I am taking some of my OLD DVDs of the OLD TV show Buck Rogers  from the  70's  and puting just some of them into  MP4  on my computer.

If I go to convert them and go to  Interlace them   the  MP4  output screen is split  the top half of screen has one video and bottum has same video and the both play.

So if I pick to De Interlace the  output  mp4  video does the same thing.

But if I pick to have  no  interlace   and no   de interlace   the output  mp4  video is ok.

I just want to know why?http://en.wikipedia.org/wiki/Interlaced_video

Essentially, with interlacing enabled, the video is playing half of two different frames at once to make the video stay at its native framerate, but with half the BANDWIDTH being used. It can also be used to double the framerate and leave the data rate unchanged. This is helpful in reducing the amount of data being sent or stored, or increasing the apparent framerate without increasing the bandwidth used. However, it doesn't seem the output is being RENDERED properly by whatever program you're using, or the MP4 encoder isn't encoding the interlaced frames correctly.I have NTSC  DVDs so I use these setings to convert to MP4

Resalotion 640x480
Frame Rate  29.97 FPS
Aspect Ratio 4:3
Audio 44.1 kHz 128 kbps
De Interlace Set to NO
Interlace Set to NO

They all comeout GOOD and play nice.  But if I set Interlace to YES  they play in split screen one image on top half and one image on bottum half scree.