[vorbis] lowpass recommendations?

[sugarcoatednapalm]

A while ago someone asked about a low-pass filter for oggenc and was told to get AFsp and filter outside of Oggenc.

Well, I got it, and am totally lost (It's way more complicated than SOX) so now can anyone briefly describe what type of filter I should set up (FIR, IIR, all-pole), why one is better than the other, and if you have filter coefficient files lying around (lowpass, 19 or 20 kHz cuttoff) PLS forward them.

I'd like a 19kHz 'brick wall' cuttoff with no ripple in the passband. : )

>From my undergrad engineering work I vaguely remember about 'too steep a cutoff gives you a lot of ripple in the passband', and other such design homilies about RC filters.

What I'm looking for here are some similar guides/rules of thumb for current digital filtering apps (AFsp, specifically).
 
PS I tested myself as having an 18kHz cutoff a few years back and I thought of using this as my cutoff but I'll keep slightly higher for when friends listen.
 
Thanks for your time.
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