capacitor at the output of the speaker driver in your PC.

Hope this helps,

 ~!paul
-- 
Paul Ziemba     zapi!gpz   gpz@3com.com   (415)940-7671

Current nemesis: CA "winters", cold enough to drive the ants indoors but
		 not cold enough to make them hibernate.