PRODUCING CORRECTION DATA FOR PRINTER

Correction data is produced for density errors in prints produced using a printer. While printing a test image, the periods of rotation of one or more rotatable imaging members arranged along a receiver feed path in the printer are measured using respective period sensors. The printed test image is measured along a selected measurement direction and a reproduction error signal representing deviation from aim density is determined. For each period sensor, the autocorrelation of the reproduction error signal for the corresponding period is determined. If the determined autocorrelation exceeds a selected threshold, the reproduction error signal is decomposed at the corresponding period to extract the variation from the measured component. The remaining error signal is separated by frequency terms. The variations from the data at measured periods and the remaining error signal are used to produce a correction signal.