Filtering without Phase Shift

The influences of phase shifts are extremely important in the determination of filtered waveforms and therefore in the interpretation of many biosignals. Low frequency shifts from high pass filtering, for example, are significant because large time displacements result from small phase deviations in the vicinity of the corner frequency. If a filter with a symmetric time response is employed, no phase shift will ensue. The time symmetry in this example is achieved with a tape recorder, first recording a high pass filtered signal and then playing back with time reversal to a second recorder through the high pass filter again. Real time processing is not achievable.