How TV Creates the Yellow 1st Down Line

Of all the great innovations that TV has introduced to sports coverage, my vote would be for instant replay. No, not the instant replay that's being used across the sports spectrum to correct/confirm calls by the officials, but rather the concept of seeing something that just happened all over again. It seems as though the replay has been around forever, but during the Army-Navy game on Dec. 7, 1963, was the first televised sports game featuring instant replay.

One of the other great technological additions to TV coverage of football is the yellow, computer-generated first down line. When did that arrive? According to Sports Illustrated:

On September 27, 1998, Sportvision debuted its yellow first down marker on the ESPN broadcast of the Week 4 game between the Ravens and Bengals. For the first time fans watching at home could see the exact moment the ball crossed the plane.

So, how do they do it?