ac vs dc (please don't kill me!)

Anything modular synth related that is not format specific.

Moderators: Kent, Joe., luketeaford, lisa

User avatar
matttech
Super Deluxe Wiggler
Posts: 9721
Joined: Wed Feb 23, 2011 1:12 pm
Location: Manchester, UK

ac vs dc (please don't kill me!)

Post by matttech » Thu Mar 31, 2011 1:05 pm

hi there

am only getting started out on this whole modular escapade and am learning more and more every day. i come from a music tech/ music background, not an electrical/ science one....and I am still a little confused about AC and DC.

1: i get that AC is alternating current - eg: one that changes polarity (lfos, oscs etc...) but am more confused about dc, and why you need to be careful with it re: speakers etc..

2: i was also just looking at an Oakley VCA which has separate inputs for AC and DC sources - what would go into the DC input?

3: is a gate voltage DC, as it goes from 0v to , say, +5v (but not from -5v to +5v - would it be alternating current if it did that?)

thanks in advance for anyone who can answer any of this (or point me in the right direction, as i guess this is one of the ultimate newb questions....i have no pride though......)
:despair: :despair: :despair: :despair: :despair: :despair:

User avatar
diophantine
Super Deluxe Wiggler
Posts: 2320
Joined: Mon Jan 10, 2011 2:50 pm
Location: San Diego, CA, USA

Post by diophantine » Thu Mar 31, 2011 1:23 pm

On the speaker issue, I'm not expert here, but my understanding is that speakers just aren't designed to handle that sort of signal. The speaker vibrates corresponding to the signal - it should go back & forth from the mid-point, not just on one side of it.

Gates and clocks are generally DC, yes. (In many applications you can use an AC square wave ok as well.) Envelopes are DC too. So is something like CV from a keyboard.

I am not familiar with the Oakley VCA, but I think that the DC switch basically just ensures that the output is DC.

Hope that helps!

User avatar
diophantine
Super Deluxe Wiggler
Posts: 2320
Joined: Mon Jan 10, 2011 2:50 pm
Location: San Diego, CA, USA

Post by diophantine » Thu Mar 31, 2011 1:25 pm

Oh, and apologies if I am stating the obvious, but VCA on a DC source just reduces the amplitude, for instance if you want to vary the amplitude of envelopes or something.

User avatar
tIB
Stainer!
Posts: 9335
Joined: Mon Jun 08, 2009 5:28 am
Location: UK

Re: ac vs dc (please don't kill me!)

Post by tIB » Thu Mar 31, 2011 1:28 pm

matttech wrote:
2: i was also just looking at an Oakley VCA which has separate inputs for AC and DC sources - what would go into the DC input?
Never understimate the power of control voltage signals through a VCA.
This may or may not be useful in explaining things: http://en.wikipedia.org/wiki/Capacitive_coupling

BTW I have an oakley classic VCA proto for sale at a very good price... :tu:

User avatar
MrBiggs
I see dead people
Posts: 3038
Joined: Mon Nov 09, 2009 3:16 pm
Location: Philadelphia
Contact:

Post by MrBiggs » Thu Mar 31, 2011 4:57 pm

You may know this -- from your question it's hard to tell. Everything relating to a modular is current. A gate is a current that is going from 0 to something and back to zero. A trigger is something that does that more quickly. Just a pulse. At its essence, that gate and pulse are exactly like any other CV source. It's just electricity sent to something in order to do something. If you slow an oscillator's square wave down enough, you get a series of gates.
A gate doesn't have to be what you're referring to as DC -- they can go pos/neg/pos/neg.

It's my understanding that when something in modular-land refers to AC, it's talking about audio-rate signals. Signals that make noise. SO...

You can send AC into the signal-in of a VCA, and control it with DC into the control-in. This effectively turns down the "volume" of the AC. However, all it's doing is attenuating the current. So if you put DC into the signal in, you're doing the same thing, you just can't hear it since it's just control voltage. But if you're using CV to open a filter, say, and you run it through the signal-in of a VCA, turning the knob or sending control voltage into the control-in will make that current open the filter less as you lower the control voltage.

I hope this makes some sense. when i started in modulars, all these CV/audio/gate/trig/ things confused the hell outta me til I realized they were all just variations of the same thing.
•Music and stuff: Dance Robot Dance.
•Current contents:Modulargrid

User avatar
matttech
Super Deluxe Wiggler
Posts: 9721
Joined: Wed Feb 23, 2011 1:12 pm
Location: Manchester, UK

Post by matttech » Thu Mar 31, 2011 6:02 pm

MrBiggs wrote:You may know this -- from your question it's hard to tell. Everything relating to a modular is current. A gate is a current that is going from 0 to something and back to zero. A trigger is something that does that more quickly. Just a pulse. At its essence, that gate and pulse are exactly like any other CV source. It's just electricity sent to something in order to do something. If you slow an oscillator's square wave down enough, you get a series of gates.
A gate doesn't have to be what you're referring to as DC -- they can go pos/neg/pos/neg.

