Marechiare wrote:
> Random noise probably would be attenuated indeed,

There is nothing probablistic about it.  The math shows quite clearly that
random noise will be attenuated, and by how much.

> but what if, for
> instance, some power SCR/TRIAC-regulated load were present on the
> line? Tops of the sine would be heavily suppressed, the filter won't
> restore the shape.

Actually it will to some extent.  Low pass filtering reduces the harmonics
compared to the fundamental.  Therefore the more filtering, the more the
result will look like a sine.  In the limit, you are left with only the
fundamental component of the distored waveform, which will be a pure sine.
Take a look at the plot I posted a few days ago.  The input to the filter
was a sine with the whole bottom cut off.  The output of the two pole filter
looked a lot more sinusoidal.

Dario is looking at the line voltage, which has very low impedence.  That
means that even with significant harmonics in the current drawn from this
line, the voltage will have little harmonic content.  The approximation that
most of the power is in the fundamental is quite reasonable for many line
voltage measuring purposes.  Remember that Dario only needs 5% accuracy.

>> True RMS requires much more computation.
>
> Looks like the statement is not true. RMS approach being a "statistic"
> approach does not require that many measurements per sine period as
> the peak-detect polling does to achieve the same precision.

Put down the dead fish and do the math instead.  This statement is just
silly superstition, unless of course you can define what a "statistic"
approach is, how RMS is one and low pass filtering with peak picking isn't,
then then show that such statistic approaches require less computation for
the same accuracy.

My recollection from past experience is that RMS requires more samples for
the same accuracy.  However, I don't feel like doing the math or writing a
simulation right now, so I can't substantiate that claim.

> Considering the fact that PIC24FJxxx got high-speed 17-bit x 17-bit
> hardware multiplier, we may conclude that the total of required
> operations per second would be much less.

Yes, the multiplies are the same cost as adds, but you conveniently left out
the square root at the end.  RMS would require one multiply and one add per
sample.  The two poles of low pass filtering I showed used 4 adds and 2
multiplies per sample.  A good chunk of the 80 extra operations per line
cycle will be offset by the square root operation of RMS.  However, both
methods will take only a tiny fraction of processor cycles, and Dario made
it clear that there were plenty of cycles available, so there is no real
distinction here.


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