% lhcheb1.m - lowpass/highpass Chebyshev type 1 filter design%% [A, B, P] = lhcheb1(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] = lhcheb1(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(1/epass) / N;W3 = Wpass * cosh(acosh(1/epass)/N);f3 = (fs/pi) * atan(W3^s);                           % 3dB frequencyP = [Wpass, Wstop, epass, estop, Nex, N, f3, a];W0 = sinh(a) * Wpass;if r==1,    G = W0 / (1 + W0);    A(1,:) = [1, s*(2*G-1), 0];    B(1,:) = G * [1, s, 0];else    G = 1 / sqrt(1 + epass^2);    A(1,:) = [1, 0, 0];    B(1,:) = G * [1, 0, 0];endfor i=1:K,    th = pi * (N - 1 + 2 * i) / (2 * N);    Wi = Wpass * sin(th);    D = 1 - 2 * W0 * cos(th) + W0^2 + Wi^2;    G = (W0^2 + Wi^2) / D;    a1 = 2 * (W0^2 + Wi^2 - 1) / 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*2, 1];end