ABSTRACT
Signals may contain many frequency components. Some of them may be of interest like frequency bands of EEG rhythms; others like power line artifacts may be undesired. To remove the unwanted part of spectral contents of the signal one usually uses filters. In order to be implementable the filter must be causal. That is, the filter response must only depend on the current and past inputs. If the filter has to depend also on the past outputs it still can be causal by introducing the feedback with appropriate delays.
