Apparatus, which consists of a plurality of modules.Each of the modules has: an insulating frame, a pair ofelectrodes fixed to the frame at respective locationsthat are spaced apart, and circuitry configured toreceive signals from the pair of electrodes and inresponse output a differential signal. The apparatusfurther consists of an insertion tube having distal andproximal ends and containing the plurality of modules inlocations spaced longitudinally in proximity to thedistal end. There is cabling running through the tubethat is connected to convey differential signals from themodules to the proximal end.
