filters - Why is a linear phase important?

If symmetry conditions are met, FIR filters have a linear phase. This is not true for IIR filters.

However, for what applications is it bad to apply filters that do not have this property and what would be the negative effect?

Answer

A linear phase filter will preserve the waveshape of the signal or component of the input signal (to the extent that's possible, given that some frequencies will be changed in amplitude by the action of the filter).

This could be important in several domains:

1. 
   

   coherent signal processing and demodulation, where the waveshape is important because a thresholding decision must be made on the waveshape (possibly in quadrature space, and with many thresholds, e.g. 128 QAM modulation), in order to decide whether a received signal represented a "1" or "0". Therefore, preserving or recovering the originally transmitted waveshape is of utmost importance, else wrong thresholding decisions will be made, which would represent a bit error in the communications system.

   
   


2. 
   

   radar signal processing, where the waveshape of a returned radar signal might contain important information about the target's properties

   
   



3. 
   

   audio processing, where some believe (although many dispute the importance) that "time aligning" the different components of a complex waveshape is important for reproducing or maintaining subtle qualities of the listening experience (like the "stereo image", and the like)