It's my understanding that when something in modular-land refers to AC, it's talking about audio-rate signals. Signals that make noise. SO...

You can send AC into the signal-in of a VCA, and control it with DC into the control-in. This effectively turns down the "volume" of the AC. However, all it's doing is attenuating the current. So if you put DC into the signal in, you're doing the same thing, you just can't hear it since it's just control voltage. But if you're using CV to open a filter, say, and you run it through the signal-in of a VCA, turning the knob or sending control voltage into the control-in will make that current open the filter less as you lower the control voltage.

I hope this makes some sense. when i started in modulars, all these CV/audio/gate/trig/ things confused the hell outta me til I realized they were all just variations of the same thing.
ah - so essentially the AC input of the oakley vca is just the "audio" (eg: osc) input, and the dc input is an input for CV's - the input you would use if you were wanting to use the VCA's CV to modulate another CV source (eg: use an envelope to control the amount of lfo modulation)

just like you would if you used a doepfer a130

so...the oakley can do both CV and Audio? is that all it is?

just never seen a "dc" input on a vca before....usually they seem to just call them "lin" or "exp"

User avatar
matttech
Super Deluxe Wiggler
Posts: 9721
Joined: Wed Feb 23, 2011 1:12 pm
Location: Manchester, UK

oakley vca

Post by matttech » Thu Mar 31, 2011 6:09 pm

tIB

what kind of price is it going for?

is this basically a lin/ exp vca? (like the doepfer a132-3, but single rather than dual?)

have got a regular a131 on the way, and am getting an a132-3 when my modular arrives - was gonna test them out and see if they do the job, but might be interested in a different one if the price is right!

Edit - sorry...that was meant to be pm!!! Not sure how that happened...
Last edited by matttech on Thu Mar 31, 2011 6:43 pm, edited 1 time in total.

User avatar
MrBiggs
I see dead people
Posts: 3038
Joined: Mon Nov 09, 2009 3:16 pm
Location: Philadelphia
Contact:

Post by MrBiggs » Thu Mar 31, 2011 6:13 pm

The dc input is the same as the CV1 or CV2 input on the A-130. It might also be labeled "control in."

To clarify, you'd not be using the VCA's CV, but whatever source you've patched INTO the VCA. Like an ADSR, which is fairly standard to control the shape or loudness of the audio. Or an LFO for doing something rhythmic or repeating.

Most VCA's, if not all of them, can handle both DC and audio AC signals -- some have certain pots or have switches to choose between linear or log/exp response depending on what you want it to do. The A-130 can. Not all of them are great at both, but they all work. Some aren't good at having audio as a control source, however.

But yeah you've got the idea.
•Music and stuff: Dance Robot Dance.
•Current contents:Modulargrid

User avatar
daverj
Vintage Video Wiggler
Posts: 8514
Joined: Fri Jun 19, 2009 11:09 am

Post by daverj » Thu Mar 31, 2011 6:20 pm

Most signals in a modular are a combination of AC and DC signals. Pure DC is a flat, constant DC voltage, such as zero volts, 5 volts, 2 volts, etc... If the voltage changes over time then it has an AC portion to the signal. For example a square wave that goes between 2 volts and 3 volts has an AC signal of 1 volt peak to peak with a DC offset of 2.5 volts. So AC doesn't just mean positive and negative, it means changing. (though a "pure" AC signal has no DC component and is centered on ground)

Most modules that have an input labeled "AC" have circuitry on that input that removes the DC part of the signal and preserves the AC part. Most often expecting signals in the audio range, so likely to ignore AC signals below a certain frequency. This means that if you feed in a square wave that goes between 2 volts and 3 volts, or one that goes between 4 volts and 5 volts they will do the exact same thing and the DC voltage will be removed.

Audio rate AC signals typically are considered to be between about 20hz and 20khz (or a bit higher and lower). But an LFO putting out a .001hz sine or square wave is putting out a non-audio rate AC signal. And a video rate oscillator might put out a 1mhz (non-audio) AC signal.

Some modules that have AC inputs do so to control just the audio portion of a signal since mixing DC into it might add distortion or bleed through from the controls.

User avatar
matttech
Super Deluxe Wiggler
Posts: 9721
Joined: Wed Feb 23, 2011 1:12 pm
Location: Manchester, UK

Post by matttech » Thu Mar 31, 2011 6:33 pm

So what is the dc input of this vca expecting? Or is it simply another name for " cv"? - seems an odd thing to call it if that's all it means...

What would be an example of a true dc signal in a modular then? A voltage offset? An amplification? Or attenuation?
Last edited by matttech on Thu Mar 31, 2011 6:46 pm, edited 1 time in total.

