A pixel density correction device for processing a signal containing pixel density values conveyed to a printing head of an electrocoagulation printing apparatus that includes a plurality of simultaneously addressable electrodes. The pixel density correction device of the invention comprises an input for receiving the signal representative of pixel density values associated with the simultaneously addressable electrodes, and a processing element for altering a pixel density value of a selected one of the simultaneously addressable electrodes, the signal processing element being responsive to pixel density values associated with electrodes other than the selected electrode to determine a corrected pixel density value associated with the selected electrode.