<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On 24 Mar 2017, at 18:13, Steve <<a href="mailto:sleepy_dog@gmx.de">sleepy_dog@gmx.de</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div><div style="font-family: Verdana;font-size: 12.0px;">
<div>Howdy @ DSP experts out there,</div>
<div> </div>
<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>
<div><br>
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>
<div> </div>
<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>
<br>
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>
<br>
Is it *ever* a good idea to omit the analog filter before sampling?</div></div></div></blockquote><br></div><div>If the signal is being picked up by a mic, I don't see how they can be so sure about the frequency content. Unless the mic is utterly dreadful, I suppose, which is possible with some piezo things I've seen. Can you put the mic output on an FFT and see what frequencies it really picks up?</div><div><br></div><div>I'd be inclined to put a simple passive filter in anyway (2 pole is only four parts) and make sure that the op-amp stages are limited to only the required frequency range. It may be that you can get enough roll-off without needing any more active stages. You could use a multiple feedback filter with gain for one of the gain stages, for example.</div><div><br></div><div>To answer the final question, I'd say no, it's never a *good* idea, but you might get away with it in a particular situation if you're clever about it.</div><div><br></div><div>HTH,</div><div>Tom</div><div><br></div><div><br></div></body></html>