[goodpoint]

> modulating the voltage but keeping that voltage always above zero

In signal transmission theory people almost always think in terms of frequency ranges.

Any DC component is simply a frequency set to 0 - that can be ignored.

Similar to the earth magnetic field when talking about radio transmission in air.

[Eelongate]

Sure, but it's still not AC. It's VDC or PDC, not AC. It's not AC if the Current isn't Alternating.