[sdiy] Dumb PWM question

[Tom Wiltshire]

On 26 Feb 2008, at 16:43, jbv wrote:

> 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)...


I've done this with digital oscillator waveforms (generalised phase  
distortion, of which PWM is a special case), but I can't see how  
you'd do it without sampling, since it is effectively a time  
distortion, and that's going to require some stored waveform.
I suppose the sampling could be done in the analogue domain (magnetic  
tape?) with the PWM applied as a modulation to tape speed at the  
input frequency.
It'd be a LOT easier on a DSP though....

Regards,
Tom