Analog filtering can produce distortion of signals by shifting phase. The signal-to-noise ratio achieved by averaging a small number of signals buried in noise will be improved greatly if that signal ensemble is averaged upon itself so that the time of signal onset varies over a predetermined series of intervals (interference filtering). The result of interference filtering is similar to the one obtained by sharp bandpass filtering, but provides zero phase shift. Transfer functions were constructed for first, second and third order interference filters. Effectiveness of interference filtering is demonstrated with synthesized signals and biological data. Distortion of signal shape and time characteristics were demonstrated in biological data where signal-to-noise ratio had been improved by analog frequency filtering. ?? 1973.
Dawson, W. W., & Doddington, H. W. (1973). Phase distortion of biological signals: Extraction of signal from noise without phase error. Electroencephalography and Clinical Neurophysiology, 34(2), 207–211. https://doi.org/10.1016/0013-4694(73)90051-5