[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

[David G Dixon]

I guess that's easier than what I was thinking of (encoder to decoder),
although not really more efficient parts-count-wise. 

> -----Original Message-----
> From: Roman Sowa [mailto:modular at go2.pl] 
> Sent: Wednesday, December 12, 2018 1:05 AM
> To: David G Dixon; synth-diy at synth-diy.org
> Subject: Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)
> 
> I'm glad I could help.
> 
> Yes, it must work in dot mode. One way of making dot mode out 
> of comparator chain is this:
> http://www.synthdiy.eu/files/dotmode.png
> with different wiring there's 1 of 8 dot mode possible or the 
> one that does what's needed for crossfader based scanner - 2 
> of 8 dot mode.
> 
> Roman
> 
> W dniu 2018-12-12 o 03:53, David G Dixon pisze:
> > Roman,
> > You're right.  Your suggestion is superior.  I was simply 
> unaware of 
> > what a complete shit-show all this digital circuitry is. This is 
> > exactly the reason why I avoid digital solutions whenever 
> possible.  
> > This module should be a pretty straightforward application of an 
> > analog multiplexer, but you are basically saying (and I 
> concur based 
> > on my experience) that I can't use such a thing for clean 
> audio, and 
> > to me, that is a sad indictment of these ICs.  I'm now 
> switching these 
> > DG408s about as cleanly as is humanly possible, and they're still 
> > misbehaving.  It just shouldn't be this hard.
> > So, I'm going to try to implement your idea, but I don't 
> have the ICs 
> > I need.  I'm also still trying to understand exactly how 
> the switching 
> > works.  I'm presuming that the 3914 must be in Dot mode, 
> because only 
> > one comparator can be on at a time for this to work, and therefore, 
> > this circuit ONLY works with 3914, and cannot be implemented with a 
> > generic flash comparator chain.
> > So, I'm going to finish off the circuit as it is, because it works 
> > pretty well, but I concede that I basically cannot use 
> these fucked up 
> > digital chips to switch active audio circuits, and will 
> build the next 
> > one your way.
> > Cheers,
> > Dave
> > 
> ----------------------------------------------------------------------
> > --
> > *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] 
> *On Behalf 
> > Of *Roman
> > *Sent:* Tuesday, December 11, 2018 1:20 PM
> > *To:* synth-diy at synth-diy.org
> > *Subject:* Re: [sdiy] Help, I'm Desperate! (Charge Injection with 
> > DG408)
> > 
> >     I know it's too late, as the circuit at this stage has 
> already gone
> >     slightly different path, and I already gave up, but 
> couldn't help
> >     drawing this little schemo that explains my idea of using single
> >     switches with that obsoleted chip. Switches should be DG444 or
> >     similar (0=closed, 1=open), or replace diodes with NAND 
> gates and
> >     use any regular dirt cheap switch like 4066.
> >     There's no switching happening while VCA is open, 
> provided that VCA
> >     control triangles match the edges of LM3914 stages. The only
> >     switched channels are the ones routed to muted VCA, so in theory
> >     there should be no clicking audible.
> >     Basicaly this is just the circuit that makes this input 
> switching
> >     sequence:
> >     VCA-A:12233445566..
> >     VCA-B:11223344556...
> >     by creating one step overlap between stages. Only 6 
> inputs in this
> >     example, but can be extended to any other number.
> >     http://www.synthdiy.eu/files/scanner.png
> > 
> >     IMHO switching active channel will always produce some 
> clicking for
> >     many reasons: not matched channels, difference between 
> transistion
> >     times H->L and L->H as Ingo said, break-before-make feature of
> >     DG407, slow response of 4532, and who knows if not also charge
> >     injection.
> > 
> >     Roman
> > 
> >     Dnia 11 grudnia 2018 18:24 Ingo Debus 
> <igg.debus at gmail.com> napisał(a):
> > 
> >             Am 11.12.2018 um 08:10 schrieb David G Dixon
> >             <dixon at mail.ubc.ca>:
> > 
> >             There is now
> >             only one tiny problem: There is still an almost
> >             imperceptible click when the
> >             CV crosses 0V (and the logic control voltage 
> crosses 2.5V)
> >             in the positive
> >             direction.  This is when the logic switches 
> from 011 to 100
> >             (i.e., all three
> >             bits change).  Interestingly, I don't hear the 
> click at all
> >             when the CV goes
> >             in the other direction (100 to 011).
> > 
> > 
> >         Couldn't this still be caused by a very brief 
> "forbidden state"
> >         during the transition from 011 to 100? In your case, the
> >         forbidden state would be 000 (MSBit changes slower that the
> >         other two) or 111 (MSBit changes faster). Probably just a
> >         difference between rising and falling slope. Can 
> you check with
> >         a scope?
> > 
> >         Ingo
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> > 
> >