function downconvert()
omega_lp = 0.48;
N_lp = 101;

N = 10000;
w = 2*pi;
n = (0:N-1);

rf = 0.4*cos(0.25*w*n);
lo_r = cos(0.25*w*n);
lo_i = -sin(0.25*w*n);
w_lp = FIRCalcLowpass(omega_lp, N_lp);

im = 2*rf .*lo_r + i*2*rf.*lo_i;
im_f = filter(w_lp, 1, real(im)) + i*filter(w_lp, 1, imag(im));

subplot(2, 1, 1)
plot(n, rf, '-*'); grid;
axis ([0 N-1 -1 1]);

subplot(2, 1, 2)
plot(n, real(im_f), '-*', n, imag(im_f), '-*'); grid;
axis ([0 N-1 -1 1]);

wavwrite(rf, 48000, 16, 'ddc_rf.wav');
wavwrite(im, 48000, 16, 'ddc_im.wav');
wavwrite(im_f, 48000, 16, 'ddc_imf.wav');