[WildUtah]

The principle of power and signal separation has been used to send multiple signals over a wire since the duplex telegraph in 1872. It's one of the few things in this world older than the headphone jack standard that we've been using since 1878.

[jessaustin]

You may be surprised when you stick a multimeter into a standard POTS jack and read 48V. (Admittedly, the voltage is lower when the line is in use, but it's still there, which is why old-fashioned phones don't need separate power cords.)