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Mixing analog voices: DC or AC ?
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KammutierSpule



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PostPosted: Fri Jan 10, 2020 1:43 pm    Post subject: Mixing analog voices: DC or AC ? Reply with quote  Mark this post and the followings unread

Most of the schematics mixers I see are mixing with DC ( a resistor divisor and then a buffer) but, does it makes sense to mix voices in AC, removing any of the offset ?
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JovianPyx



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PostPosted: Fri Jan 10, 2020 2:35 pm    Post subject: Re: Mixing analog voices: DC or AC ? Reply with quote  Mark this post and the followings unread

KammutierSpule wrote:
Most of the schematics mixers I see are mixing with DC ( a resistor divisor and then a buffer) but, does it makes sense to mix voices in AC, removing any of the offset ?


Yes.

In audio mixers, I've seen only AC coupling. There is no way to know whether a device plugged into the mixer contains DC or not. Since DC has no sound, removing it causes no problems. A DC mixer is used in a synthesizer to mix CVs. If a DC mixer is ever used for audio, it's done because the user ran out of AC mixers. DC in an audio signal reduces dynamic range.

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Grumble



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PostPosted: Fri Jan 10, 2020 3:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

One can often see switches at the input so you can choose between AC and DC input configuration per input so you can mix audio with CV
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VA1



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PostPosted: Sat Jan 11, 2020 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

With DC you get clipping fast.
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Grumble



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PostPosted: Sat Jan 11, 2020 4:47 am    Post subject: Reply with quote  Mark this post and the followings unread

If you e.g. one channel set to DC (for Control Voltage) and the other(s) to AC (for Audio) you can modulate the CV.
Great stuff!

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efluon



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PostPosted: Sat Jan 11, 2020 1:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
If you e.g. one channel set to DC (for Control Voltage) and the other(s) to AC (for Audio) you can modulate the CV.

Excellent idea, need to make some passive highpasses now!
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KammutierSpule



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PostPosted: Sun Jan 19, 2020 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Let me clarify my question. There are two scenarios:

- On a voice, how do you mix the OSC?
Eg: you have a Square Osc, sub-Square Osc, etc how do you sum it? in DC or AC?

- How do you mix multiple voices in a synth?
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Moosgummi



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PostPosted: Mon Jan 27, 2020 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

All your examples are in the audio range, so AC it is.
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KammutierSpule



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PostPosted: Tue Jan 28, 2020 3:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks is makes sense!

Another related basic question.

While summing all OSCs / or Voices, at some pointing it is possible that it starts clipping / saturate the values.

I believe the correct way is to SUM all without any attenuation and let the user adjust levels so it will stop saturate.

But are there any other concerns?
Should it reduce any levels in SUM (eg multiply by 0.5)?
Or should the SUM handle some "head room" to allow mixing with less saturation?
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ixtern



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PostPosted: Tue Jan 28, 2020 4:28 am    Post subject: Reply with quote  Mark this post and the followings unread

All depends on context. If mixing external signals, AC coupling is reasonable.
If mixing internal known signals e.g. typical eurorack +-5V without DC offset, then DC may be better (no low frequency signal drop, less parts).

So, if such mixer is internal part of thoughtfully designed signal path, it may be DC coupled. If not, AC coupling is better.
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Grumble



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PostPosted: Tue Jan 28, 2020 5:44 am    Post subject: Reply with quote  Mark this post and the followings unread

A thoughtful designer would put a small switch in his/her design to choose between AC and DC Cool because you never know what is connected to the input...
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ixtern



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PostPosted: Tue Jan 28, 2020 5:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
A thoughtful designer would put a small switch in his/her design to choose between AC and DC Cool because you never know what is connected to the input...

Don't know if it is a good idea if you don't know what is connected to the input.
AC and DC path may require different design decisions.
E.g. if you are designing AC path, you may use op cheap op amps with relatively big offset input voltage and temp drift. in CV DC path you shouldn't do that.
In AC path, if signal volume is low, you may choose low-noise op amps and low input impedances (e.g. input pots 10k, not 100k as for CV).
etc. etc.
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Grumble



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PostPosted: Tue Jan 28, 2020 6:09 am    Post subject: Reply with quote  Mark this post and the followings unread

A thoughtful designer can tackle all those problems Rolling Eyes Very Happy
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