Interlace

Interlace is a method of smoothening the video picture moving by having double the amount of frames than the video's fps value suggests.

Basically, how the interlacing is done, let's say in PAL format, is that each frame is split into two separate pictures, which both are missing half of the picture information. The split is done by its horizontal lines. The PAL video contains 576 horizontal lines -- let's say that the numbering of these lines begins from the top of the frame being the line number 1. The frame is being split to half-frame A and half-frame B -- the half-frame A would contain lines 1, 3, 5, 7, ... and the half-frame B would contain horizontal lines 2, 4, 6, 8, 10, ...

So, when the video is shown with PAL that has frame frequency (fps) of 25fps, there are 50 "half frames" per second that are being updated. First comes the first full frame's "half A" that fills horizontal lines 1, 3, 5, .. leaving other ones blank (assuming that we start from a "zero state" that output doesn't have anything shown before we start counting this). Then comes first full frame's "half B" that fills the missing lines 2, 4, 6, ... and leaves the existing "half A"'s lines showing as well -- now we have the frame no.1 fully shown on screen. Then comes frame no.2's "half A" that then wipes off the frame no.1's "half A", but leaves the frame no.1's "half B" showing. So, now we have a mixture where lines 1, 3, 5, 7, .. are being filled by frame no.2 and lines 2, 4, 6, .. are being filled by frame no.1.