n u2212 1 th field data d n u2212 1 that is asl n u2212 1 is determined by the automatic power controller 120 at the point of time when the input of n u2212 1 th field data d n u2212 1 is completed asl n that is the asl of the nth field data d n and asl n 1 that is the asl of the n 1 th field data d n 1 are determined by the method used for determining the asl n u2212 1 the asl controller 130 from fig5 compares the number of sub fields that is determined by the asl of previous field data with the number of sub fields that is determined by the asl of the current field and outputs the number of sub fields that is determined by the asl of the previous field data when the number of sub fields that is determined by the asl of the previous field data is not equal to the number of sub fields that is determined by the asl of the current field therefore in fig6 when it is assumed that the number of sub fields that is determined by the asl of the n u2212.