Bass / Treble Shelving Filter

Generates coefficients for a 2nd order IIR digital filter

Derive coefficients for a shelving filter with a given amplitude and cutoff frequency. All coefficients are calculated as described in Udo
Zolzer's DAFX book (p. 50 -55).

Usage: [B,A] = shelving(G, Fc, Fs, Q, type);

G is the logrithmic gain (in dB)
FC is the center frequency
Fs is the sampling rate
Q adjusts the slope be replacing the sqrt(2) term
type is a character string defining filter type
Choices are: 'Base_Shelf' or 'Treble_Shelf'

Typically this filter is used for multi-band equalization of audio. For example a 3-band equalizer (Bass/Mid/Treble) might use:

Bass = Bass shelving Filter
Mid = Peaking Filter
Treble = Treble shelving Filter