% Program 10_9
% Frequency Responses of Johnston's QMF Filters
% 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
%  Compute the gain responses of the two filters
[H1z, w] = freqz(B1, 1, 256);
h1 = abs(H1z); g1 = 20*log10(h1);
[H2z, w] = freqz(B2, 1, 256);
h2 = abs(H2z); g2 = 20*log10(h2);
%  Plot the gain responses of the two filters
plot(w/pi, g1,'-',w/pi, g2,'--');grid
xlabel('\omega/\pi'); ylabel('Gain, dB')
pause
% Compute the sum of the squared-magnitude responses
for i = 1:256,
   sum(i) = (h1(i)*h1(i)) + (h2(i)*h2(i));
end
d =10*log10(sum);
% Plot the amplitude distortion
plot(w/pi,d);grid;
xlabel('\omega/\pi'); ylabel('Amplitude distortion, dB')