All electrical signals are analog, period. Digital is a representation that is useful only in signal transmissions (that requires back and forth conversions), storage, and computations.
Here is, again, an example of a typical eye-diagram (for 2-levels binary, i.e. NRZ modulation. There are modulation schemes that use more than 2 levels, such as PAM4). It shows the electrical signal that represents some digital signal being transmitted.
This is an example of what the electrical signal may represent in digital form.
View attachment 556886
When translating the electrical signal to its digital representation, the absolute voltage levels do not matter a lot. For example, for 5V TTL logical levels, an input level of >2 V is considered logic high; and an input level of <0.8V is considered logical low. In-between high and low levels is considered to be an error when encountered. 3V is the same 4 V is the same as 5 V. They are all logic high. Timing is similar. Jitters in the signal timing are unimportant when they are within the window of timing tolerance. That why the eye doesn't need to look anywhere close to perfect square waves. If your eye-diagram "looks really good", you have left a lot of bandwidth on the table.
But when you want to convert the PWM pulses back to the analog waveform by passing it through a low pass filter, then everything matters. The exact timing, the exact levels, all will affect the outcome. That's why PWM are not digital. It is no longer just two discrete levels of 0 and 1. 0.0 is different from 0.1 is different from 0.2, etc. Timing is again similar. A pulse train with rise time of 10 ns will give you different analog results after filtering than a pulse train with a rise time of 11 ns. For digital, they are identical as long as they are within spec.
View attachment 556888
Therefore, you cannot treat PWM as anything other than analog.
Back to your earlier question (post #40) regarding where the DAC process in the chain is optimal. High power amplifiers driving difficult loads will require feedback for high performance. The feedback signal, optimally, has to come from the amplifier output, which is an analog signal. Therefore, it is most practical to operate the power amplifier fully in the analog domain, regardless of it being class A, AB, B, D, or something else.
The actual conversion from digital to analog inside a DAC require high precision voltage or current sources. (The highest performance DAC's are usually current output DAC's.) It is extremely difficult to construct high precision sources that can drive difficult loads at high power levels. It is much easier (= practical) to make high precision sources that are only required to drive a benign and stable load such as that of an op-amp input. Therefore, the optimal arrangement (with the technology available at this time) is to have DAC's with line level outputs to drive conventional analog amplifiers.
