[sdiy] Audio Anti Aliasing Filter

[Richie Burnett]

Elliptic filters are a good choice to use for anti-aliasing and anti-imaging filters. They can be designed with an arbitrarily flat passband and have a *very* steep transition from the passband to the stopband. This is particularly important if you want to pass audio cleanly up to 20khz with a 44.1kHz sample rate. With these figures an anti-aliasing filter doesn't have much room in which to work its magic!

These days most audio ADCs have digital anti-aliasing filters built in. Likewise audio DACs often have internal digital anti-imaging filtering. The latter is sometimes referred to as a "reconstruction filter."  Some more info might help clarify if you could use one of these chips.

If you dont want the complexity of designing an elliptic filter, something like a 6th or 8th order Butterworth made from cascaded 2nd order Sallen-Key filters isn't a bad alternative. You can see this circuit preceding and following the BBD chips in old Roland Juno service notes. Just simulate tge circuit as-is, then scale capacitor values to shift the cutoff frequency exactly where you want it to be.

-Richie,

Sent from my Sony Ericsson Xperia ray

Scott Gravenhorst <music.maker at gte.net> wrote:

>
>What are people here using for audio anti-aliasing filters?  
>
>I'm looking for something simple.  In my own mind, I'm thinking state
>variable lowpass with unity Q and cutoff of some 20 kHz for sample rates of
>44.1 kHz and above.  
>
>Are there better or simpler?
>
>-- ScottG
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