resonz i f rq

A two pole resonant filter with zeroes at z = +/- 1. Based on
K. Steiglitz, "A Note on Constant-Gain Digital Resonators,"
Computer Music Journal, vol 18, no. 4, pp. 8-10, Winter 1994.  The
reciprocal of Q is used rather than Q because it saves a divide
operation inside the unit generator.

     i - input signal
     f - resonant frequency (in hertz)
    rq - bandwidth ratio (reciprocal of Q). 
         rq = bandwidth / centerFreq

> import Sound.DF

> do { n <- white_noise_m
>    ; r <- resonz (n * 0.5) 440.0 0.1
>    ; audition [] (out1 r) }

Modulate frequency

> do { n <- white_noise_m
>    ; f <- fmap (\x -> x * 3500.0 + 4500.0) (lf_saw 0.1 0.0)
>    ; r <- resonz (n * 0.5) f 0.05
>    ; audition [] (out1 r) }