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

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Wed Dec 12 15:51:35 CET 2018

I haven't done the calculations, but I'm left wondering whether or not 
you can do this "multi-input crossfade scanner" thing well enough using 
just fast PWM of the analogue switches themselves, and ditch the VCA's 


On 2018-12-12 13:42, Martin Klang wrote:
> 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
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