[leoedin]

Radio waves down a wire are by definition AC. Otherwise they don't work. The voltage must change, thus the current must change, thus it is AC. Just because it's not 50/60Hz AC like a powerline doesn't change that.

There might be a DC component, but ultimately all the ADSL modem cares about is the AC part.

[Eelongate]

Couldn't you send an amplitude modulated signal down a DC line by modulating the voltage but keeping that voltage always above zero? If the current never actually reverses, it wouldn't be AC as I understand it. It would be PDC, pulsed direct current.

[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.