y(n) = (1 - a) * y(n-1) + a * x(n-1)
transformed into z domain:
Y = (1 - a) * z^-1 * Y + a * X
Y + (a - 1) * Y * z^-1 = a * X
Y * (1 + (a - 1) * z^-1) = a * X
Y / X = a / (1 + (a - 1) * z^-1)