"Digital" is a misnomer when it comes to Class D amps. The don't work with digital data. They take a continuous analog wave form as input and compand that with a high frequency triangle wave which produces a square wave of varying duty cycle. That square wave is used to open and close the output stage valve, finally that output is sent to a low-pass filter to remove the remaining high frequency artifacts.

Class D amps are very efficient because they are either all the way on or all the way off (which combined with the classification of 'D' is why many people call them digital).

I've always liked this note from Audioholics when they were reviewing a Class D amp:
 Quote:
What's important to note is it can be MORE expensive to properly design a Class D amplifier than a conventional linear one. You still need a good, high current power supply in both designs, but the Class D requires costly power inductors for the reconstruction filters and fast switchers being clocked at high frequency to handle the high current demand and also keep the harmonic content pristine within the audio band.


For more information (and there is a LOT of it out there) just Google "Class D amp"

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris