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

David G Dixon dixon at mail.ubc.ca
Wed Dec 12 23:36:35 CET 2018


Sounds cool, Martin.

I already had a scanner with separate VCAs for each channel, and it worked
great, but involved too much circuitry.  It did some of the things you
mention -- there's a video of it on Youtube -- but I wanted something
simpler, and the multiplexer thing seemed really elegant.  I didn't realize
that these logic issues were such a pain in the ass. 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Martin Klang
> Sent: Wednesday, December 12, 2018 5:42 AM
> To: synth-diy at synth-diy.org
> Subject: Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)
> 
> I understand you don't do microcontrollers David, but some of 
> us are less fussy!
> 
> 
> This is something I'm planning to prototype:
> 
> 8 input channels going into two analogue switches, into two 
> VCAs, with 
> switches and VCAs controlled by an MCU. The MCU decides to switch an 
> input only when the corresponding VCA is at -inf (ie when the 
> other VCA 
> is fully on).
> 
> Using an MCU means it's easy to do a bunch of things that 
> would be very 
> hard with analogue comparators:
> 
> - configurable cross-fade slopes using lookup tables
> 
> - cross-fade to start function (let the last input be 
> crossfaded back to 
> the first one so that a ramp input will produce a continuous, 
> circular scan)
> 
> - adjustable range of active inputs
> 
> - LEDs to show which inputs are active and audible
> 
> - different modes: tracking, triggered, cycling
> 
> 
> I was going to use two VCAs of a V2164 for the cross fading 
> between the 
> two multiplexed inputs. Driven by two 12bit DACs, controlled 
> by CV read 
> by a 12bit ADC. I think I'll be able to run both ADC and DAC 
> at a pretty 
> high rate, to achieve something like 16x oversampling at audio rate, 
> which should give performance / dynamic range nearer to 16 bits.
> 
> 
> The second two VCAs on the 2164 I was going to use to add a 
> regular VCA 
> with adjustable linearisation. With the input normalled to 
> +10V and CV 
> to the output from the scanner, this would let you control the 
> 'exponentiality' of the output. Alternatively you could use 
> the scanner 
> in triggered mode as an 8-stage envelope and VCA.
> 
> 
> Martin
> 
> 
> 
> On 12/12/2018 11:41, Tom Wiltshire 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
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> 





More information about the Synth-diy mailing list