Here we use the LPC function and an FIR filter simply to come up with parameters we will use to create the autoregressive signal we will work with. The use of FIR1 and LPC are not critical here. For example, we could replace d with something as simple as [1 1/2 1/3 1/4 1/5 1/6 1/7 1/8] and p0 with something like 1e-6. But the shape of this filter is nicer so we use it instead.