[sdiy] ADC - anti-alias filter

[Eric Brombaugh]

If you *know* that there's no signal content in a certain frequency 
range then there's no need to filter that out. It's certainly proper to 
omit the anti-aliasing filter if there are no signals that could alias.

The trick is *knowing*.

Eric

On 03/24/2017 11:13 AM, Steve wrote:
> Howdy @ DSP experts out there,
> I was wondering:
> 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.
>
> 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.
> 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.
> Is that really feasible?
> 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.
>
> Now even if the sound source picked up and the frequency response curve
> of the piezo make sure that the mentioned upper limit holds.
> 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?
>
> Is it *ever* a good idea to omit the analog filter before sampling?
> - Steve
>
>
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