and ch b reach the predetermined phase difference this removes the influences caused by the phase error the phase error compensation parameter is 1 u2212 2c 2 2c 2 in step 230 the width error compensation parameter and the phase error compensation error obtained in step 220 are used to adjust the encoder output signals in order to eliminate the width and phase errors in step 240 the printing apparatus controls a printing process according to the adjusted encoder output signals since the width and phase errors in the encoder output signals are already compensated by the disclosed method the high frequency bandings can be effectively avoided in the printed images thus the invention helps improving the printed picture quality from the above description it is clear that when there are errors in multiple encoder output signals the disclosed encoder signal compensation method can perform width error compensations for the signals in individual channels afterwards phase compensations are performed according to the phase differences in different channels consequently the disclosed encoder signal compensation method is not limited to the use of a quadrature encoder any multiple encoder can be used in the disclosed method without departing from the spirit of the invention.