Apparatus, which consists of a plurality of modules. Each of the modules has: an insulating frame, a pair of electrodes fixed to the frame at respective locations that are spaced apart, and circuitry configured to receive signals from the pair of electrodes and in response output a differential signal. The apparatus further consists of an insertion tube having distal and proximal ends and containing the plurality of modules in locations spaced longitudinally in proximity to the distal end. There is cabling running through the tube that is connected to convey differential signals from the modules to the proximal end.
