in duration and have relatively diffused waveform transitions this is clearly shown in fig2 a and 2 b by comparison of the respective signals accordingly it is advantageous to pre process the near field ventricular waveform before it is used to improve the correlation of the noise reference signal to the expected far field interference one way to achieve this is by using a fixed coefficient fir filter of suitable design in conducting simulations of this technique the inventors have noted that a simple moving average filter can significantly improve the interference reduction advanced implementations may employ other pre processing methods using adaptive but slowly varying coefficients as appropriate for these reasons the waveform shown in fig2 b is not the actual near field ventricular signal but is instead a first estimate of the far field interference derived from the actual near field ventricular signal turning now to fig2 c the benefit of using the invention in the waveforms of fig2 a and the derived waveform of fig2 b is illustrated as a post processed atrial signal that is an the output signal of the filter block of fig1 this post processed atrial signal was generated using a matlab simulink system simulation described in more detail below finally fig2 d shows the enabling signal used in the matlab simulink simulation for lms filter adaptation this signal is triggered from the near field ventricular iegm signal which as previously noted is not shown while the preceding example shows the.