Infinite Impulse Response Filter
A filter with more than 1 poles.
For any finite input signal (limited number of elements) the output of the filter could be see page 5, infinite due to the filter having feedback loops unlike a IIR-filter.
See [see page 6, here] for advantages vs disadvantages. Highlights:
- Depends on previous outputs of the filter (recursive-design/feedback-loops).
[see page 6, Examples] of IIR filters which all approximate the ideal Brickwall Filter include:
| Type | Description | Pole/Zero count |
|---|---|---|
| Butterworth | No ripples. | Many |
| Chebyshev (type 1) | Ripples at the highband (let through) region. | Less |
| Chebyshev (type 2) | Ripples at the lowband (block) region | Less |
| Elliptic/Cauer | Ripples at the both locations | Very few |
Warn: [see page 7, higher order] IIR filters can easily become unstable so we often implement them as a cascade of biquads sections.