An Infinite Impulse Response (IIR) filter produces an output, y(n), that is the weighted sum of the current and past inputs, x(n), and past outputs.
The Linear Predictive model is a specical case of an IIR filter and shown in figure 7.
The general IIR filter (figure 8) is given by:
Figure 8: The general linear filter
If p = 0 then the system represents a finite impulse response (FIR) filter. If p is not zero, then the system is an infinite impulse response (IIR) filter.
An example is the two pole resonator with center frequency 
and
bandwidth related to r is:
Common types of IIR filter:
For more details see [4].
