by convention voltages measured at the right arm left arm and left leg is known as the wilson central terminal wct the wct was initially described in wilson n f johnston f e macleod a g barker p s u201c electrocardiograms that represent the potential variations of a single electrode u201d am heart journal 1934 9 447 458 whose disclosure is incorporated herein by reference the wct is often used as a reference voltage for the v1 v6 signals and is therefore available for additional signal processing line interference that is induced in the body extremities is typically prevalent in the wct ecg processor 28 exploits this aspect of the wct to determine the line interference frequency as described further hereinbelow fig2 is a block diagram that schematically shows elements of ecg processor 28 in accordance with an embodiment of the present invention ecg processor 28 may comprise a general purpose computer running any suitable operating system with suitable input interfaces not shown for receiving ecg signals and software for performing the processing functions that are described hereinbelow this software may be downloaded to the processor in electronic form over a network for example or may be stored on tangible media such as optical magnetic or electronic memory media alternatively or additionally the ecg processor may comprise a special purpose processing device such as a programmable signal processor or a customized hardware control unit elements of ecg processor 28 may also be implemented as several separate processing devices in an embodiment of the present invention a raw ecg signal such as the avr.