I applied the following FIR comb filter in real-time:
Since this is an FIR, the group delay is D=(N-1)/2=20 samples. After applying the filter to a signal, I tried to use cross correlation between the filtered and unfiltered signal, to reproduce D computationally by determining the argmax of the cross correlation (I do have a need to the delay this way). The issue is that I get too peaks in the cross correlation, one at zero lag and another at 20 lag. But the peak at zero lag is the maxima which means the peak at 20 lag which is the correct lag is ignored. This method work really well with other filters like averaging filters.
Does anyone know while I get a the peak at zero which is overshadowing the real peak? Is this normal for comb filters? Is there another method to compute delays using the filtered and unfiltered signal other than cross correlation?