One such case is if you are targeting hardware which has constrained the filter order to a specific number. There are many practical situations in which you must specify the filter order. FIR Lowpass Designs - Specifying the Filter Order However, the use of minimum-phase and multirate designs can result in FIR filters comparable to IIR filters in terms of group delay and computational efficiency. IIR filters also tend to have a shorter transient response and a smaller group delay. IIR filters are generally computationally more efficient in the sense that they can meet the design specifications with fewer coefficients than FIR filters.
IIR filters (in particular biquad filters) are used in applications (such as audio signal processing) where phase linearity is not a concern. FIR filters are also used in many high-speed implementations such as FPGAs or ASICs because they are suitable for pipelining. FIR filters also tend to be preferred for fixed-point implementations because they are typically more robust to quantization effects. You generally choose FIR filters when a linear phase response is important. When designing a lowpass filter, the first choice you make is whether to design an FIR or IIR filter.
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