[sdiy] (Ab)use DAC as mixer?

[rsdio at audiobanshee.com]

I had the same idea that you describe, only it was about 30 years ago. I was thinking about a digital signal, though, while using the Vref as the volume control. I assume that feeding the audio signal into the Vref would not work very well.

Since then I’ve learned about the MDAC, or Multiplying DAC. I haven’t found any specific part numbers (or, if I have, I’ve forgotten). An MDAC should be perfect for volume control of a signal passing through.

The thing you need to look out for is that the Vref input is probably designed for a DC signal with lots of bypass filtering. If you send an audio bandwidth signal into the Vref pin, you’ll probably discover that it has very poor performance.

Brian Willoughby


On Jun 2, 2018, at 12:40 AM, sleepy_dog at gmx.de wrote:
> 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