> Olin, why is it impossible to guarantee response time or design a
> filter for that? Could you please explain in more detail?

The way I understood it, you were dithering the input signal with a random
number, meaning white noise in signal processing terms.  If your random
number generator is really good, then each output value has no correlation
to any prevous output values and has an equal probability of being anywhere
in the output range.  This means it contains all frequencies (up to what can
be represented by the sample rate) equally.  In other words, you are
dithering with a signal that contains 1Hz, .1Hz, .01Hz, etc.

Let's look at this in the time domain instead of frequency.  Assume your
desired duty cycle is 50% and your random numbers range from 0 to 1.  There
is no guarantee how long a continuous string of random numbers might be
below 1/2.  It gets increasingly unlikely for longer strings, but there is
no guaranteed upper limit.

Look at this another way.  The reconstruction filter will remove frequencies
above some value, but the dither signal will always contain frequencies
below that value.  Therefore, there will always be low frequency noise on
the reconstructed signal.

Does that make sense?


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@embedinc.com, http://www.embedinc.com

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