[sdiy] (Ab)use DAC as mixer?

[Michael E Caloroso]

Would a Gilbert multiplier be a good candidate?  Those are high
fidelity, not sure how good their AC feedthrough is though.

MC

On 6/3/18, Tom Wiltshire <tom at electricdruid.net> wrote:
> A capacitor will shift a steady DC offset, but the problem arises when you
> change the DAC value from “big” to “small” and get a large step in the DC
> offset. That step will have higher harmonics that will get past a capacitor.
>
> A DAC that takes a bipolar Vref, or equivalently Roman’s solution for making
> one on the cheap, are better plans.
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
>> On 3 Jun 2018, at 13:59, sleepy_dog at gmx.de wrote:
>>
>>
>> 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>
>>> 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> 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> 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
>>>
>>> _______________________________________________
>>> 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
>