Hi, I did this with an AVR a year and a half ago.  There are some 
niggles.  Depending on how accurate you'd like to be, how fast you can 
sample, and the voltage range of the incoming signal, how accurate you 
need to be for 100Hz vs. 1Khz vs. 10Khz frequencies, you may or may not 
need to:

1. amplify or cut incoming signal to scale to the A/D range
2. use a peak detector.  I used the precision half-wave rectifier sample 
circuit from the National Semi LM3916 datasheet.
3. calibrate for unit-to-unit variation in peak detector output for a 
given input signal level
4. do some tweaking of averaging of samples to get a 
reasonably-responsive VU display

The device this was for had small size and power consumption as critical 
parameters, and I decided to do it in firmware.  The original plan was 
to sample the internal digital audio stream (it is a mic pre with A/D, a 
specialty portable pro audio thing) but the AVR couldn't do it for 96Khz 
or 192Khz audio sample rates, a mistake I made from not reading the AVR 
datasheet closely enough and no space/time for a CPLD or other external 
helper digital logic.  So this was all something of a (successful) hack, 
unfortunately.

I am able to get -30db (relative to about 2.5Vpp fullscale input) 
reliably.  But each unit requires calibration, a hassle.

Cheers,
J





David Duffy (AVD) wrote:
> Has anyone here coded a VU meter on a PIC ? I'm designing a product that
> has three (for 2.1 output) 10 segment LED bars.
> 
> I could go done the old LM3915 route, but thought a micro with
> multiplexed output would be better in terms of ease of scale /  response
> customisation.
> David...
> 
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