TDA8920 TDA8920B TDA8920TH TDA8922 Understanding Faulting troubleshooting Repair

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From The Datasheet..
2. Features\\\\\\
The TDA8920B is a high efficiency class-D audio power amplifier with very low dissipation.
...
From Datasheet also
The audio input signal is converted into a digital pulse width modulated signal via an analog input stage and Pulse Width Modulation (PWM) modulator.

SO AFAIK
The audio input signal is turned into a 'digital' carrier?.. and then amplified and the carrier? is then filtered off with HPF and you are left with HIGH power output.

Trying to understand the tda8920 operation better to assist in repairing amplifier.

i.e. what signal should we see at output pin21
     also post filter ?
     whats with the push pull thing ?
   and the 90% efficiency that is really impressive...
   and where is the bias is that now the boot pin15&22 job ?
what is the difference in the PSU pins..
VSSP VDDP VDDA VSSA. what do those abbreviations mean ?
 

SO questions comments..

http://www.farnell.com/datasheets/1503711.pdf

http://www.farnell.com/datasheets/1503711.pdf  circuit page 21

TDA8920 circuit diagram from data sheet Att.