Apparatus, including a set of N electrodes (22), configured to be located in proximity to an epidermis (24) of a subject, and to acquire signals generated by electric sources within the subject. The apparatus also includes a set of M channels, configured to transfer the signals, where M is less than N, and a switch (40), configured to select, repetitively and randomly, M signals from the N electrodes and to direct the M signals to the M channels. The apparatus further includes a processor (28), configured to activate the switch, and to receive and analyze the M signals from the M channels so as to determine respective positions of the electric sources within the subject.