[sdiy] 20ms delay? analog allpass filter?

[Tom Wiltshire]

I had a go at this problem using two DG406 1-to-16 switches and an AS3360 dual VCA to provide crossfading from one signal to the next. The relevant 4-bit addresses and two VCA CVs were provided by a PIC. For the delay, I used the LC delay line that JH designed for his Hammond Scanner project.

It worked, but there was a notable volume modulation at the scanning frequency because I didn't get the crossfade to be equal power. The CVs were simple linear ramps, but there needs to be another lookup table stuck in-between so you get a sin/cos cross fade to make it smoother. At least, I *think* that's where the problem came from.

Tom


> On 8 Feb 2023, at 12:57, René Schmitz <synth at schmitzbits.de> wrote:
> 
> HI David,
> 
> On 08.02.2023 00:12, David G Dixon wrote:
>> That's cool, but why not use a 4-bit counter and a 16-channel digital
>> switch?
>> 
> Because that would be hard switching, while the scanner does a crossfade
> 
> (plus some high pass action) as the rotor plates move over the stator plates.
> 
> The whole thing is a complex variable capacitor.
> 
> 
> Best,
> 
>  René
> 
> 
> --
> synth at schmitzbits.de
> http://schmitzbits.de
> 
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