In an image forming device an apparatus is provided that supplies a continuous flow of an electrical charge between a photoconductive drum and a power supply. This is achieved by analyzing each of the pieces that make up the tesserae according to a large number of parameters, including the existence of marks and their position in the pieces, and assigning a storage location to each of them. A bit error test unit receives the sampled comparator output signal and determine therefrom an indication of a bit error in a sequence of the sampled comparator output signal.