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About the different DC offsets with unipolar Vref,<br>
<br>
maybe I didn't get where the problem exactly happens and it's a
stupid question,<br>
but wouldn't it help to couple the 4 DAC outputs via capacitors
before mixing them together?<br>
<br>
- Steve<br>
<br>
Am 03.06.2018 um 09:22 schrieb Roman:<br>
</div>
<blockquote cite="mid:8b18be40040a4c049c39e9663af53889@grupawp.pl"
type="cite">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.<br>
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.<br>
<br>
Roman<br>
<div><br>
</div>
<div class="nh_extra">
<p>Dnia 3 czerwca 2018 00:45 Tom Wiltshire <<a
moz-do-not-send="true" href="mailto:tom@electricdruid.net">tom@electricdruid.net</a>>
napisał(a):<br>
</p>
<blockquote class="nh_quote" style="border-left: 2px solid #999;
padding-left: 8px; margin: 0;">
<div id="gwp030750d6">
<div>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).<br>
</div>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>HTH,<br>
</div>
<div>Tom<br>
</div>
<div><br>
</div>
<div>==================<br>
</div>
<div> Electric Druid<br>
</div>
<div>Synth & Stompbox DIY<br>
</div>
<div>==================<br>
</div>
<div><br>
</div>
<blockquote is-minimized="">
<div>On 2 Jun 2018, at 15:44, Roman <<a
moz-do-not-send="true" href="mailto:modular@go2.pl">modular@go2.pl</a>>
wrote:<br>
</div>
<div><br>
</div>
<div>2 reasons why you shouldn't do it:<br>
</div>
<div>- zipper noise<br>
</div>
<div>- altering DC bias with volume control (thumping)<br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>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.<br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>Roman<br>
</div>
<div><br>
</div>
<div>Dnia 2 czerwca 2018 09:46 Roman <<a
moz-do-not-send="true" href="mailto:sleepy_dog@gmx.de">sleepy_dog@gmx.de</a>>
napisał(a):<br>
</div>
<div><br>
</div>
<div>Hey list,<br>
</div>
<div><br>
</div>
<div>this has been floating around in my mind for some
years and I just<br>
</div>
<div>remembered it again, so maybe some of you know
something about this:<br>
</div>
<div><br>
</div>
<div>Would it be feasible to "abuse" a quad DAC with 4
independant Vref<br>
</div>
<div>inputs as a 4 channel, digitally controlled mixer?<br>
</div>
<div><br>
</div>
<div>I.e., you feed in 4 analogue voltages into the 4 Vref
inputs, and<br>
</div>
<div>control the volumes by writing digital output values.<br>
</div>
<div>The DAC I had in mind (TLV5620, price OK) only has
8-bit resolution and<br>
</div>
<div>can stomach only 1/2 Vcc (1.65V I believe),<br>
</div>
<div>and it's positive-voltage-only, but that would do
away with so much<br>
</div>
<div>effort, in comparison to other solutions I'm aware
of...<br>
</div>
<div>So If I have up to 4 audio sources which are already
positive-only,<br>
</div>
<div>*some* of which might come out of a DAC and not yet
put through a<br>
</div>
<div>reconstruction filter,<br>
</div>
<div>I could mix them all that way, and put only one
reconstruction filter<br>
</div>
<div>for the mix?<br>
</div>
<div><br>
</div>
<div>Yeah, it's for something like waveform mixing. Not
planning on a high<br>
</div>
<div>update rate for the digital volume levels (so far),
just manual knob<br>
</div>
<div>adjustment. Although I could imagine it might be cool
to modulate that<br>
</div>
<div>somewhat, if that doesn't get too noisy.<br>
</div>
<div><br>
</div>
<div>But the layman I am, I probably don't know about some
horrible side<br>
</div>
<div>effects that may have :-D<br>
</div>
<div><br>
</div>
<div>What can you say about the feasibility of those two
aspects (and then<br>
</div>
<div>others I may have overlooked):<br>
</div>
<div>1) using a DAC this way in general<br>
</div>
<div>2) my desire to skimp on the number of reconstruction
filters, filtering<br>
</div>
<div>the mix (I'm not sure I'd be using > 1 DAC output
for audio, but keep it<br>
</div>
<div>in mind as an option, when DAC resolution is
moderate)<br>
</div>
<div><br>
</div>
<div>- Steve<br>
</div>
<div><br>
</div>
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