Direct Digital Amplifiers

Bombaywalla, Class D amp doesn't convert anything to digital. It converts analog voltage to analog duty cycle of square wave and back to analog voltage thru filtering. Word "digital" would imply limit of resolution while it is unlimited - completely analog from input to output. People often confuse switching (where duty cycle is analog quantity) with digital.

Signal chain might be simpler but class D consist only of modulator and output Mosfets vs. multiple stages of gain in traditional amplifier.

Atmasphere, that's true but output power is very limited while in class D it depends only on power supply and size of output Mosfets.

Bombaywalla, Adjusted analog quantity is duty cycle. For instance, when you adjust oscillator's frequency by turning knob it doesn't matter if output signal is square, triangle or sinewave - your adjustment is still analog. The fact that voltage jumps between two level's doesn't make it digital if adjusted quantity is duty cycle. Also, amplifier doesn't make any use of these levels other than converting to output voltage by taking average value (filtering).
Distorted electric guitar (square wave) dosn't make it digital. FM radio is not digital in spite of voltage moving between two levels.

It is not discrete time system since discrete time system has countable number of states by definition. The states here are percents of duty cycle (quantity of interest) that changes with infinite number of states in similar fashion to change of frequency in analog Frequency Modulation. Yes I do design and use SMPS in my work. I'm surprised that you're confused by two levels in class D amp but not in FM radio signal. Again, for system to be "digital" at any point it has to have limited resolution (countable number of states) of quantity of interest - duty cycle, which class D amplifier isn't, being completely analog with unlimited resolution. In similar way FM radio signal has two states of voltage, but quantity of interest - frequency has unlimited number of states.

Very early primitive class D amps had digital modulator but these days almost everything is purely analog with unlimited resolution (Icepower, Hypex, NuForce etc.) Analog input voltage turns into analog duty cycle to become again analog output voltage by means of taking average value (filtering) of square wave (50% duty cycle representing 0V). Whole thing is continuous time system and has unlimited resolution unless, of course, amplifier has digital input which imposes resolution limit by definition.