[sdiy] PWM noise in analog ground in Optigan-like system

[cheater cheater]

Just a hunch - totally unscientific - sorry, should have marked this :-)

The idea is that the pulses will be uneven, and some of them might be
bigger (in amplitude) than others; this would be happening randomly
creating a noise-like wide band signal. However that signal is still
'synced' to the pulses coming through from the PWM. But all that is
just boogy-woogy and handwaving.

On the more scientific side:

> If that frequency
> is also outside the range that you can hear, any remaining noise won't be
> perceptible.

Bear in mind non-linearities and VCAs can shift that down easily.
Putting junk in supersonics is not a safe practice.

D.

On Mon, Mar 29, 2010 at 19:00, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> On 29 Mar 2010, at 17:50, cheater cheater wrote:
>
>>> Could you try an oblique strategy, like change the PWM frequency to be
>>> outside the audio range? If you could get it up to 30KHz, all your
>>> problems
>>> might magically disappear. That'd be my (typically hacky) approach!
>>
>> I think this would still generate sub-harmonics
>
> Please explain. Harmonics above, for sure (switching on/off is a
> pulse/square wave, therefore lots of third, maybe second, harmonic), but
> below? How come?
>
> Any lowpass filtering you do (caps on the rails, ferrite beads, whatever) is
> going to be *more* effective the higher the PWM frequency. If that frequency
> is also outside the range that you can hear, any remaining noise won't be
> perceptible.
>
> Thanks,
> Tom
>
>