User avatar
DGTom
Super Deluxe Wiggler
Posts: 2615
Joined: Tue Aug 19, 2008 2:42 am
Location: Pt.Adelaide, Sth.Aust.

Post by DGTom » Thu Mar 31, 2011 6:45 pm

The DC input just won't remove any offset, it will pass it thru the VCA, whereas the AC input won't.

which you can use to your advantage.

User avatar
Joe.
Wired for sound
Posts: 5027
Joined: Sun Jan 30, 2011 10:58 pm

Post by Joe. » Thu Mar 31, 2011 6:49 pm

Don't get used to the idea that all oscillators or LFO's are AC.

If a signal has a DC offset that is greater than half the "Peak to peak" value then the voltage is never going to drop below 0 volts; Its a DC signal.

AC refers to switching polaritys, not just "audio rate signals" as another poster put it.

User avatar
Spanningtree
Loop free patching via 802.1d
Posts: 918
Joined: Fri May 21, 2010 6:03 pm
Location: LV,NV

Post by Spanningtree » Thu Mar 31, 2011 7:09 pm

Great thread! Learning a lot! :party:

User avatar
Synthbuilder
Super Deluxe Wiggler
Posts: 3051
Joined: Tue Mar 24, 2009 9:12 am
Location: Cumbria, UK

Post by Synthbuilder » Fri Apr 01, 2011 2:05 am

In the case of the Oakley VCA the DC and AC inputs are labelled as such to indicate coupling. The DC input is DC coupled while the AC input is AC coupled. Strictly speaking it has nothing to do with whether the signal has a polarity change or not.

AC coupling allows signals that change sufficiently quickly over time to pass through that input. But it blocks those signals that remain constant, or do not move up or down fast enough. Think of it functioning as a high pass filter with a sub-audible cut-off frequency. So it won't pass a steady voltage, but it will pass audio.

DC coupling allows all types of signals and CVs to pass. There is no high pass filtering effect.

Tony

User avatar
matttech
Super Deluxe Wiggler
Posts: 9721
Joined: Wed Feb 23, 2011 1:12 pm
Location: Manchester, UK

Post by matttech » Fri Apr 01, 2011 4:32 am

Synthbuilder wrote:In the case of the Oakley VCA the DC and AC inputs are labelled as such to indicate coupling. The DC input is DC coupled while the AC input is AC coupled. Strictly speaking it has nothing to do with whether the signal has a polarity change or not.

AC coupling allows signals that change sufficiently quickly over time to pass through that input. But it blocks those signals that remain constant, or do not move up or down fast enough. Think of it functioning as a high pass filter with a sub-audible cut-off frequency. So it won't pass a steady voltage, but it will pass audio.

DC coupling allows all types of signals and CVs to pass. There is no high pass filtering effect.

Tony
cool....think I'm understanding this a bit better now. thanks for all the input!

User avatar
chamomileshark
Super Deluxe Wiggler
Posts: 3455
Joined: Sun Jan 11, 2009 8:24 am
Location: UK

Post by chamomileshark » Fri Apr 01, 2011 2:14 pm

Excellent thread and great responses, also no one gets killed here - three more reason why this is my favourite forum.
Key Gear: Wiard 300 Series, EMS VCS3, Frac Modular (Blacet, Wiard, Bananalogue & Synthesis Technology).

Pastoral Music: Mark Ellery Griffiths
Music: https://markgriffiths.bandcamp.com/
Website: https://markellerygriffithsmusic.blogspot.com/


Experimental & IDM: Mark Dalton Griffiths
Music https://markdaltongriffiths.bandcamp.com/
Website:https://markdaltongriffithsmusic.blogspot.com/

Pensive
Learning to Wiggle
Posts: 2
Joined: Sun Oct 18, 2015 9:52 am

Post by Pensive » Tue Nov 03, 2015 5:31 am

Hello, I was just reading your post and I too am just getting into modular synths, I would like to create sound effects with it. I would like to be able to send audio signals through the synth and modify the sound, is this possible? I also heard that i would have to change the AC to DC, is this correct?

Thank you

User avatar
milkshake
Super Deluxe Wiggler
Posts: 1489
Joined: Mon Jul 22, 2013 6:19 am

Post by milkshake » Tue Nov 03, 2015 7:00 am

Pensive wrote:Hello, I was just reading your post and I too am just getting into modular synths, I would like to create sound effects with it. I would like to be able to send audio signals through the synth and modify the sound, is this possible? I also heard that i would have to change the AC to DC, is this correct?

Thank you
You can send outboard audio signals through your modular synth. Most soundcards with a +4dBu output will be sufficient.
If you don't have a high level output at your disposal, you need a preamp module. Something like this, or this will do.

