% lhcheb2.m - lowpass/highpass Chebyshev type 2 filter design%% [A, B, P] = lhcheb2(s, fs, fpass, fstop, Apass, Astop)%% s = 1, -1 = lowpass, highpass% design parameters:% P = [Wpass, Wstop, epass, estop, Nex, N, f3, a];% A, B are Kx3 matrices of cascaded second-order sectionsfunction [A, B, P] = lhcheb2(s, fs, fpass, fstop, Apass, Astop)Wpass = tan(pi * fpass / fs);  Wpass = Wpass^s;Wstop = tan(pi * fstop / fs);  Wstop = Wstop^s;epass = sqrt(10^(Apass/10) - 1);estop = sqrt(10^(Astop/10) - 1);Nex = acosh(estop/epass) / acosh(Wstop/Wpass);N = ceil(Nex);  r = rem(N,2);  K = (N - r) / 2;a = asinh(estop) / N;W3 = Wstop / cosh(acosh(estop)/N);f3 = (fs/pi) * atan(W3^s);P = [Wpass, Wstop, epass, estop, Nex, N, f3, a];W0 = sinh(a) / Wstop;                              % reciprocal of textif r==1,    G = 1 / (1 + W0);    A(1,:) = [1, s*(2*G-1), 0];    B(1,:) = G * [1, s, 0];else    A(1,:) = [1, 0, 0];    B(1,:) = [1, 0, 0];endfor i=1:K,    th = pi * (N - 1 + 2 * i) / (2 * N);    Wi = sin(th) / Wstop;                          % reciprocal of text    D = 1 - 2 * W0 * cos(th) + W0^2 + Wi^2;    G = (1 + Wi^2) / D;    b1 = 2 * (1 - Wi^2) / (1 + Wi^2);    a1 = 2 * (1 - W0^2 - Wi^2) / D;    a2 = (1 + 2 * W0 * cos(th) + W0^2 + Wi^2) / D;    A(i+1,:) = [1, s*a1, a2];    B(i+1,:) = G * [1, s*b1, 1];end