[sdiy] Adding Tap Tempo to an LFO

[Don Tillman]

   > From: "drheqx" <drheqx at heqx.com>
   > Date: Mon, 18 Jul 2005 10:26:39 -0400
   > 
   > I am having trouble visualizing the method for adding tap tempo to an
   > LFO. I really love that feature on my Lexicon processor. While playing
   > live you just hit the tap tempo button a few times in synch with the
   > drummer. I do it just before I need the delay effect and like magic it
   > locks the delay to the beat.
   > 
   > So if I wanted to add that feature to an LFO where would I start?

That's a very neat idea for an LFO.

Use a linear ramp to measure the time between taps.  Some logic starts
the ramp at the first tap and S/H's the ramp voltage and resets it on
subsequent taps.  So this delivers a linear voltage proportional to
the time between taps; perhaps 1V/sec.

Take the VCO's 2-transistor-and-opamp exponential converter and modify
it to perform a reciprical function.  (I guess you wouldn't want
exponential voltage control and tap control at the same time,
interracting.)  An inverting opamp and first transistor convert the
tap time voltage into a negative log voltage, and the second
transistor turns that into the VCO tuning current.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com