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The magic first down line - Jason Lindquist
Idle ramblings of an idle mind
The magic first down line
One of the world's greatest technological innovations is the television system that overlays a yellow (usually) line on the field showing the line-to-gain (the line the offense needs to cross to earn a first down.) This isn't easy because a) the cameras are not locked down, and are still free to pan, tilt, and zoom, and 2: the line is drawn in a way that it appears to be part of the field--it's not drawn across players or officials who are standing between the camera and a segment of line, it stops so it appears to be behind/beneath them.

Via slashdot, there's a video and a brief (undetailed) writeup describing the process. They capture the camera parameters exactly the way I speculated they do--rotary encoders on the pan and tilt axes, and they pull the zoom and focus data out of the lens hardware. What surprises me is how they move that data around. The camera parameters are relayed to the production truck not as raw digital data, but as an audio signal using a half-duplex modem. A computer on the receiving end decodes that data and does not draw the line. It encodes the parameters of how to draw the line as "dots at the top of the video frame", according to the video. In the analog NTSC world, this would be the Vertical Blanking Interval. I don't know what the deal is in HD.
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