[sdiy] Unusual BP filter - Voltage Control

[harrybissell]

It depends on how fast you like to modulate the center frequency

Vactols are good for slower motion, matching is tough but once you do that
you could pass anything through it.

EL panels could have higher speed, otoh harder to drive.  I'd think whis would

be second choice.

PWM of analog switches would (imho) change the character of the filter a lot.
For a LPF maybe, for a BPF I'd avoid it.

Now an interesting idea would be to use the PWM to drive the LEDs in the
Vactrols... have 8 separate PWM outputs and a linearization table for each
Vactrol. Make the matching REALLY good.   The Vactrol slowness would
probably filter the PWM WAY WAY better than the analog switch method

H^) harry

david wright wrote:

> 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