Message-Id: <33FBAD28.242E@kih.net>
Date: Wed, 20 Aug 1997 22:51:20 -0400
From: "Michael Biel" <mbiel@kih.net>
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To: vinylphiles@graphics.cornell.edu
Subject: Pitch Shifting  was Re: The future IS VINYL
References: <970818225029_75613.1265_GHT111-2@CompuServe.COM> <33FB93ED.3EE7@worldnet.att.net>
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Robert Cohen wrote:
> 
> Steven R. Rochlin wrote:
> >
> > Bob,
> >
> > >>>A point I've meant to make for a long time.  Many mid-fi turntables
> > from the 70's and 80's had pitch control, or speed control (some hi end
> > TT's have the same).  You can play drums to anything, but not so with
> > piano or guitar (I play guitar).  I used to spend many hours learning
> > new songs by playing along with a record.  You *must* have a pitch
> > control to do that;  CD and cassette cannot be used for that purpose.
> > As good a reason as any to keep vinyl alive.<<<
> >
> >     Well, um, hate to say this but there are quite a few CD players with 'pitch'
> > control.  With DJ CD players, a upper end ($220) Technics, a Dennon...  you
> > actually change the speed BUT the music's frequency pitch doesn't change (that
> > also depends on the  CD player in question).  In other words the song slows down
> > BUT the pitch of the music remains as it was recorded.
> 
> How the hech do they do that?  And, anyway, if you can't change the
> pitch, you still can't play along with the music, so the feature is only
> good if you want to change the speed of dancing, right?

What he describes should be called a "tempo control."  A speed control
would adjust both the tempo and pitch simultaneously.  A pitch shifter
adjusts only the pitch.  
Unfortunately the industry seems to want to call everything a pitch
control even if it isnt.

Once you are in the digital domaine you can change anything.  By adding
some additional cycles to a tone you can raise its pitch.  By skipping
some cycles in a tone you reduce its pitch.  If you speed up a
recording--which raises its pitch--if you reduce the pitch with a pitch
shifter by the same amount, you have a faster recording at the original
pitch.  And vice versa.

Now, let me tell you how the Whirling Dirvish did it in analogue.  The
machine uses a tape with a stationary recording head but the playback is
through a head wheel with four heads that rotate either in the direction
of tape movement or opposite direction of tape movement.  The wheel is
set up so that the tape contacts it for 90 degrees of its
circumference.  So one--and only one--of the four heads is always in
contact with the tape.  

If the head wheel is rotating in the same direction of the tape it will
reduce the pitch because while the tape is recorded at a certain speed
it is being played back at a lower speed.  The RELATIVE SPEED of the
tape vs. the moving head is lower than the relative speed of the tape
vs. the stationary recording head. But since a new head will pick up at
an earlier place than where the old head leaves the tape, it will appear
that there will be little pieces of tape that played twice.  But the
tape and the wheel are moving VERY fast, so these little pieces are
ALMOST unnoticable.  

If the head wheel is rotating against the direction of tape movement,
the pitch will be raised because the relative speed of tape vs. play
head has been raised. The new head will pick up at a later point on the
tape than the old head left and will be cutting out little pieces of
tape as unplayed. 

These extra or missing pieces are not unnoticable.  There are smoothing
circuits which help cover them over, and you can adjust the tape speed
and the head wheel speed to find a frequency of the cuts that won't be
objectionable.  But it is better for speech than for music, and the more
the pitch is varied from normal, the more noticable it is.

Sounds like voodoo, but it works.

Mike Biel  mbiel@kih.net