[sdiy] (Ab)use DAC as mixer?

[sleepy_dog at gmx.de]

About the different DC offsets with unipolar Vref,

maybe I didn't get where the problem exactly happens and it's a stupid 
question,
but wouldn't it help to couple the 4 DAC outputs via capacitors before 
mixing them together?

- Steve

Am 03.06.2018 um 09:22 schrieb Roman:
> There are 4-quadrant multiplying DACs out there still and usually they 
> cost a few times more than regular DAC with external Vref. It's 
> easier/cheaper to use digital pot.
> One way to get rid of thumping is to feed 2 DACs with the same digital 
> word, literally tie all digital pins together in 2 chips. One DAC is 
> actual attenuator of Vref input that we want to work with, the other 
> DAC has Vref tied to DC voltage at half the amplitude of AC signal fed 
> to first DAC. Outputs of both DAC then go to differential amplifier 
> and you get nice digitally-amplitude-modulated signal with 0V DC bias 
> at the output, no thumping regardless of attenuation ratio changes. 
> That's one DAC and opamp more to do the job, but still probably half 
> cheaper than decent MDAC of the same reslution.
>
> Roman
>
> Dnia 3 czerwca 2018 00:45 Tom Wiltshire <tom at electricdruid.net 
> <mailto:tom at electricdruid.net>> napisał(a):
>
>     There used to be plenty of MDACs that accepted bipolar Vref
>     inputs. Are there any of those these days? That’s the obvious
>     solution for one of these problems (thumping). Using a unipolar
>     MDAC with biased bipolar signals is going to cause trouble
>     (although as Roman says - not entirely unmanageable).
>
>     This won’t solve zipper noise, and for an 8-bit DAC that might be
>     a significant factor if you try and do level modulation in the
>     digital domain. I’d look at getting the DAC up to 10 or 12 bit
>     before I tried that.
>
>     HTH,
>     Tom
>
>     ==================
>          Electric Druid
>     Synth & Stompbox DIY
>     ==================
>
>         On 2 Jun 2018, at 15:44, Roman <modular at go2.pl
>         <mailto:modular at go2.pl>> wrote:
>
>         2 reasons why you shouldn't do it:
>         - zipper noise
>         - altering DC bias with volume control (thumping)
>         But since you want to tuse just for static mix of waveforms,
>         both those reasons are not valid anymore. Go for it. It's been
>         done like this for decades.
>
>         Best if your input waveforms are not bipolar, so you don't
>         have to bias it. And if you need to remove thumping, just add
>         another set of DACs loaded with the same value, and subtract both.
>         Yes you can use the one filter for the mix of 4, but you don't
>         actually need any filter. You treat it as digital
>         potentiometer. No reconstrucion is needed as no sampling is
>         happening. You only change the attenuation ratio for each
>         input from time to time, hence zipper noise, but if I
>         understand your description properly, it's only done once when
>         changing patches or something.
>
>         Roman
>
>         Dnia 2 czerwca 2018 09:46 Roman <sleepy_dog at gmx.de
>         <mailto:sleepy_dog at gmx.de>> napisał(a):
>
>         Hey list,
>
>         this has been floating around in my mind for some years and I just
>         remembered it again, so maybe some of you know something about
>         this:
>
>         Would it be feasible to "abuse" a quad DAC with 4 independant Vref
>         inputs as a 4 channel, digitally controlled  mixer?
>
>         I.e., you feed in 4 analogue voltages into the 4 Vref inputs, and
>         control the volumes by writing digital output values.
>         The DAC I had in mind (TLV5620, price OK) only has 8-bit
>         resolution and
>         can stomach only 1/2 Vcc (1.65V I believe),
>         and it's positive-voltage-only, but that would do away with so
>         much
>         effort, in comparison to other solutions I'm aware of...
>         So If I have up to 4 audio sources which are already
>         positive-only,
>         *some* of which might come out of a DAC and not yet put through a
>         reconstruction filter,
>         I could mix them all that way, and put only one reconstruction
>         filter
>         for the mix?
>
>         Yeah, it's for something like waveform mixing. Not planning on
>         a high
>         update rate for the digital volume levels (so far), just
>         manual knob
>         adjustment. Although I could imagine it might be cool to
>         modulate that
>         somewhat, if that doesn't get too noisy.
>
>         But the layman I am, I probably don't know about some horrible
>         side
>         effects that may have :-D
>
>         What can you say about the feasibility of those two aspects
>         (and then
>         others I may have overlooked):
>         1) using a DAC this way in general
>         2) my desire to skimp on the number of reconstruction filters,
>         filtering
>         the mix (I'm not sure I'd be using > 1 DAC output for audio,
>         but keep it
>         in mind as an option, when DAC resolution is moderate)
>
>         - Steve
>
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