Harald Bode Barberpole Phaser Questions

[Sean Costello]

Hi all you synth type techies out there:

I have a few questions regarding the recent discussion of the Harald Bode
patent for the Shepard Tone/Barberpole/Infinite/Motorized/Always Going Up
(Or Down) phaser.  For all who are interested, this is US Patent No.
4,399,326, and can be viewed online at http://www.patents.ibm.com/.

1) Anybody have any ideas for the construction of the all-pass filter that
is used in series (or in parallel) with the dome filters in the frequency
shifter? The allpass filter is used to add notches to the phase shift effect
in the audible areas of the spectrum, around 300 to 1500 Hz.  Any idea what
resistor and capacitor values should be used in the circuit?  Should the
values be the same for each op-amp stage, or should they be staggered in
frequency?

2) Is it necessary to have a fully functioning, lab-quality frequency
shifter for this effect, or is it possible to cut corners if all you want is
a barberpole phasing effect?  Do you need such a complicated setup for the
dome filter, or could you construct a dome filter with less stages and less
precise components? How about substituting a manually adjustable quadrature
function generator for the quadrature through-zero VCO in the Bode patent?
And how about using cheaper OTA's for the multipliers?  If you keep the
quadrature oscillator frequency below the audio range, would a squelch
circuit be needed?  Is the precise nature of the circuit essential to having
the small shift amounts needed?

Obviously the Bode design is great, if you want a fully-functional frequency
shifter in addition to a phasing effect.  However, it might be nice to have
a less complicated circuit that was more like a traditional phaser (i.e.
controlled by a manual oscillator, with a frequency range from 0.05 Hz to 20
Hz or so), but with the barberpole effect.  It would make a great stompbox,
if you could make it in such a way as to price it competitively with the
boutique effects out there (the Lovetone gear, the Univibe clones).  I
really doubt that you could make the design as it stands, and expect to sell
it for less than $1000 (which is what the Serge Frequency Shifter sells for,
as an example).

Any and all advice welcome.  I'd love to own such a device.  It seems much
easier to build than my Motorized Barberpole Phaser idea - although it won't
look as cool. :)

Sean Costello

P.S.  I'd like to build a Frequency Shifter, too.  I just wouldn't feel too
good about having a frequency shifter in a stompbox - that's a lot of
precise circuitry to be stepping on.  A cheaper, easier circuit would be nice.

P.P.S.  I love the term "Klangumwandler" for Frequency Shifter.  Is there a
similarly cool German word for the Barberpole Phaser?