Re: Low pass filters revisited

["Jason Whorton" <jason at microxl.com>]

Here's a note to remember.  Odd-order filters are at -3db at the crossover
frequency and even-order filters are at -6db at the crossover frequency.
    I can't hear Mac above 18K, so I am in the clear.

Thanks,
Jason Whorton
jason at microxl.com


"Dennis Doms" <SynABit@everest.invalid> wrote in message
3dceb367_5@corp-goliath.newsgroups.com">news:3dceb367_5@corp-goliath.newsgroups.com...
> Supertimer wrote:
>
> > Some years ago, some newsgroup readers made
> > a big fuss that there was a low pass filter on the
> > Apple IIGS.  The IIGS has a low pass filter at 26kHz
> > frequencies.  What those people kept arguing was
> > that this halved the sampling rate compared to CD.
> > This is total nonsense.  The maximum frequency
> > that can be accurately captured by a given rate of
> > sampling is half the sampling rate.  This is denoted
> > by the Nyquist theorem.
> >
> > Fact is, practically all digital audio equipment has
> > low pass filters.  CD players have them and PC
> > sound cards have them.
> >
> > Take a look at Tom's Hardware's review of the new
> > Audigy2 card from Creative.  It specifically points
> > out a low pass filter.
> >
> > I'm just thinking how funny it was that people were
> > blaming Apple for putting in the filter when in fact
> > there's nothing to blame.  Without such a filter all
> > you get is extra noise above 1/2 the sample rate.
> > I even recall that someone came out with a IIGS
> > emulator and promoted it saying that it lacked the
> > low pass filter.  Shouldn't the Mac's audio out
> > have one too?  It probably does.
>
> I missed the earlier part of this discussion, but my $0.02...
>
> Assuming a sampling rate of N, you have to have an analog filter that
> removes any frequency over N/2, the Nyquist criteria you refer to. It's
> not a matter of noise; what happens is that any frequency above the
> Nyquist threshold that that reaches the digitizer will be "folded" to a
> different frequency when reproduced. Best explanation of this I have
> seen is in Hal Chamberlin's _Musical Applications of Microprocessors_,
> in my mind a superb book for anyone really interested in the subject.
> (Chamberlin also built a wavetable synth card for the original Apple,
> maybe the first such consumer computer-based synth. Unfortunately I got
> rid of mine years ago.)
>
> CD sampling frequency is set a bit above 44KHz so the Nyquist cutoff is
> 22KHz. People can argue (and have argued) about whether this is
> suffcient, but it's what the industry is locked into. So the input
> filter should cut any input off below this, and more bandwidth on
> output shouldn't matter (if you sampled properly at 44KHz, anything
> about 22Kz on out put has to be noise or distortion.
>
> Another problem is that an analog filter isn't going to cut off sharply
> at the defined frequency; it's actually going to effect a "rolloff"
> (hopefully rapidly) toward that frequency. It's just the physics of how
> things work. On the input side (analog-to-digital) any signal above the
> Nyquist cutoff is a potentially severe source of distortion, so the
> rolloff may have to sacrifice some accuracy below the cutoff to avoid
> that (some people, including me, think CDs should have been
> standardized at a higher sampling frequency; they probably went with an
> assumption that most people don't hear well above 10-12K which I also
> disagree with). On the output side (digital back to analog), 26K may be
> intended to let the entire digitzed signal knowing they may be allowing
> a little "noise" through in the 22K-26K range. (That is, they're
> allowing the filter a little "headroom" so the rolloff doesn't affect
> the high frequencies of the original signal as much; this is safe
> because folding isn't involved in this direction.)
>
> --
> Dennis Doms SynABit@Everest.invalid
> [ObSpamFoil: replace ".invalid" with "kc.net" to send real mail.]
> "We aren't here to rule. We aren't here to bring chaos or anarchy.
>  We're here to end the reign of criminals." - Batman, _The Dark Knight_
>
>
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