What DC Coupling Actually Governs
A DC-coupled path passes all frequencies down to zero hertz — which, in practice, means it passes steady voltages unchanged. A gate sitting at five volts stays at five volts. A slow LFO triangle that spends most of its cycle below zero crosses that zero line faithfully. An envelope that holds at its sustain level doesn't sag.
AC coupling cuts that low-frequency content by inserting a capacitor in the signal path. The capacitor blocks the DC component and rolls off anything below its corner frequency — typically somewhere in the range of a few hertz to a few dozen hertz, depending on component values. Above that corner the signal passes intact. Below it, the signal is progressively attenuated. At DC, nothing gets through at all.
This is completely harmless for audio you intend to listen to. A high-pass filter with a corner frequency below 20 Hz removes nothing audible. Recording interfaces, mixers and speaker amplifiers routinely use AC coupling at their inputs precisely to block unwanted offset voltages that could stress drivers or waste headroom.
The problem arrives when you route voltage that is also a control signal into one of those AC-coupled inputs. An envelope sent to an external interface for recording will arrive with its sustain level riding toward zero: the capacitor doesn't care that the flat top of the envelope is musically meaningful, only that it looks like a slowly changing DC value. A slow LFO will lose its low-frequency components and emerge as something thinner and phase-shifted, no longer tracking the shape you set. A CV offset — a steady voltage you've deliberately dialed in to transpose an oscillator — disappears entirely.
Within a properly designed modular system, patch points are DC-coupled throughout. The break in that convention almost always happens at the boundary: when signal leaves the case. If a mixer, audio interface or effect unit on the receiving end is AC-coupled, any control-rate or offset content in your signal won't survive the crossing. Know where your DC coupling ends, and route accordingly.