.de BR
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..
.Th                              BANDREJECT
.S2 SUMMARY:
BandReject filter traces

BandReject [options] 

where options is one or more of the following:

[Butter | BEssel | C1 ] [ Corner fc ] [ Npoles npoles ] [ Passes npass] [ Tranbw tranbw ] [ Atten atten ]

.S2 INPUT:
Butter   : Butterworth filter
.BR
BEssel   : Bessel filter
.BR
C1       : Chebyshev Type I filter
.BR
Corner   : Corner frequencies (R) range 0 \- Nyquist
.BR
Npoles   : Number of poles  (I) range 1 \- 10
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Passes   : Number of passes (I) range 1 \- 2
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Tranbw   : Chebyshev transition bandwidth fraction (0.3 default)
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Atten    : Chebyshev stop band attenuation (30 default)

.S2 DESCRIPTION:
Highpass filter using a BI\-LINEAR Z\-transformation implementation of
a highpass filter. A bi\-linear method is chosen since this is easily
implemented algebraically. Passes = 1 gives a causal filter while 
Passes = 2 gives a zero\-phase filter with a 6db point at the corner 
frequency.
.sp
The lowpass filter design of the Chebyshev Type 1 filter is based on the information in  Hamming (1997) equation (13.5.4) and Figure 13.6.1, This filter attempts to approximate a sharp lowpass filter. In reality a transition band is defined by Fp and Fs, where Fp is the lowpass corner frequency and Fs is the stopband, where Fs = (1 + tranbw)*Fp.  The amplitude level of the stop band (f > Fs) is 1/atten.  Actual implementation requires a parameter epsilon, eps, which defines the lowpass ripple, which varies between 1 and 1/sqrt(1 + eps*eps).
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Hamming, Richard W. (1997). Digital Filters (3rd edition), Dover Publications, 296 pp, ISBN 048665088X
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Given npoles and eps, the poles and zeros are given by 
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http://www.answers.com/topic/chebyshev-filter
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The correct normalization amplitude together with poles and zeros is given in
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Digital Filter Designers Handbook with C++ algorithms, C.  Britton Rorabaugh 2nd Edition, McGraw Hill, New York, 479 pp 1997  Chapter 5
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.S2 EXAMPLES:
Bandreject with corner frequencies at 1 and 10 Hz, zero phase, 2\-pole
.BR
     BR BUTTER C 1 10 P 2 NP 2

.S2 "SEE ALSO":
LOWPASS, HIGHPASS, BANDPASS

