Hellfire, Serge, Doepfer ..waveshaping PWM

[ADSR at aol.com]

Now, the Doepfer Waveshaper looks very similar to the Serge, any major 
differences?(aside from not being  a  "triple")
But the Hellfire looks quite a bit different and perhaps technically you can 
achieve more waveshapes with the Serge/Doepfer... but what's it going to 
sound like?My impression is that  the sound difference will be very 
different... haven't heard so I don't know. 
I'm actually, really curious what the PWM does to other signals.  Is it just 
a pulse waveform LFO the modulates the input?
something very unique?
-phil