[sdiy] Idea for PT2399 chorus

[Tom Wiltshire]

Hi all,

I was reading about Scott Swartz' PT80 delay pedal for guitars last  
night:
	http://www.generalguitargadgets.com/index.php? 
option=com_content&task=view&id=125&Itemid=26

I was also finding out about the various ways of adding voltage  
control to the PT2399 delay chip on which it is based. The delay time  
is set by a resistance on pin 6 which sinks current from the pin and  
sets the clock VCO speed. There are three basic methods;

1) Current sink
	http://www.homebuilthardware.com/index.php/projects/pt239x-delay/
2) FET used as variable resistance.
3) Vactrol
	http://home.comcast.net/~sbernardi/elec/og2/og3_echo.html

The trouble with Scott Bernardi's vactrol solution is that it uses a  
current source to control the brightness of the vactrols LED, so in  
fact it finishes up being more complicated than the current sink on  
its own. Also the output of an LED isn't that linear when fed from a  
current source.

Then it occurred to me - why not use PWM to control the LED  
brightness? The vactrols LDR response is slow enough that it would  
provide PWM filtering, and PWM provides a much more linear output  
from the LED. Seeing as I already have a PIC chip which acts as an  
LFO and produces a PWM output, I thought I could easily knock  
together a quick chorus.

I'll let you know how I get on, but it seems like a promising idea.  
I'm also keen to try with a slightly larger chip. For instance, the  
16F767 has three separate PWM outputs, so you could program up a  
triple chorus, and either have independent LFOs for each delay, or  
phase-shifted delays like in many string synths. Since all the clever  
bit (the LFOs) is then done in software, the actual circuit would  
amount to one PIC, three vactrols and three delay chips with  
associated Rs and Cs.

T.