From: bill@bilver.uucp (Bill Vermillion)
Subject: Re: polarity inversion
Organization: W. J. Vermillion - Winter Park, FL
Date: Tue, 5 Jan 1993 04:08:31 GMT

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!


Just something I happened to think of today after remembering
the polarity inversion talks.

It's all well and good to match polarity along the line - but
in many cases it won't make much difference.

"Why?" you ask.

Simple.   In the real world there are a good many signals that
are not symetric.    These signals can have a different sound
depending on polarity.

In the early days of broadcast - I'm talking the 1930's era,
ribbon mikes were quite often THE mike in the studios.

And many stations had a mike polarity reversal switch in the
mike line, or mike pre-amp line.

"Why?" you ask.

Well simple.  Voices are not symetric.  In the early days,
before such items as the 'Symetra-Peak' and it's ilk, - devices
that made peaks symetrical, or others that force more positive
peaks than negative to get more modulation of the carrier, the
engineers would check voices and set the polarity of mike so
that on non-symetric items the positive peaks were greater than
the negative peaks.

And what happened when you had a discussion?  Well you test all
the speakers, grouped them into two groups one with predominant
positive peaks in their voices, and the other with negative
peaks in their voice.

You set the ones with positive peaks facing the front of the
mike, and those with negative peaks on the back of the mike
facing the others.  Voila!  All voice now have predominant
positive peaks.

Fast forward a few years. 

jasdf#as%9f98^2e*709asdf 098   (that's the sound a tape makes
when it is fast forwarded :-) ).

I cut a session with a screamin' trumpet.  (I kid you not - if
it wasn't below high-C and triple forzando - it wasn't worth
playing  :-)/2  Ex lead trumpter for Maynard Ferguson's band).

The tape were sent to one of the generic custom LP plants
(against my recommendation to the producer), and they tried and
tried to cut a master, but they all sounded poor - had a
'ripping' sound when the trumpet was cutting loose.

So I had them sent to a mastering engineer I really trusted.
He called the next day, and had the same problem - but he
pinponted for me.

Two items - there was, as John put it, "significant energy at
and above 17KHz" and it was truly non-symetric.

So I got out the master, and the scope and isolated the trumpet
track.   I was running 80% positive peaks in the 5k - 9k range
during the solos.     Just try to picture what that would be
doing to a record groove.

Remix time.   To cure this one, without doing damage to the
rest of the tracks, I band-passed the trouble-some range - the
5k - 9k range.   Took this signal in through a separate route
into the console, inverted the phase, and brought the signal up
in that range enough to give me a useable signal for LP,
without destroying the sound of the trumpet.  Then I ganged
that fader to the master trumpet fader, and remixed it.
Problem solved.

So when it comes to absolute phase, I'd say that in the
many cases, depending on source and mikes, you won't hear any
differnce.  And in many other cases, you will hear differences
and some instruments will sound good with phase one direction,
and other with the phase reversed.   This should also be
noticeable in such things as MS miking, or other Blumlein
techniques if any of the instruments native peak modes differ
from the others.

The above trumpet problem would only give problems mastering an
LP - because of the physical limitations.  Never heard the
problems on the master, or reference tapes.   And it wouldn't
show up in CD's either.     

In the studio it seems to always be trading off something you
want for something you need.

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