sequences are started at each 1 v period since the signal pg appears at this period during the steady state operation the control unit 60 has a timer adapted to count a period of 270 h the timer may be implemented in software or hardware adapted to count the number of reference clock signals supplied from the reference generator 62 for counting the period equal to 270 h in terms of the number of these clock signals the 270 h timer is decremented at each 1 h period and upon the issuance of the signal pg step 300 the counting operation ceases step 302 the control unit 60 operates to store a prevailing count in the memory step 304 reset the 270 h timer to its initial state or 270 h step 306 and to restart the decrementing operation at each 1 h period step 306 the control unit 60 reads out the count just stored in the memory step 308 to check whether the value falls within the range of about 7 h as its center or median value with a predetermined allowance for example of u00b1 5 h step 310 since the 1 v period is equal to 262 5 h in the present system it is checked in the decision box 310 whether the time elapsed since the preceding pg signal until the current pg signal is about equal to 1 v that is within the range of 270 h u00b1 5 h it should be noted that since the vertical sync signals vsync 506 included in the video signals to be sensed by magnetic head 26 are recorded during steady state operation with a phase.