Use of notch filters in signal averaged ECG applications is avoided because they introduce ringing following the QRS. The authors therefore studied the use of a line interference subtraction (LIS) filter by which a scaled and shifted version of a signal at line frequency (line reference signal, LRS) is subtracted from the ECG signal in order to reduce the level of line interference. The frequency of the LRS must be the same as that of the line interference. For this, hardware is developed such that line frequency common mode interference is recorded simultaneously with the ECG. When applied to late potential detection it is seen that line interference is completely removed and no additional noise is introduced so that Simson's criteria derived from the vector magnitude are not changed. Similarly it is shown that despite high level of line interference notchy details of signal averaged P waves are completely recovered
