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Interlacing

Back in the 1930s, television faced two technical problems: bandwidth to broadcast video and sound, and the slow decay rate of phosphors in the cathode ray tubes. If the phosphors weren’t given time to lose their brightness, the image would smear where light and dark edges meet.

To correct these problems, both American (NTSC) and European (PAL) designers hit upon the same solution: interlaced video, or the broadcast and display of only half of every frame, alternating between the odd and even lines. Interlacing allows NTSC video to effectively broadcast at 60 frames per second (50 for PAL), but each frame contains only half the total image. Fast movement is observed at 60 frames per second while the bandwidth bill is cut in half.

If you have content originally destined for TV, it is quite likely interlaced. Interlaced video looks fine when displayed on a TV from eight feet away, but on a computer it looks terrible. Computer displays are designed for fast refresh from a short distance; at this range, the eye sees the flicker as rows illuminate and fade to black. For viewing on a computer, we want to recombine the lines into a full-scan, or progressive, frame.

One solution is to combine consecutive frames’ alternating lines into a non-interlaced video that, on your computer, isn’t obviously interlaced. It may not look substantially better, however, as the two consecutive frames do not represent a single snapshot in time. Therefore, anything in motion is in a different location within each of the two frames, and the combination (merging them to a single point in time) looks ragged. Some video software interpolates the new frame by compensating for motion between the interlaced frames; frames are combined when when there is little motion, and interpolated when there is. Interpolating eliminates the ragged look in favor of a chunky and distored frame. Better, but not great.

Most DVDs with content designed for the theatre has a progressive, or non-interlaced, display. Your ripping software may provide an option to deinterlace a progressive video stream; this process blurs and muddles the image, similar to the interpolated sample above. Before you decide to de-interlace, rip an unaltered section and look at the results. If the interlacing is apparent, then choose to de-interlace. Otherwise, leave it be.