Interlace Gone Wrong
UPDATE: Since first posting this page, I've figured out that the problem is probably caused by an interruption in TV reception or recording. If the data that is lost happens to include the metadata that sets the frame dominance, the TV set can incorrectly interpret the video it receives after the interruption.
As technology gets more pervasive, life gets more complicated and there are more ways for things to go wrong. Here's an example, taken from a TV broadcast of the 2009 NHK Gran Prix of figure skating.
All through the men's short programs, moving objects flickered and looked double-exposed— not the best way to watch a skating competition. To explain what happened, I have to talk about two aspects of video:
(1) interlaced video, and
(2) field dominance.
Interlaced Video
Here's a detail showing the video interlace. Because the skater was moving, his image is displaced from one field to the next. In this case, the displacement is much larger than normal; the next part explains why. (If you'd like to read more about interlaced video, you can click here.)
Field Dominance
Field dominance determines the order in which the two fields of each frame are displayed. The pictures below show the individual fields separately so you can see what's going on. (Frames are labeled 1 and 2; fields are labeled a and b.)
When everything is working properly, the fields follow each other in the order in which they were scanned, as shown in the following sequence. You can see that the skater's right hand is moving a little farther to the viewer's right from one field to the next.
The next sequence shows how the video actually came in: The field dominance is reversed. If you look closely at the skater's right hand, you can see it move a little to the left, then jump to the right, then move to the left again. The flicker is caused by these reversals of direction and big jumps; the de-interlacing algorithm in the TV can't cope with them.
Last updated on 1/27/2012
Copyright © Allen Watson III