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## Copyright (C) 2000 Paul Kienzle
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2 of the License, or
## (at your option) any later version.
##
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this program; if not, write to the Free Software
## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
##
## Completed by: Laurent S. Mazet
## Compute chebyshev type I filter order and cutoff for the desired response
## characteristics. Rp is the allowable decibels of ripple in the pass
## band. Rs is the minimum attenuation in the stop band.
##
## [n, Wc] = cheb1ord(Wp, Ws, Rp, Rs)
## Low pass (Wp<Ws) or high pass (Wp>Ws) filter design. Wp is the
## pass band edge and Ws is the stop band edge. Frequencies are
## normalized to [0,1], corresponding to the range [0,Fs/2].
##
## [n, Wc] = cheb1ord([Wp1, Wp2], [Ws1, Ws2], Rp, Rs)
## Band pass (Ws1<Wp1<Wp2<Ws2) or band reject (Wp1<Ws1<Ws2<Wp2)
## filter design. Wp gives the edges of the pass band, and Ws gives
## the edges of the stop band.
##
## See also: cheby1
cheb1ord <- function(Wp, Ws, Rp, Rs) {
if (length(Wp) != length(Ws))
stop("Wp and Ws must have the same length")
if (length(Wp) != 1 && length(Wp) != 2)
stop("Wp,Ws must have length 1 or 2")
if (length(Wp) == 2 && (all(Wp>Ws) || all(Ws>Wp) || diff(Wp)<=0 || diff(Ws)<=0))
stop("Wp[1]<Ws[1]<Ws[2]<Wp[2] or Ws[1]<Wp[1]<Wp[2]<Ws[2]")
T <- 2
## returned frequency is the same as the input frequency
Wc <- Wp
## warp the target frequencies according to the bilinear transform
Ws <- (2/T) * tan(pi * Ws / T)
Wp <- (2/T) * tan(pi * Wp / T)
if (Wp[1] < Ws[1]) {
## low pass
if (length(Wp) == 1) {
Wa <- Ws/Wp
type <- "low"
} else {
## band reject
type <- "stop"
stop("band reject is not implement yet")
}
} else {
## if high pass, reverse the sense of the test
if (length(Wp) == 1) {
type <- "high"
Wa <- Wp/Ws
} else {
type <- "pass"
## band pass
Wa<-(Ws^2 - Wp[1]*Wp[2]) / (Ws*(Wp[1]-Wp[2]))
}
}
Wa <- min(abs(Wa))
## compute minimum n which satisfies all band edge conditions
stop_atten <- 10^(abs(Rs)/10)
pass_atten <- 10^(abs(Rp)/10)
n <- ceiling(acosh(sqrt((stop_atten-1) / (pass_atten-1))) / acosh(Wa))
FilterOfOrder(n = n, Wc = Wc, type = type, Rp = Rp)
}
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