[sdiy] Buchla 266 Quantized Random Voltage

Martin Fay martin at martinfay.com
Sat Dec 14 12:41:02 CET 2013


Hi Pete,

Does sound like the feedback is missing as values are there for about  
the expected length of the shift register. This is from memory right  
now but I think the feedback is a couple of taps taken from the  
register and XORed. I've only cloned this in code, but you could probe  
the in of the first register chip?

Martin

Sent from my iPhone

On 14 Dec 2013, at 06:17, Pete Hartman <pete.hartman at gmail.com> wrote:

> I'm troubleshooting one of Roman's 266 clones, and while I understand
> what the QRV is supposed to DO, I don't yet grasp entirely HOW it is
> supposed to do it.
>
> The behavior I see is that on the start of my clock, I get quantized
> voltages, correctly varying among a number of steps based on the input
> pot, for 20 clocks.  After that I get nothing, for as long as I've
> been able to observe.
>
> I understand that we're looking at a shift register.  I've read Dave
> Brown's discussion of the startup voltage to start things off with all
> 1's....what I suspect is happening is that after those ones are
> shifted out, I'm only getting zeros in.  But I'm not grasping (at
> midnight anyway ;-) ) how I might test that hypothesis, nor how it is
> that it should be getting things that aren't zeroes shifted back in.
>
> I've googled a couple of things trying to find something discussing it
> in step by step detail, but found nothing.  I scanned Bernie's
> Electronotes article about random voltages from issue #64, but nothing
> jumped out at me as matching the topology (though I do see he talks
> about shift registers, so when I'm more awake I will try to read it
> more closely).
>
> But if anyone knows of a resource where this is discussed in more
> detail, I would very much like to read it.
>
> Thanks!
>
> Pete
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