Harold Hallikainen wrote:

> Thanks for all the comments! Regarding Nyquist, I agree I'd have to
> sample at twice the highest frequency or more if I want to reconstruct
> the signal. But, I don't want to reconstruct the signal, I just want an
> approximation of its amplitude. As has been pointed out, if I sample at
> 100 Hz and a 101 Hz tone is presented, I'd swing from zero to some value
> at 1 Hz. However, it seems that frequencies very close to the sampling
> rate would be a small portion of the overall audio. Frequencies further
> from the sampling rate would be filtered out by the averaging filter.

But still... a single bass tone that happens to be close to your sampling
frequency and in the right phase /will/ make your meter jump way beyond it
is expected to jump.

> As I read the specs on VU meters, they are pretty much an averaging
> device. They are supposed to reach 99% of their ultimate value within
> 300ms and overshoot by 1 to 1.5%. A National ap note says this is
> equivalent to a damped second order response with a resonant frequency
> of 2.1Hz and a Q of 0.62.

Yes, VU meters are not peak detectors. There are digital versions that have
both (often a bar for the VU value and a single dot for the peak). 

> Finally, the people responsible for the hardware side of this project
> decided to do it all in hardware. They're going to rectify and filter
> the audio and give the PIC a varying DC level that I'll measure whenever
> I need to update the display level. Should work fine, but uses lots more
> parts...

But better. IMO there's no shortcut -- wanting to reproduce or not, if you
want to have reliable data about AC, you need to sample at twice the
bandwidth.

Gerhard
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