Only if you want to control your modular with a DAW, you need DC outputs. I'm talking about automaticaly turning the knobs of your synth here. For example changing the cutoff of a filter, or pitch of an oscillator. You can do this by hand or with other modules like LFO's, ADSR's or any module that creates/changes a waveform.
So if you just want to send audio signals through your modular, you don't need DC outputs, any soundcard or other sound device will do. Just make sure you have enough level output.

But some people prefer to control their synth with a DAW. You need pluginns like Silent Way and a DC soundcard or some Expert Sleepers hardware. I don't have experience with this, so can't help you further.
Shall I ... blablabla ... to thee.

The more you pay for something, the less likely you are to critise it.

ContraPoints

calaveras
Super Deluxe Wiggler
Posts: 3495
Joined: Sun Dec 21, 2014 2:02 pm

Post by calaveras » Tue Nov 03, 2015 11:50 am

I've been noticing that a lot of people seem to get confounded on the AC and DC aspects of modular. I have a little background in electronics so it hasn't been as hard for me making the transition.
For instance my uncle got me started on Heathkit electronics projects when I was pretty young, like 8 or 9. And back in the 90's I had a live sound company that I ran for a while. Which meant my soldering skills were pressed into service on a regular basis so that the 'show could go on' so to speak. (ever fix a microphone while running the mixer during an all day punk show?)

I would point out that while an envelope or a positive going square pulse may be technically termed AC, for the sake of this discussion that is a technical distinction only. That is not something that you would ever want to pass to a speaker, so it is not AC the same way an audio signal is AC. When we talk about DC in modular land we don't generally mean steady, flat DC like you see in a well regulated power supply. But rather control signals that are not bipolar.

I think that is the easiest way to understand it. AC is bipolar, the wave swings positive and negative in relation to ground and is often a periodic signal, meaning it repeats over a certain period. Say 60 cycles per each second such as you have with North American wall socket power.
A DC signal may change over time (you generally want them to) but they usually stick to one side of ground, going postive or going negative. Of course there are bipolar things like LFO's that seem to be DC since you can use them in a lot of places that you might use a DC source from a sequencer or envelope. But the fact that you can take them up into audio rate, or that many Oscillators can be LFO's indicates that they are AC.
It is also useful to point out that audio is AC. If you zoom in on an audio clip in your computer you can see the positive and negative waves with the center line representing a ground reference.
Last edited by calaveras on Mon Feb 15, 2016 3:38 pm, edited 1 time in total.

Pensive
Learning to Wiggle
Posts: 2
Joined: Sun Oct 18, 2015 9:52 am

Post by Pensive » Wed Nov 04, 2015 7:32 pm

I think I'm beginning to understand now; Time for me to start looking around for parts :)

Thank you for the reply It's been a big help.

droningspaghettimonster
Veteran Wiggler
Posts: 602
Joined: Sun Nov 22, 2015 11:18 am

Post by droningspaghettimonster » Mon Feb 15, 2016 3:19 pm

felt like resurrecting this thread since i'm also a beginner.

could somebody describe to me the difference between AC and DC coupled modules? i know my cold mac is AC and does therefore not mix CV signals, why is that? i'd like detailed instructions by someone who has the time. my MMG also has AC and DC inputs, and sometime people talk about DC coupled filters. thank in advice

calaveras
Super Deluxe Wiggler
Posts: 3495
Joined: Sun Dec 21, 2014 2:02 pm

Post by calaveras » Mon Feb 15, 2016 3:40 pm

in usage terms an AC coupled module will only pass audio, or very fast CV.
A DC coupled module will pass CV or audio.
This is mostly of concern with mixers and VCAs.
Though you can also use filters to modify CV.

droningspaghettimonster
Veteran Wiggler
Posts: 602
Joined: Sun Nov 22, 2015 11:18 am

Post by droningspaghettimonster » Mon Feb 15, 2016 4:04 pm

thanks!! what is the concern with vcas?

calaveras
Super Deluxe Wiggler
Posts: 3495
Joined: Sun Dec 21, 2014 2:02 pm

Post by calaveras » Mon Feb 15, 2016 4:49 pm

droningsphagettimonster wrote:thanks!! what is the concern with vcas?
Well this was news to me when I first heard it, but it is the coolest thing ever.
With DC coupled VCAs and Mixers you can run CV through.
So say you have an LFO and pass it through a VCA. Then you can wiggle the CV with another LFO, or an envelope, or any other arbitrary function.
Likewise you can use a DC coupled mixer to merge several CV signals. You can do the same with stackcables or whatever, but with a mixer you have more control over the ratio of signals.

droningspaghettimonster
Veteran Wiggler
Posts: 602
Joined: Sun Nov 22, 2015 11:18 am

Post by droningspaghettimonster » Tue Feb 16, 2016 11:34 am

thx. :)

Post Reply

Return to “Modular Synth General Discussion”