The frame number modulo 2 (for NTSC and SECAM) or modulo 4 (for PAL) should be preserved across cuts in order to avoid phase jumps in the chrominance subcarrier. Bits 35, 55, and 75 differ between 25 frame/s time code, and 30/29.97 frame/s.[1]:20[3] The bits are: Field flag (bit 35 for 29.97/30 frame/s, bit 75 for 25 frame/s): This is an additional least-significant bit for the frame number, distinguishing the two interlaced fields in one video frame. It is set to 0 during the first field of a frame, and to 1 during the second.