[sdiy] (Ab)use DAC as mixer?

sleepy_dog at gmx.de sleepy_dog at gmx.de
Sat Jun 2 09:40:22 CEST 2018

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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