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

john slee indigoid at oldcorollas.org
Thu Dec 13 00:15:29 CET 2018


It is a lot of hardware, yes. However it would let you have any arbitrary
mix of inputs mixed into the output, rather than being a scanner.

I’m thinking here of the Radiate control on the Make Noise RxMx, where the
“channel span” of the scanner is voltage-controlled. AFAIK their
implementation is all analog, so clearly it doesn’t need to be digital.

John


On Wed, 12 Dec 2018 at 22:41, Tom Wiltshire <tom at electricdruid.net> wrote:

> Nine VCAs and DAC channels seems like too much hardware.
>
> There’s nothing wrong with the switches, but I’d do it with two switches
> to select the inputs to two VCAs controlled by two DACs, and then a uP to
> control the switches and DACs. You can easily make sure that the VCA is
> shut off when the multiplexer switches, so there shouldn’t be any clicks
> even if the switch generates one.
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> > On 12 Dec 2018, at 10:02, john slee <indigoid at oldcorollas.org> wrote:
> >
> > 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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