[sdiy] Unusual BP filter - Voltage Control

[david wright]

hello JH and all - some intresting ideas coming together. 

i think i'm going to end up hand matching
some LDRs as the cheapest way to go about this.
but i got a crazy idea from paul perry (hi paul!)
about using a PIC to do PWW - what do you folks
think of this? I think that might be too high tech for
this project as i'm trying to keep that 1950's 'coffee n cigerettes'
sound - i do have a PIC burner though. 

i have several of the electroluminence 'night lights' in my house.
maybe this would be a good 'long light strip' for the ldrs? that
or the light of the 15 tubes inside ;) (jk)

thanks for your response

here's what paul said:

"The idea with a 'switched resistor' is this:
if you have a 10K resistor with a switch in series with it,
and the switch is "on" (conducting) half the time, then
the 'average' resistance is 20K (because, only half as
much current gets thru as if it were a 10K resistor on all the time.

And if the switch was on 25% of the time, you get the effect
of a 40K resistor and so on.

Now, if you switch the reistor on & off much faster than audio frequencies
(say 50KHz)
then, so far as any audio signal is concerned, the resistor looks exactly
like a
20K or 40K or whatever.

The business of making the control voltage generate the appropriate
pulses is called PWM (pulse width modulation).
Which you get by (typically) running a triangle wave and a control
voltage to the 2 imputs of an analog comparator.

There is a bit on this hidden away in geofex, where he hints at fun things
to do with PIC
microprocessors."

> No better idea. (Although I'd live to see a VCF built from
> remote cutoff tubes connected a variable gm differential
> amps - but *not* in this circuit. (;->) )

me to! if someone designs one - they will come. 
 

> (See http://www.oldcrows.net/~jhaible/opto_compressor/jh_opto_comp.html

amazing again


dave wright