Motorized Barberpole Phaser (was Re: Another kind of Phasing)

[Sean Costello]

At 04:25 PM 1/13/98 -0800, you wrote:

>   "Motorized" phase shifter?  Could you explain this term?  I mean,
>   is there a device that produces "barber pole" phasing that actually
>   USES some little motor, chugging away?  If so, do you have any more
>   details?
>
>Sorry, I was being cute.  By "motorized" I simply meant that instead
>of a control voltage controlling the amount phase shift, that a
>control voltage would control the rate of change of phase shift, and
>that the phase shift could increase continuously, as if there were a
>motor in between.  (Except for the fact that there's no actual motor,
>I think the term is very descriptive.)
>
>A barber pole phase shifter is where you have two or three parallel
>phase shift stages, a multi-phase LFO controlling them and an
>electronic fader fading between them all to make a phase shifter that
>would appear to go up (or down) forever.  The notches travel up in
>pitch, and as they get higher they fade out and new notches fade in at
>low pitches.
>
>I don't know if anybody has actually built one of these beasts.  I've
>always wanted to, it's been one of my dream projects for a while, but
>I've never had the time.

Harold Bode made one.  I have no further details; I read a little blurb
about it in some tall, thin and expensive paperback at the Stanford
Bookstore.  I wish I had bought that book now, just for that scrap of
information.  I can't find much on the Internet about Harold Bode.  I need
to go to the UW Engineering Library, and do a search on him.

As far as a barber pole phase shifter, the only way I have heard of is to
have a Shepard tone generator (a tricky circuit, that) controlling 8 phase
shifters and 8 VCA's.  The Shepard tone generator creates 8 triangle
functions that are 45 degrees out of phase with each other (well, you know,
0, 45, 90, 135, 180, and so on).  It seems like a pretty complex circuit.  I
was thinking about building a circuit someday (in the future) where there
are 8 4-stage phase shifters; each 4-stage section could conceivably be
built from a single LM3900.  It would be nice to have the capability of
using the 8 phasers, not only in a Shepard generator, but also in a version
of the Electronotes Waveform Animator that uses 8 different LFO's with 8
phase shifters to create a very rich chorused waveform.

However, the idea of a "motorized phase shifter" got me thinking...

What if you constructed an actual "motorized phase shifter," where the
amount and speed of the phase shifting is actually constructed by a motor?
It would be simple enough to do; a very simple implemenation would use 4
light-dependent resistors and the usual op-amp phase shift stages, where the
LDR's act as the variable resistors in the circuit.  The 4 LDR's would be
placed around a rotating drum, which contains a light source (the LDR's,
drum and light source would be enclosed in something dark, of course).  The
drum would have slits in it, which start off small, grow large, and shrink
down to nothing again, like this: <>  As the drum revolves, the slits move
past the LDR's, letting in increasing then decreasing amounts of light on
the LDR's, which would result in a very gradual increase and decrease of the
resistance on the LDR's.  This would create a phase shifting effect.  I
think that a similar circuit may have been used to create vibrato in some
older organs. Think of it as a cross between a Univibe (the photocells) and
a Leslie (the rotating drum).

Now, what if you extended upon this "motorized phase shifter" so that there
were 8 independent phase shifters (of 4 LDR's each) that were powered off of
the same drum?  And what if you created the drum in such a way as to stagger
the light that hits the LDR's of each phase shifter, so that the modulation
"voltages" of the phase shifters are essentially 45 degrees out of phase
with each other?  

Take the above circuit, and add 8 VCA's, also based around photocells (much
like the Buchla Lowpass Gates).  Have the VCA's controll the outputs of the
8 phasers, in such a way as to get a staggered Shepard tone effect.  The
result is a Motorized Barberpole Phaser!

Of course, there are a few details to be worked out:

- What is the configuration of the cylinder?  How are the slits staggered to
get the proper effect?
- What is the relationship of the phaser LDR's to the VCA LDR's?
- What sort of motor would drive such a monstrosity?
- How would the LDR's be arranged?
- How would you afford the 40 LDR's needed?  Are that many LDR's needed to
get a good phase-shifting effect, or are less needed?

Still, I think it would be a very cool device indeed.  Perhaps the cylinder
could be made out of clear plastic, with actual barber pole-type stripes
painted on to it (or better yet, a transparency could be made, with the
proper stripes pattern, and wrapped around the cylinder).  Trying to figure
out how this device would work is taxing my exhausted brain, so I will stop
now.  

Would this work?  Would it be worth constructing?  I think it would sure be
a cool device to own: some little chugging monstrosity that creates a
beautiful sound.  You could slow down, stop, and reverse the motor, without
any of the strain that comes from trying to design a voltage-controlled
Shepard function generator with through-zero FM.  Besides, it would look
great: some weird little mystery tube that is incorporated into your setup.
It would be great to have the tube visible, and surrounded by rings that
contain the LDR's, with the motor and belt driving the cylinder also out in
the open.  (My vision of this also incorporates lots of brass and walnut, a
la scientific equipment of the late 1800's.)

Any and all advice on this idea is greatly appreciated.  I am also
interested in any and all electro-mechanical sound modifiers that have ever
been used (the Hammond vibrato, the weird Morley/Fender electrostatic oil
drum echoes, Leslies, and so on).

Thanks, 

Sean Costello