[sdiy] (Ab)use DAC as mixer?

sleepy_dog at gmx.de sleepy_dog at gmx.de
Sat Jun 2 10:33:49 CEST 2018

rsdio at audiobanshee.com wrote:
 > 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

Ah! Darn, this makes too much sense ;)

Although now reading the TLV5620 datasheet, under applications, it says:
- "Digitally Controlled Amplifiers/Attenuators"

And in operating characteristics:
- Reference input bandwidth: 100 kHz

Sounds good?

- Steve

Am 02.06.2018 um 10:14 schrieb 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

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