% Program 14_2
     % Frequency Responses of Tree-Structured QMF
     % Filters
     %
     clf;
     % Type in prototype lowpass filter coefficients
     B1 = input('Filter coefficients = ');
     B1 = [B1 fliplr(B1)];
     % Generate the complementary highpass filter
     L = length(B1);
     for k = 1:L
     B2(k) = ((-1)^k)*B1(k);
     end
     % Determine the coefficients of the four filters
     B10 = zeros(1, 2*length(B1));
     B10([1: 2: length(B10)]) = B1;
     B11 = zeros(1, 2*length(B2));
     B11([1: 2: length(B11)]) = B2;
     C0 = conv(B1, B10);C1 = conv(B1, B11);
     C2 = conv(B2, B10);C3 = conv(B2, B11);
     % Determine the frequency responses
     [H00z, w] = freqz(C0, 1, 256);
     h00 = abs(H0z);
     M00 = 20*log10(h00);
     [H01z, w] = freqz(C1, 1, 256);
     h01 = abs(H01z);
     M01 = 20*log10(h01);
     [H10z, w] = freqz(C2, 1, 256);
     h10 = abs(H10z);
     M10 = 20*log10(h10);
     [H11z, w] = freqz(C3, 1, 256);
     h11 = abs(H11z);
     M11 = 20*log10(h11);
     plot(w/pi, M01,'-',w/pi, M01,'--',w/pi, M10,'--',w/pi,M11,'-');
     xlabel('\omega/\pi'); ylabel('Gain, dB');grid