1/a * y' + y = x
y' = a * (x - y)

Discretized in time, approximation by using the first two terms
of the Taylor series (-> explicit Euler method). Dt means
delta t.

y(n+1) = y(n) + Dt * y'(n)
y(n+1) = y(n) + Dt * a * (x(n) - y(n))
y(n+1) = (1 - Dt * a) * y(n) + Dt * a * x(n) 

For signal processing, Dt is usually simplified (normalized)
to 1. This is different from numeric simulation, where playing
with the step size might become important.

So the <b>difference</b> equation that approximates the
<b>differential</b> equation is:

y(n+1) = (1 - a) * y(n) + a * x(n)