[sdiy] Dumb PWM question

[jbv]

Hi list,

This might be a really stupid question, but anyway I was wondering
if there was any analog & digital circuitry that allows to apply
REALTIME
pulse width modulation on ANY input signal (cyclic or not)... providing
of course that it makes any sense to apply such processing on signals
other than square waves...
The basic idea would be, for instance, to "compress" the lower part of a

sine period by a certain percentage while "expanding" the upper part of
the
same period, but the freq of the sine would remain the same...
I guess that it would be somewhat more tricky on natural sounds in which

the lower & upper parts (thru-zero) of the signals may be difficult to
identify...

Thanks in advance (and apologizes for cluttering the list with such a
question
if it happens to be really stupid)...

JB