Two points:
1. We have started discussing AC vs DC signals.
AC usually implies periodic cycling or repeating of the voltage
pattern, such as a VCO waveform. However, an envelope generator that
normally puts out a transient DC voltage can put out an AC voltage if
being repeatedly triggered from an LFO, for instance.
If a cyclic waveform is symmetrical around zero volts, it is pure AC.
If not, it is AC with a DC component.
2. Think of DC coupled inputs and outputs as having the ability to
handle DC and/or AC. They pass everything on exactly as it came in.
An AC coupled input or output will filter out the DC component in the
signal but pass on the AC component. Apply a pure DC waveform to an
AC coupled input and it will have no effect, because it is completely
filtered.
Moe
http://www.hotrodmotm.com