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http://en.wikipedia.org/wiki/Pulse_wave

A pulse wave form can simple be constructed by adding two saw waves; the phase difference will control the pulse width.

        double pw = pulseWidth / 2. + 1.;
        double y1 = ((int)(f * i) % n) / n1;
        double y2 = ((int)(f * i + pw) % n) / n1;
        double y = (y1 - y2) * (1 + pulseWidth / n1);

http://en.wikipedia.org/wiki/Comb_filter

The frequency response of a discrete-time system expressed in the Z domain is obtained by making the substitution z=exp(j*omega)

        double omega = 2 * pi * f/n;
        double y = sqrt((1. + pow(alpha, 2.)) + 2 * alpha * cos(omega * K)) / 2;