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<div>Howdy @ DSP experts out there,</div>
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<div>I was wondering:<br/>
if done by the book, if you're doing ADC conversions of an audio signal and like it to be clean, you put a nicely steep LPF before it that, immensely reducing anything >= fsample/2.</div>
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So somebody told me recently, he wants to analyse an audio signal which pretty much only has frequencies up to 4kHz, maybe 6 but very quiet, and he's interested only in 1..2 kHz of that.</div>
<div>Hence, he wants to omit the LPF to save parts & PCB space, sample at a rate like 15..25 kHz and use a digital filter and then decimate to 4 kHz or so.</div>
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<div>Is that really feasible?<br/>
The thing is, the signal is picked up by an analog mic (piezo) with 2 opamps after another, each has an integrated PGA of up to 16x gain, because the signal get get really quiet.<br/>
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Now even if the sound source picked up and the frequency response curve of the piezo make sure that the mentioned upper limit holds.<br/>
Could that stuff not pick up noise in some environments which introduce frequencies above 1/2 of his higher samplerate and hence pollute the spectrum?<br/>
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Is it *ever* a good idea to omit the analog filter before sampling?</div>
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<div>- Steve<br/>
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