Use the Interlace tool to convert a sequence of progressive frames to an output of interlaced video frames. Each pair of input frames result in one output frame.
To convert progressive frames to interlaced frames:
The output is always at the same frame rate as the input, and the action will play twice as fast. You can always use a Rate Convert node before the Interlace if needed. The following table shows how frames are interlaced. The input frames are numbered consecutively from 1 as shown in the top part of the table. Each output frame shows two numbers, which indicates the input frame indices for field 1 and field 2 respectively.