From: bill@bilver.uucp (Bill Vermillion)
Subject: Re: polarity inversion
Organization: W. J. Vermillion - Winter Park, FL
Date: Fri, 1 Jan 1993 14:45:01 GMT

In article <shetline-311292015736@128.89.19.85> shetline@bbn.com (Kerry Shetline) writes:
>In article <92364.114116U37426@uicvm.uic.edu>, U37426@uicvm.uic.edu
>writes...

>>   Just how does polarity inversion affect the sound of a stereo?  I have
>>never had a system that had this feature.  I have no idea what it does.
>>Could somebody explain?  thanks!

>Sound waves in air consist of radiating patterns of compression and
>rarefaction (increases and decreases in pressure).   .....

>Well, so what? If one channel in a stereo pair is in phase and the other is
>not, it makes a big difference. The two speakers fight each other and end
>up canceling out much of the sound. But if both match...

It won't cancel all frequencies, just the ones that are of the
wave-length between the speakers or listening position.

>With the way many recordings are made, only minimally-miked classical
>recordings are likely to even have recoverable absolute phase anyway. With
>something that has been multi-miked, multi-tracked, mixed, edited, and
>recorded in separate sessions with different setups, absolute phase is
>completely jumbled and mixed with inverted phase -- different for different
>voices and instruments, on different tracks or parts within tracks.

It even boils down to the way the studios are put together.  Too many
studios don't take proper care in design, layout, and construction.
When we put the studio up that I was CE for many years ago - we tested
absolute at every point of the way.

We had a device that we plugged into the mike-pre and hit a button.  A
quick 'pop' and then an LED lit up showing phase one way or the other.
We verified every single mike line back to the console - there were
many of them.   Then we verifed every mike cable.  Then we verified the
signal path through every piece of external peripheral equipment.

And about every 6 months or so we verified mike performance by setting
a pair of indentical mikes up, inverting phase on one, and listen to
what did not cancel.    Did this with the Neumann, AKG & Sony
condensors, the dynamics normally didn't match that well.  Odd man out
went back to be refurbished.  Running a studio where everything is
'right' isn't cheap!

Many people just plug and go.   We found many pieces of 'pro' gear that
had phase inversion.  Our multi-track inverted phase in sync mode but
not in normal play.   That could be a problem for ping-pong, but we
usually had enough tracks.   (Our staff producer really looked forward
to that, as we constantly had to ping on the 16 track machine.  He was
incredulous during our first big 32-track session when I said "George,
we don't have any tracks".  He was so intent on miking as much stuff in
true stereo as possible we ran out in the second day of tracking!)

Of course anytime you use any analog EQ units you are going to get some
phase shifts too.   And before digital, it was not uncommon to see
phase shift (group delay) in the upper ends of the tape recorders.


>So, except for the stereo channel mismatch mentioned above, absolute phase
>comes down to (IMHO):

>1) YOU probably can't hear it.
>2) Maybe no one can hear it.
>3) If you can hear it, you probably won't care.
>4) If you can hear it, and you do care, you often can't get it.
>5) If you can hear it, you do care, and you can get it, you may not know
>when you've got it. (Do I push the little button, or don't I?)

Agree on 1-4, and on 5 it's easy.  Push the button until it sound right
to you!

>There are, of course, people who make extravagant claims as to the enormous
>effect of polarity. Some person on this news group recently spoke of CDs
>that went from unlistenable to wondrous with the touch of a polarity
>switch. The answer to this phenomenon is not likely to be found in audio
>literature, but somewhere is DSM III...

One man's "unlistenable" is another's "sounds fine to me".

And there have been times when I have gotten out of an elevator a few
floors early because the wow&flutter in the system affected me too much.


-- 
Bill Vermillion - bill@bilver.oau.org  bill@bilver.uucp
                - ..!{peora|tous|tarpit}!bilver!bill