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

[john slee]

I suspect a more-digital solution would be easier to get right, for people
more inclined toward such things, just more expensive

9 VCAs, 9 DACs to control the VCAs, and a microcontroller to control the
DACs.

No need for switching at all, if I understand your intent correctly?

I readily acknowledge that digital isn't for everyone, though.

John

On Wed, 12 Dec 2018 at 13:57, David G Dixon <dixon at mail.ubc.ca> wrote:

> 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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