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.

Regarding use of the PIC internal clamp diodes instead of an external
diode to get rid of the negative swing, I've found in the past (especially
with the positive clamp diode) that current in the PIC clamp diodes causes
leakage into adjacent A/D channels. I don't know if this is the case on
the negative side, but I've seen it on the positive side. I haven't
checked it recently, so I don't know if that's the case with current
chips.

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.

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...

Again, THANKS for all the comments!

Harold

-- 
FCC Rules Updated Daily at http://www.hallikainen.com
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist