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

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flo
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Post by flo » Tue Feb 16, 2016 11:42 am

calaveras wrote: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.
No. Do not mix with stackcables or passive mults!

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Post by calaveras » Tue Feb 16, 2016 7:46 pm

flo wrote:
calaveras wrote: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.
No. Do not mix with stackcables or passive mults!
I guess I phrased that badly, yeah you arent supposed to patch things so that an output is pushing volts at another output.

blooma116
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Post by blooma116 » Wed Jun 15, 2016 6:03 pm

A few thoughts/questions:

What is the lower frequency threshold using a DC signal in an AC Coupled input? Conversely, someone else mentioned also that some modules are not well equipped for handling audio rate signals, could someone explain the technical limitations between the design of modules here?

Also are there modules for converting a DC signal to AC or vice versa, or would the best (or only) way to do this be with a mixer and offset generators? What's the difference between DC coupled mixer and a regular mixer? Is this related to whether they have a linear/logarithmic response or totally separate? Asking cause I'm aware linear is better for CV and logarithmic is better for audio, but they aren't mutually exclusive. I know some people mentioned some modules which have AC inputs which remove any DC offset, but are these available as standalone modules? Do modules exist which do the opposite, i.e. take an AC signal and shift it entirely into the positive voltage realm?

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Post by thermionicjunky » Thu Jun 16, 2016 8:24 am

blooma116 wrote:A few thoughts/questions:

What is the lower frequency threshold using a DC signal in an AC Coupled input? Conversely, someone else mentioned also that some modules are not well equipped for handling audio rate signals, could someone explain the technical limitations between the design of modules here?

Also are there modules for converting a DC signal to AC or vice versa, or would the best (or only) way to do this be with a mixer and offset generators? What's the difference between DC coupled mixer and a regular mixer? Is this related to whether they have a linear/logarithmic response or totally separate? Asking cause I'm aware linear is better for CV and logarithmic is better for audio, but they aren't mutually exclusive. I know some people mentioned some modules which have AC inputs which remove any DC offset, but are these available as standalone modules? Do modules exist which do the opposite, i.e. take an AC signal and shift it entirely into the positive voltage realm?
You are conflating DC-coupled with unipolar. It's a common mistake. Some CVs are unipolar and some are bipolar. Though AC-coupling will make a unipolar audio-rate signal bipolar, an DC-offset generator is generally a more useful tool for shifting signals around.

Though audio level controls tend to be log, it has nothing to do with whether or not the circuit is AC-coupled.

Anyway, AC-coupled inputs can usually pass a signal down to a few Hz, though the shape tends to distort near the cutoff.

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Post by nigel » Thu Jun 16, 2016 8:48 am

blooma116 wrote:A few thoughts/questions:

What is the lower frequency threshold using a DC signal in an AC Coupled input?
It depends on the design of the circuit. There is no sudden threshold - as the frequency gets lower, less and less of the signal will enter the circuit, until it is essentially ignored. Most circuits will want to handle low bass though, so there will usually be little effect above (say) 30 Hz. On the other hand, a 1 Hz signal will probably be severely reduced.
blooma116 wrote:Conversely, someone else mentioned also that some modules are not well equipped for handling audio rate signals, could someone explain the technical limitations between the design of modules here?
For example, some circuits use vactrols, which are quite slow to respond to a change in voltage. Feeding a 1kHz tone into them will probably have no effect at all.
blooma116 wrote:Also are there modules for converting a DC signal to AC or vice versa, or would the best (or only) way to do this be with a mixer and offset generators?
This is almost a meaningless question - there is no such thing as a DC or AC signal. The DC part of a signal is just the average value over a long-ish time, the AC part is the amount of variation of the signal from that average. AC coupling a signal means that the average value of the signal (inside the circuit) over (say) a second or two will be zero volts. DC coupling means that the signal can vary between (for example) three and four volts for hours.
blooma116 wrote:What's the difference between DC coupled mixer and a regular mixer?
A DC coupled mixer will let you mix control voltages (which may be at a non-zero value for many seconds) without affecting them. The output of an AC coupled mixer will (over a few seconds) be centered around zero volts.
blooma116 wrote:Is this related to whether they have a linear/logarithmic response or totally separate?
Totally separate.

