MOTM

Here is a simple explanation:  DC (direct curent) means a steady, 
unchanging voltage, which could be any voltage level.  If a circuit is 
DC-coupled it means that the level of an unchanging voltage at the input 
will be passed through.  If a circuit is AC coupled then only changing 
voltages will be passed and there will be a "high pass" function 
applied, i.e. there will be a frequency below which signals won't pass.  
This cutoff frequency is normally about 20 Hz  for audio processing.  If 
you look at a circuit diagram, you will see a capacitor in series with 
the signal path if it is AC coupled.  Both inputs and outputs can be AC 
coupled.

If you feed a DC voltage, say +5V, into an AC coupled input, the output 
will be zero.  You can always feed an AC signal into a DC coupled 
input.  That works fine.  So why not always just use DC coupling?

The benefit of AC coupling is that it removes any DC offset (constant) 
voltage.  For example, the signal input of a VCA is sometimes AC 
coupled, because a DC offset at the signal input can result in a popping 
sound when it is multiplied with the control input.  The control input 
of a VCA has to be DC coupled, or else a prolonged unchanging voltage, 
such as the sustain portion of an envelope, would not produce the 
desired constant output level.

Good question about the EG output.  It is DC coupled, for the reason 
above.  However, an envelope is a type of AC signal, because it changes 
over time.  The interesting thing about an envelope is that it combines 
different frequencies in one cycle.  Attack is typically fast, decay and 
release slower.  An attack time of 20 milliseconds is 1/50 of a second, 
i.e. 50 Hz, up in the AC range for sure.  But a 500 millisecond release 
time is only 2 Hz.  So it is an AC signal with some very low frequency 
parts.

-Richard Brewster


Robert van der Kamp wrote:

>Hi all,
>
>could someone try to explain in layman's terms what it means 
>when an input is either AC or DC coupled?
>
>I know it means it assumes either an AC or DC signal. But 
>what if I feed it the other type of signal?
>
>And a related question: what is the definitions of an AC and 
>DC signal, in modular context? For example, when I an EG 
>ouputs a through-zero DC signal, isn't that actually an AC 
>signal?
>
>Confused...
>
>- Robert
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