www.andyware.com/abox2/library/filters/iir_filters.txt

    These filters are implementations of common higher order filters.
    Each filter has: 
        
        X  input, signal to filter
        F  input, desired 3db cutoff frequency 
        =  output, filtered signal
        !! output, clipping indicater changes 0 to -1 if 
                    internal values are being clipped

    See the inside of each filter for it's specifications

Naming Scheme

    classification
    filter type
    order

    Classification

        states the general flatness of the frequency response curve

        B   Butterworth, 
            maximally flat passband
            mediocre initial cutoff
            good for use as tone controls

        C1  Chebychev
            1db dips in pass band
            sharp initial cutoff
            good for use in antialiasing

        BE  Bessel            
            maximally flat group delay
            poor initial cutoff 
            optimized for pulse/edge transmission without ringing

        Not yet included:

        C3  Chebychev
            3db dips in passband
            sharper initial cutoff

        E#  Elliptical
            sharpest cutoff
            peaks in stopband

        And there are many more besides these ...

    Filter Type

        intended frequencies to pass through the filter

        LP  Low Pass
        HP  High Pass
        BP  Band Pass

    Order

        digit indicating the number of reactive nodes in the filter
        corresponds directly to frequency rolloff        
        corresponds to number of peaks in pass band
        corresponds to number of phase transitions in pass band

        3   3rd order     18db/octave
        4   4th order     24db/octave
        5   5th order     30db/octave
        6   6th order     36db/octave
        
Example:

    B_LP6

        Butterworth
        Low Pass
        6th order