Don't have time to address the rest of your post right now, sorry.

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Post by thermionicjunky » Thu Jun 16, 2016 9:01 am

nigel wrote:
blooma116 wrote:Conversely, someone else mentioned also that some modules are not well equipped for handling audio rate signals, could someone explain the technical limitations between the design of modules here?
For example, some circuits use vactrols, which are quite slow to respond to a change in voltage. Feeding a 1kHz tone into them will probably have no effect at all.
To clarify, a vactrol responds slowly to the control signal. Audio passing through the LDR is fine. A typical vactrol circuit (VTL5C3 or similar) can be modulated at lower audio rates, though the modulator will be lowpass filtered. It will gradually be attenuated as the frequency rises.

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daverj
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Post by daverj » Thu Jun 16, 2016 1:30 pm

The problem here is one of semantics and context.

For the most part people that understand the signals and the terminology speak in shortcuts, because being specific and accurate in the terminology is long winded and boring. And people that understand the signals, understand what somebody else is talking about based on the context of what they are saying.

The problem comes when people who don't understand the signals read those statements and doesn't know the words that are missing based on the context, so doesn't understand the differences of how those same terms are being used in each context.

It starts with the terms themselves.

DC = Direct Current
AC = Alternating Current

The first problem is that we don't measure signals in a modular system in current. We measure them in voltages. So for the most part when somebody says "DC" they mean "DC voltage" and when they say "AC" they mean "AC voltage".

A DC voltage is a constant flat voltage. 5 volts is a DC voltage. So is 12 volts. It doesn't change. It has no frequency. It has 0Hz frequency. You can have a "slowly changing DC voltage" where a voltage slowly changes from one DC voltage to another. But if it goes back to the original voltage and then back to the second voltage and then back to the original voltage over and over, no matter how slowly, that changing voltage is now an AC voltage.

An AC voltage is one that changes in a cycle. It doesn't have to be a constant speed cycle. It doesn't have to be a fast or a slow cycle. It doesn't have to start and end at exactly the same voltages each time. But if it goes up, then down, then up, then down, over and over, it is an AC voltage.

An AC voltage can be extremely slow. It could be one cycle per day and still be AC. It could be extremely fast. It could repeat a billion times per second and it would be AC. An AC signal generally can have one or more specific frequencies in it, or it could be completely random.

An AC voltage can also contain a DC voltage. If an AC signal is a pure sine wave that goes from -5v to +5v then it's average DC voltage is 0v and it is said to not have any DC voltage. In fact it still does, it's just that it's DC voltage is 0v. If an AC signal is a pure sine wave and goes from 0v to +5v then it's average DC voltage is +2.5v. So in that case it is said to be a 5v AC signal with a +2.5v DC signal or +2.5v "bias". I gave examples here using pure sine waves only because a symmetrical waveform will have an average DC value that is half way between it's top and bottom. More complex shapes or waves that are not 50% duty cycle will have their average DC value somewhere else off center, and are more complex to calculate. (you could use an O'Tool Plus to do it for you :hihi: )

When an input on a module is defined as an "AC input" or a "DC input", this is again a shortcut in semantics. What that really is saying is that it is an "AC coupled input" or a "DC coupled input".

A DC coupled input allows the full signal through, with both it's AC and DC components. So a fixed DC voltage will get through and so will an AC signal, as well as a combined signal with an AC voltage sitting at a specific DC bias.

An AC coupled input blocks lower frequency signals from getting through, which include DC signals (since DC is 0Hz, it is the ultimate low frequency). The fact that it is marked as an AC coupled input does not describe what frequencies are blocked and what are allowed through. Only the specs from the manufacturer can answer that, if they actually tell you. In general, if it is an input designed for an audio signal then you can usually expect it to pass through signals within the audio range. So it should allow signals down to maybe 20Hz, or maybe 40Hz, and block ones lower than that. Usually it is a very soft transition. So it can take several octaves before the signals are fully attenuated.

