Re: Q: Applied Engineering Sound Card

[Eric Jacobs <no@no.no>]

MJMahon wrote:
 <snip>
> 
> There ain't no such thing as a free lunch--and you can't do better
> than the Nyquist frequency!  A lowpass simply acknowledges
> reality and makes for a proper design.
> 
> -michael
> 
>  Email:  mjmahon@aol.com
>  Home page:  http://members.aol.com/MJMahon/

Well said. And it might also be worthy to note that even if you don't
have the low-pass filter, putting that signal into an audio amp and a
speaker is definitely going to cut out those high frequencies. The low
pass filter simply makes for the most accurate waveform on a 'scope. It
does very little to the audible quality of the sample.

I think much of this confusion results from the ambiguity of kHz, which
can either refer to "thousands of samples per second" (sampling rate) or 
"thousands of cycles per second" (which is frequency.) You can't sample
a 22 kHz frequency, at 22 thousand samples per second. You're asking to
represent this:

| _____
|/     \
|-------\-------/--
|        \_____/
|
 ^   one cycle  ^

in one digital sample! It doesn't make sense to put a whole cycle of
anything into one digital number, whether it's an 8-bit or a 16-bit.
You'll just get zero.

You'll have more success if you try to sample that frequency at 44 kHz.
Then you'll get two samples per cycle. This will at least give -some-
variation in the sample. You'll get +1,-1,+1,-1,+1,-1,.. etc. This is
of course, a square wave, which is a very poor approximation for the
sine wave it is intended to represent.

Unless you add the low pass filter. The low pass filter will cut
out those excess harmonics above 22 kHz, transforming the square wave
output from the DAC into a (relatively) smooth sine curve.

The bottom line is this: in digital sampling and playback, any output
frequency above 1/2 the sampling rate is distortion and needs to be
filtered out for the most accurate result. In this example, the
sampling rate was 44 kHz and any frequencies above 22 kHz are
extraneous, introduced by the digital sampling process.

Even the 26 kHz low pass filter in the GS may be bit excessive. Humans
can't hear anything above about 20 kHz or so. The whine you sometimes
hear from TV's scan circuitry is 15.7 kHz. Most of the useful audio we
hear is way, way below that.

So the 26 kHz low pass filter on the GS motherboard wasn't put there to
lower the sound quality. It's merely part of proper digital sound
design. Other computers can get away without the filter because they
expect the audio will be fed into an audio amp or mixer, which will cut
off the high frequencies anyway.

Or maybe they were just engineered by aliens from outer space who
have exceptionally high frequency vocal cords.

:)

As a human, I find the GS serves me well. As for those OTHER machines,
.. well..


-ej