Disclosed are methods of controlling operation of a printer, apparatus and computer-readable medium. The first delay line receives a first pulse and a first delay selection signal, and delays the first pulse for a first delay period according to the first delay selection signal to output a first delayed pulse. If the extracted signal is a bullet falling point signal, a control circuit then uses the horizontal synchronous signal H_sync and the vertical synchronous signal V_sync of the video frequency signal to compute the coordinates of the bullet.