Likewise, an AC or DC coupled input does not by it's nature describe the top end of the frequencies that can go into it. That again is up to the specs of the manufacturer, if they happen to publish them (many don't). So an input intended for an audio signal might accept signals as high as the full audio range, maybe up towards 20KHz. One designed for a control input might also accept signals up to the top of audio range, but might not. They might only accept signals at a fraction of the audio range, depending on the circuitry that they are going into. There's no way of being sure if the manufacturer doesn't give specs, or you don't know the full principals behind the circuitry.

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Post by blooma116 » Thu Jun 16, 2016 8:30 pm

Thanks! This is great info, I think I'm getting a better understanding of all of this. So, I guess what I'm wondering: are there modules which convert bipolar signals to unipolar signals or vice versa? Is a high pass filter set to 20 hz going to universally remove all DC offset? So if you put a fixed DC +5v into a high pass filter would the output be 0v? Regardless of whether it is attached to an alternating signal? What if it's -5v? Cause theoretically a fixed DC signal is 0 Hz, so any high pass filter regardless of what frequency it's set at would remove this? I guess if you are making a unipolar LFO bipolar with this method the high pass filter would have to be below the frequency of the LFO for this to work? Am I totally on the wrong track here? Is there a simpler solution? I know you can add a DC offset to an AC signal and shift it positive or negative by mixing the cv's, but is there like some kind of logic module which is simply bipolar in/unipolar out? What about unipolar in/bipolar out? Like, something that figures out the average voltage a signal is centered around and then shift that to 0? Would something like this need to be digital? Does it exist at all?

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Post by daverj » Fri Jun 17, 2016 3:26 pm

Bipolar and unipolar are separate from AC and DC. Bipolar is a signal that goes above and below zero volts. So it contains positive and negative voltages relative to zero volts (or a common voltage reference often called "ground" though zero volts is more accurate).

A unipolar signal is one that simply does not go negative below zero volts.

In either of those you can have AC signals and DC signals. An AC coupled signal is usually considered to be bipolar, but it could be AC coupled into a fixed DC bias that makes the whole signal sit above zero volts, and therefore it could become a unipolar signal.

In most synths there aren't modules to convert between unipolar and bipolar, unless there is a specialty module that is used to convert between a specific brand of synth that is generally unipolar into a signal for a different brand of synth that is generally bipolar. Within any given synth you usually convert between unipolar and bipolar simply by mixing in a DC voltage of an appropriate amount.
Is a high pass filter set to 20 hz going to universally remove all DC offset?
Yes. DC is 0HZ, therefore it is below the pass band of the filter and is removed.
So if you put a fixed DC +5v into a high pass filter would the output be 0v?
Yes
Regardless of whether it is attached to an alternating signal?
Yes
What if it's -5v?
Removed
I guess if you are making a unipolar LFO bipolar with this method the high pass filter would have to be below the frequency of the LFO for this to work?
If an LFO is set very low and is run through a high pass filter that is set much higher, the output is nothing. The LFO is gone. DC and AC. If the Highpass is set low enough then it will pass the AC of the LFO but won't pass the DC of the LFO. If it did then it wouldn't be a highpass. Because no matter how low you set it, the low end still has to be higher than 0Hz for it to be a highpass.
Like, something that figures out the average voltage a signal is centered around and then shift that to 0?
That would be a lowpass filter. But set extremely low. So low that no AC signal is getting through, only the DC signal. Then if you inverted that signal and added it to the original (invert and add is the same as subtracting) you would be subtracting the DC voltage from your original.

But that's even more complex than running it through a highpass filter. And much more complex than simply adding a fixed DC voltage with a mixer.

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Post by blooma116 » Fri Jun 17, 2016 5:39 pm

Wow, thanks! This is starting to make a lot more sense!

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Post by ratskull » Sat Jun 18, 2016 12:15 am

This is a very helpful thread. Thanks to everyone who has taken the time to share their knowledge.

