The Cyndustries List

The Euro format Zeroscillator has both AC and DC coupled Linear FM inputs.

LINEAR FM INPUT = DC Coupling, (which allows both AC and DC Inputs). The
DYNAMIC LINEAR FM INPUT = AC Coupling, (Which allows audio only by
stripping any DC out of the signal).

The reason for AC coupling is simple. 
When a modulation signal is applied to the linear input, the frequency is 
supposed to modulate symmetrically about the center (note). If there is any 
DC on the modulation signal, the note will "pull" to one side or the other
(depending on the polarity of the DC) as the modulation signal is applied. 
You will hear this as a detuning or chirp of the tone if there is an envelope 
gating the modulation. For these reasons, we carefully null the modulation 
VCA (balanced multiplier) for any residual DC and we use a capacitor on the
input to block any DC from entering from the horrible, corrupted outside 
world.

The beauty of linear modulation at audio rates is that the note stays put 
and only the timbre changes, if everything is done right. This is not 
possible with exponential modulation because it is an inherently 
non-linear process.

Cynthia

http://www.cyndustries.com/