Harald Bode Barberpole Phaser Questions

[Don Tillman]

   Date: Thu, 15 Jan 1998 22:14:55 -0800 (PST)
   From: Sean Costello <costello at costello.seanet.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? 

Juergen will probably have some useful words here, but in the
meantime... 

The easiest approach is to just copy the MXR Phase 90 (or any other
known circuit) from Leper's schematics.

If I were going to do it from scratch and be completely uptown about
it I'd use a UniVibe style phase shifter, mostly because there are
fewer parts in the signal path.

   Should the values be the same for each op-amp stage, or should they
   be staggered in frequency?

I tried both many years ago (around 1975) and thought that the
difference was very subtle but that the effect was a little more
pronounced when all the stages were tuned the same.  I should try that
experiment again with my older, wiser and more experienced ears.
(Surprisingly I can still hear high frequencies just fine!)

   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? 

What's the effect of a less accurate quadrature filter?  It means that
some falling barberpole phasing will be mixed in with your rising
barberpole phasing.  That's not really a bad thing, probably not very
audible.

The accuracy of the quadrature filter is more important with
audio-frequency frequency shifting, because having a little shift-down
mixed in with your shift-up can be a problem.

Wait a second...  What if it was a *completely* inaccurate quadrature
filter?  Well, that means you'd have both up-shift and down-shift
mixed together.  But that's just a simple ring modulator.  If you
replaced the frequency shifter with a simple ring modulator you'd
effectively have a rising barberpole mixed with a falling barberpole.
That might be nice effect.  Sure it's cooler to have rising and falling
barberpoles as a stereo pair, but if you're on a budget this might not
be a bad thing.  Hey Juergen, you're set up for this experiment right
now...

   How about substituting a manually adjustable quadrature
   function generator for the quadrature through-zero VCO in the Bode
   patent?

I think this would be fine.  You might even like it better.

   And how about using cheaper OTA's for the multipliers? 

Better to use 1496 modulators, and overdrive them just a bit with a
triangle wave LFO to do your triangle-to-sine shaping.

   If you keep the quadrature oscillator frequency below the audio
   range, would a squelch circuit be needed?  

Nope.

  -- Don