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daverj
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Post by daverj » Sat Jun 18, 2016 1:21 pm

Here's a graphic I made a couple of years ago explaining about using an attenuator and a bias control (adding a DC voltage in a mixer) to turn a bipolar signal into a unipolar signal:

Image

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Post by Moon Indigo » Mon Aug 28, 2017 4:05 pm

daverj wrote:The problem here is one of semantics and context.

For the most part people that understand the signals and the terminology speak in shortcuts, because being specific and accurate in the terminology is long winded and boring. And people that understand the signals, understand what somebody else is talking about based on the context of what they are saying.

The problem comes when people who don't understand the signals read those statements and doesn't know the words that are missing based on the context, so doesn't understand the differences of how those same terms are being used in each context.

It starts with the terms themselves.

DC = Direct Current
AC = Alternating Current

The first problem is that we don't measure signals in a modular system in current. We measure them in voltages. So for the most part when somebody says "DC" they mean "DC voltage" and when they say "AC" they mean "AC voltage".

A DC voltage is a constant flat voltage. 5 volts is a DC voltage. So is 12 volts. It doesn't change. It has no frequency. It has 0Hz frequency. You can have a "slowly changing DC voltage" where a voltage slowly changes from one DC voltage to another. But if it goes back to the original voltage and then back to the second voltage and then back to the original voltage over and over, no matter how slowly, that changing voltage is now an AC voltage.

An AC voltage is one that changes in a cycle. It doesn't have to be a constant speed cycle. It doesn't have to be a fast or a slow cycle. It doesn't have to start and end at exactly the same voltages each time. But if it goes up, then down, then up, then down, over and over, it is an AC voltage.

An AC voltage can be extremely slow. It could be one cycle per day and still be AC. It could be extremely fast. It could repeat a billion times per second and it would be AC. An AC signal generally can have one or more specific frequencies in it, or it could be completely random.

An AC voltage can also contain a DC voltage. If an AC signal is a pure sine wave that goes from -5v to +5v then it's average DC voltage is 0v and it is said to not have any DC voltage. In fact it still does, it's just that it's DC voltage is 0v. If an AC signal is a pure sine wave and goes from 0v to +5v then it's average DC voltage is +2.5v. So in that case it is said to be a 5v AC signal with a +2.5v DC signal or +2.5v "bias". I gave examples here using pure sine waves only because a symmetrical waveform will have an average DC value that is half way between it's top and bottom. More complex shapes or waves that are not 50% duty cycle will have their average DC value somewhere else off center, and are more complex to calculate. (you could use an O'Tool Plus to do it for you :hihi: )

When an input on a module is defined as an "AC input" or a "DC input", this is again a shortcut in semantics. What that really is saying is that it is an "AC coupled input" or a "DC coupled input".

A DC coupled input allows the full signal through, with both it's AC and DC components. So a fixed DC voltage will get through and so will an AC signal, as well as a combined signal with an AC voltage sitting at a specific DC bias.

An AC coupled input blocks lower frequency signals from getting through, which include DC signals (since DC is 0Hz, it is the ultimate low frequency). The fact that it is marked as an AC coupled input does not describe what frequencies are blocked and what are allowed through. Only the specs from the manufacturer can answer that, if they actually tell you. In general, if it is an input designed for an audio signal then you can usually expect it to pass through signals within the audio range. So it should allow signals down to maybe 20Hz, or maybe 40Hz, and block ones lower than that. Usually it is a very soft transition. So it can take several octaves before the signals are fully attenuated.

Likewise, an AC or DC coupled input does not by it's nature describe the top end of the frequencies that can go into it. That again is up to the specs of the manufacturer, if they happen to publish them (many don't). So an input intended for an audio signal might accept signals as high as the full audio range, maybe up towards 20KHz. One designed for a control input might also accept signals up to the top of audio range, but might not. They might only accept signals at a fraction of the audio range, depending on the circuitry that they are going into. There's no way of being sure if the manufacturer doesn't give specs, or you don't know the full principals behind the circuitry.
This is a really excellent explanation, thank you.

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