Vasile Surducan wrote:

>> I am trying to take an analog signal (out of an MP3 player) and
>> determine its signal amplitude with the PIC's A2D input.  I would like
>> to keep the sampling frequency down (to reduce the clock speed and save
>> power) so I thought of adding a large cap outside the PIC in parallel
>> with a resistor to ground to hold the level.
> 
> The sampling frequency must be in agreement with the Nyquist theorem,
> you must have a bigger sampling frequency than 2x of your maximum analog
> signal frequency. 

That's only if you want to sample the original /waveform/. Since the OP
wants to sample the volume, the situation is different.

> Adding a cap will create a crude low pass filter at the AD input, egual
> with loosing your frequency bandwith of the analogic signal 

Yes, but that's exactly what e.g. the VU meters on tape recorders do (even
the electronic ones that wouldn't have to do it). They do it because for
certain applications, what you want to know is a running average of the
volume.

Gerhard
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