Hello, I too was confused by the use of this term.
Receivers using class D amplifiers, are sometimes classified using phrases like 'fully digital.'
What this means, is that digital (PCM) signals are converted to a PWM signal which is used to switch the power transistors on and off.
Compare this to a Class amplifier whose inputs is an analog signal, which is converted to a PWM signal.
A power amplifier that could pull the same trick, would require digital inputs. If it takes analog inputs, it would not be consider to be 'digital' by this definition.
If a receiver using a class D amplifier has to convert digital signals to analog to drive the amp stage, then it is not considered fully digital.
We could get into complex arguments over what constitutes a digital circuit, but there's no point to that.
The main points are -
* Class D does not mean digital - it's just a convenient letter like A, B or C
* A class D amplifier which accepts PCM signals and directly converts them into the PWM signal without a need to convert the PCM to analog, is consider to be a 'digital' amplifier (or whatever terms people are using for this concept.)