The Interlacing Legacy
Interlacing in video goes back to the 1930s. The shorter story behind it is that when TV standards were being established they wanted to display 50-60 frames per second rather than the standard 24 frames per second of feature film. Live broadcasts would look much smoother this way, and since the technology at the time did not support such frame speeds, they used interlacing to record twice the amount of ‘frames’ by dividing each frame into two ‘fields’, each taken at different points in time, to combine into one frame per second. So 60 fields per second became 30 frames per second.
What Are Those Lines?
Each frame of recorded video (even modern digital videos, though it is more rare now) is actually two fields, which interlock like putting your fingers together. Using this metaphor, your left hand would be the image recorded in one moment of time and your right hand recorded at a different moment. If the scene were perfectly still, you wouldn’t see any difference between the two, but if there were fast movement, the difference would be obvious. Old tube-style monitors were built to play back interlaced footage properly so that your eyes did not perceive these interchanging fields, but modern displays do not. There are methods to de-interlace footage, but the best method is different for every video, and the algorithms are always improving. So leave them in their original, interlaced state when transferring footage. Many players and software will automatically de-interlace, so if you don’t see lines that is the reason.