[sdiy] Configurable Phase Shift Project

[Michael Bacich]

On Mar 11, 2006, at 11:19 AM, <scottnoanh at peoplepc.com> wrote:
The higher value of capacitance always brought a certain amount of pitch
bend on the lower end of the sweep.  I started wondering if that was
because, as the VTL5C3/2 goes higher in resistance, the tolerance of
resistance begins to vary more greatly from device to device.  Taking  
that
into account, along with a suggestion from a list member, I lowered  
the LDR
parallel resistance from 1.5M to 1M, and that seems to have improved  
things
on the lower end quite a bit.

With both phasers and flangers, there is always more apparent pitch  
bend at the low end of the modulation.  This becomes more apparent at  
faster modulation rates.  Adding lots of juicy regeneration/feedback  
seems to make this worse, unfortunately.  You can always minimize  
this unwanted pitch shifting by reducing the overall modulation  
depth, but where's the fun in that?  We all love deeeeeeep phasing  
and flanging modulation.

There are two ways that I can think of to keep the modulation fat and  
deep, but reduce the unwanted pitch bending:

1.  Use an LFO that changes its frequency (and hence, its shape) as  
it goes through its cycle.  Craig Anderton famously did this on his  
PAIA Hyperflange design.  He used a CEM3340 VCO for his LFO, and he  
fed a little bit of the LFO's triangle wave output back into the  
3340's modulation CV input, so that the LFO would speed up during the  
triangles peak, then slow down at the low part of its cycle.  He had  
a special name for this type of LFO -- a "hyper-triangle", or  
something like that.  The end result was that you could use much  
greater modulation depths on the flanger, because the relative LFO  
speeds were better matched for the high and low frequency ranges of  
the flanger's comb-filtering.  Another way of describing this is that  
the modulation intentionally moves more slowly while it is working on  
the frequency range that provides the most dramatic "action" -- which  
is very useful with a deep, resonant flange effect, especially at  
very sloooooooow LFO rates.  I'm sure that this method would work  
just as well with a phaser.  One possible down side to this method is  
that the up and down modulation sound will have a kind of lop-sided  
quality to it -- this may or may not sound good at certain LFO rates.

2.  Create an LFO scheme that would allow your modulation depth to  
change as the LFO goes up and down.  That could be done by using the  
raw triangle LFO itself to modulate a VCA/attenuator (such as a  
LM3080).  Run the LFO's output through the VCA, which would, in turn,  
output a triangle wave whose amplitude varies in direct proportion to  
its relative phase (that is, higher amplitude when the wave is  
peaking, and lower amplitude when the wave is low).  Use this self- 
amplitude-modulated LFO to modulate your phaser.  This should also  
help minimize the pitch-shifting, still allowing fairly deep apparent  
modulation depths.  The effect would be similar to Anderton's idea,  
but with a more triangular overall modulation shape, which might  
sound less lop-sided at certain LFO rates (particularly at faster  
rates).

I have tried neither of these methods using actual LFO's, but I have  
simulated their effects by manually adjusting LFO speed and depth  
while the LFO cycles, and both methods seem to work pretty well.  My  
apologies if I have described the actual electronic methods used not  
so accurately -- please refer to the Craig Anderton Hyperflange  
schematic for better details.

Michael B.


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