DM365 BT.656 输入模式下一帧图像产生两次中断如何理解？

NOTE: In the case of BT.656 input mode, there is a VSYNC at the beginning of each field. Therefore,
there are two interrupts for each frame (i.e., one for each field).  关于这段话不知如何理解，还请各位指点一二，谢谢！