[sdiy] (Ab)use DAC as mixer?

Tom Wiltshire tom at electricdruid.net
Sun Jun 3 15:15:29 CEST 2018


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
>> 
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