From: utzoo!decvax!harpo!seismo!hao!hplabs!sri-unix!VaughanW@HI-MULTICS
Newsgroups: net.physics
Title: re: Digital vs. the Audiophile
Article-I.D.: sri-unix.4934
Posted: Sat Dec 25 13:20:43 1982
Received: Mon Dec 27 03:29:04 1982

From:  VaughanW at HI-MULTICS (Bill Vaughan)
Though I can't hear the difference between .01 and .001 percent
THD (or I don't think I can), I would be careful about saying
people are hallucinating.  There are several scenarios I can think
of which would explain why one might be able to hear low levels of
distortion through a filter having more distortion.

1) Suppose the coloration of the amplifier's distortion differs
significantly from that of the speaker?  e.g. the amplifier has
more odd harmonic distortion at certain frequencies, the speaker
has more even harmonic distortion at other frequencies, etc.  Odd
harmonic distortion is easier to hear than even HD (or so I've
read somewhere).  This now begins to sound like a classical
problem in signal processing, i.e retrieving a weak signal (the
distortion of the amplifier) from strong interference (the
distortion of the speakers).

But that's unlikely, because the amp. and speakers are likely to
be used together a lot & you wouldn't be able to tell what
coloration came from what box -- unless you were doing an A-B test
in an audio showroom (the best way to buy a system anyway).

2) Maybe the reference was to intermodulation distortion rather
than THD.  I wouldn't be surprised to find that people can hear
the difference between .01% and .001% IMD.  IMD is particularly
annoying - flutter on a turntable or tape machine is an example.

3) Don't forget that you can see colors through colored
sunglasses.  The brain is a far far better signal processor than
any test equipment yet invented.  The problem lies in telling just
what it is processing and how.

4) We tend to measure the characteristics of audio equipment under
steady-state conditions, though what we listen to through the
equipment is far from steady-state.  (Kind of like looking for a
lost coin under a streetlamp on the wrong side of the street.)  A
box that has very good steady-state specifications may have very
poor transient response.  Maybe the .001% and .01% figures are
both wrong under the actual conditions.
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