To begin with, he used off-the-shelf subscriber line interface circuits (SLICs) to implement the correct voltages, currents and impedances to drive analog phones.

We started out with an old modem and ended up with a useful circuit to convert phone audio to regular line audio. This little hack is useful for those of us wanting to record Voip telephone calls ...

The system contains automatic switchover circuits based on a priority system, ensuring that important messages are never missed.

At the heart of the control scheme of a classic switching regulator is the analog modulator, of which one typical implementation is shown in Figure 21.