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From: joe@cornell.UUCP (Joseph Bates)
Newsgroups: net.audio
Subject: to demolish: phase shift argument
Message-ID: <1545@cornell.UUCP>
Date: Mon, 6-May-85 15:16:29 EDT
Article-I.D.: cornell.1545
Posted: Mon May  6 15:16:29 1985
Date-Received: Fri, 10-May-85 08:29:58 EDT
Distribution: net
Organization: Cornell Univ. CS Dept.
Lines: 31

CD players with sharp anti-aliasing filters exhibit a frequency dependent
phase shift of hundreds of degrees at high frequencies.  This has been
suggested as a cause of harshness in their reproduction.  The argument
below raises doubt as to the significance of such phase shifts.  I would
like to learn where (if) the argument fails.

Consider a conventional speaker with the tweeter roughly one foot above
the woofer.  Suppose one is sitting with the woofer at ear level and that
in this situation the music is being reproduced "correctly", that is the
signals possess the proper phase relationship.  Suppose further that one
is roughly seven feet from the speaker.  [The details here are not critical.]

Suppose now that one raises one's head so that it is at the level of the 
tweeter.  Sound from the tweeter reaches the ear somewhat sooner than before
(since the tweeter is a bit closer), while sound from the woofer arrives a
little later.  By my calculations the tweeter is about an inch closer, the 
woofer an inch further.  The cumulative change of two inches is about the
length of one sound wave at 6KHz.  At 12Khz it is two waves, at 18KHz it is
three waves.  Thus, the improper positioning of one's head (not its motion,
but its improper positioning) appears to introduce a frequency dependent
phase shift of many hundreds of degrees at high frequencies.

This argument suggest that the vertical positioning of one's head is critical
to within inches to avoid a frequency dependent phase shift from the "correct"
signal, for otherwise harshness will result.  I have not heard of such a
problem.

I await enlightenment.

joe bates  (joe@cornell.arpa)