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attempt to define Filter types and related Kyma components.

Contributions by: BenPhenix, DavidMcClain, CarlaScaletti

Butterworth (Kyma IIR HP/LP filters): maximally flat amplitude response, no consideration of phase linearity or group delay variations. Filter has no amplitude ripple in the passband or stopband.

Here's an interactive tool for designing Butterworth filters.

Another interative tool for Butterworth, Bessel and Chebyshev filters.

Bessel: most linear phase response of IIR filter class, no consideration of frequency response. constant group delay.

Linkwitz Riley, serial Butterworth filters: A 1-pole Butterworth into 2nd 1-pole Butterworth gives you a 2-pole Linkwitz filter. 2 2-pole Butterworths give you a 4-pole Linkwitz, and so forth. Great for creating cross-overs where keeping phase alignment is critical, common in speaker and audio components.

Biquad, bi-quadratic: Dual path filter operating on the in-phase and quadrature-phase.

DavidMcClain 's microsound 2-pole filters: discussion surrounding his DirectIIR filters

Bessel: retains nearly constant group delay through the passband with little consideration to amplitude response.

Reson: to quote DavidMcClain, "Reson filters have the desirable property of exhibiting constant peak amplitude across the entire frequency range, and the peak frequency can be directly specified. They have a post-gain section to bring their outputs back up to where you want them." example no longer in the forum, perhaps David or someone can post the Reson filter example here.

Chebyshev: produces passband, or stopband, ripples contrained within fixed bounds, such as 1 dB, 2 dB, and 3 dB of ripple. Ripples are either in the passband (Type I) or stopband (Type II). Subject to a high degree of nonlinear phase response.

Elliptic (Cauer filters): very sharp roll-off, very poor phase linearity, and have ripples both the passpand and stopband.

All-Pass (Kyma IIR Allpass): passes all frequenices with equal gain. Kyma filters have a non-linear phase response see AllPass Discussion.

Finite Impulse Response Filter (Kyma FIR): defines a class of digital filters that has only zeros on the z-plane. FIR filters are stable, and have linear phase responses. (<<<---necessary but not sufficient)

The reason for the name is that if you feed an impulse (a single "1" followed by an infinite string of "0s") into an FIR, it eventually dies away. Its impulse response has a finite duration. An FIR is made up of delays and attenuators but _no feedback. As anyone who's stuck a microphone in front of a speaker knows, feedback can "blow up" or grow without bounds (like a "pole" in an IIR filter). An FIR, not having feedback, can't blow up in the same way; hence its reputation for stability.

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