A system for performing an electrocardiograph, ECG, the system comprising: a plurality of leads configured to be attached to a subject in order to capture electric signals; a signal processor configured to process the captured electrical signals by: applying a comb filter to the captured electrical signals to emphasize interference at a plurality of selected frequencies; estimating an amplitude and a phase of the applied filtered signals; performing a first stage of processing the applied filtered signals to identify power line interference features using feature detection; performing a second stage of processing the applied filtered signals to determine if the performed first stage misdetected a feature to further identify power line interference features using feature detection; estimating interference using the identified features, amplitude and phase; and subtracting the estimated interference from the signal to generate a signal substantially free of power line interference; and an output device (1665; 1670; 1660) configured to output the processed captured electrical signals. a plurality of leads configured to be attached to a subject in order to capture electric signals; a signal processor configured to process the captured electrical signals by: applying a comb filter to the captured electrical signals to emphasize interference at a plurality of selected frequencies; estimating an amplitude and a phase of the applied filtered signals; performing a first stage of processing the applied filtered signals to identify power line interference features using feature detection; performing a second stage of processing the applied filtered signals to determine if the performed first stage misdetected a feature to further identify power line interference features using feature detection; estimating interference using the identified features, amplitude and phase; and subtracting the estimated interference from the signal to generate a signal substantially free of power line interference; and applying a comb filter to the captured electrical signals to emphasize interference at a plurality of selected frequencies; estimating an amplitude and a phase of the applied filtered signals; performing a first stage of processing the applied filtered signals to identify power line interference features using feature detection; performing a second stage of processing the applied filtered signals to determine if the performed first stage misdetected a feature to further identify power line interference features using feature detection; performing a first stage of processing the applied filtered signals to identify power line interference features using feature detection; performing a second stage of processing the applied filtered signals to determine if the performed first stage misdetected a feature to further identify power line interference features using feature detection; estimating interference using the identified features, amplitude and phase; and subtracting the estimated interference from the signal to generate a signal substantially free of power line interference; and an output device (1665; 1670; 1660) configured to output the processed captured electrical signals.