From: wood@uk.tele.nokia.fi (Peter Wood)
Newsgroups: rec.audio.high-end
Subject: Re: Digital processing of sound
Date: Wed, 11 Nov 1992 10:32:00 GMT
Organization: Nokia Telecommunications, Transmission Product Development,
	Cambridge, UK
  (5.65c/IDA-1.4.4 for <rec-audio-high-end@fuug.fi>); Wed, 11 Nov 1992 14:35:23 +0200
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

>>>>> On 9 Nov 1992 23:59:58 GMT, bill@rsphy1.anu.edu.au (Bill Alford) said:

Bill> Finally I've been able to read the interesting article in the New York
Bill> Times....

Bill> ...mentioned that in the real recording world pre-filtering of the
Bill> signal the cut-off filter for removing anything above 22.05KHz affects
Bill> quite vital overtone information in that it affects the signal below
Bill> 22.05KHz. Does anybody have any further information on this? I've
Bill> expanded out the sentence in the article that mentioned this, but it is
Bill> something that has bothered me for sometime because, for example,
Bill> digitally recorded pianos don't sound quite right (compared to analog
Bill> and the real world) because of their complex overtones.

This one can run and run, but this is how I had it explained to me.  I must
stress that the psycho-acoustic bit at the end is well outside my engineering
field, so I just pass it on...

I think we all agree that digital audio only works at all because a brick
wall filter is inserted at around 22kHz in the recording process.  This is
necessary for Nyquist's results to be valid.  Therefore no frequencies above
this can possibly be recorded.  No problem, say the proponents, you can't
hear above this frequency anyway.

However, we may be able to show that the process of filtering at 22kHz can
inject substantial noise and distortion back down into the audio band.
Consider a tone of 20kHz, being amplitude modulated at 7kHz.  This might be a
_very_ simple representation of a cymbal.  Ignoring the time domain, the
spectrum of this signal (a 20kHz tone regularly varying in amplitude) will
consist of three spikes - one at 20kHz, where we would expect, and one at
each of 20kHz +/- 7kHz, ie 13kHz and 27kHz.

In the ordinary course of events, playing back these three signals will
result in the original signal. This happens in the human sensing mechanism,
because the body is still sensitive to the 27kHz tone, it combines all three
and we 'hear' the original cymbal.

However, pass this signal through a brick wall filter at 22kHz, and we now
have only two tones, one at 20kHz and one at 13kHz.  No matter how well these
are then reproduced by the playing equipment, there is no 27kHz sideband to
combine with the 13kHz sideband to modulate the 20kHz carrier.  The result is
a distorted version of the original 20kHz tone, and a new and completely
unrelated 13kHz tone that wasn't in the original signal!

Proponents of Nyquist (but who haven't studied his work) will say that his
theorem states that all signals up to half the sampling frequency can be
reproduced. This is precisely what's happening in this case, but the
information content of the signals below half the sampling frequency is being
eroded, and noise is being added.

It is this injection of noise back into the audio band that is subjectively
the most objectionable component of digital sound.  The ear is apparently
tolerant of noise and/or distortion, as long as it is a harmonic of the
original signal.  It is extremely sensitive to signals that are not harmonic.
I was told that this is easy enough to demonstrate in a laboratory using
signal generators - when a test tone is playing, harmonic signals (several %)
may be introduced with very little subjective effect, but fractional
percentages of unrelated signals are extremely easy to detect.  The ear is
very good (because it practices every day, in the world around us) at picking
out small sounds from background signals.

Cheers
Pete

--
-----------------------------------------------------------------------------
- Peter Wood,   Nokia Telecommunications,  Transmission Product Development -
-                    Cambridge, UK.  Tel:  +44 223 423 123                  -
- External Email: wood@uk.tele.nokia.fi         Internal Email: NTL02::WOOD -
-----------------------------------------------------------------------------