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If you attend a Super Bowl party on Sunday, youll probably hear at least one casual football viewer ask, How do they get that yellow first-down line on the field? While magic is a fine answer in its own right, the real explanation is a bit more technologically intense. Lets have a look at the background and mechanics behind every football fans shining beacon: the yellow first-down line. According to Allen St. Johns 2009 book
The Billion Dollar Game: Behind the Scenes of the Greatest Day in American Sport - Super Bowl Sunday, the first-down line actually emerged from the ashes of one of sports broadcastings bigger debacles: the FoxTrax system for hockey, which was designed by a company called Sportvision. which hockey fans no doubt remember as the much-maligned technopuck that debuted in 1996employed a system of cameras and sensors around a hockey rink to place a little blue halo around the puck. FoxTrax wasn't a great fit for NHL broadcasts: Hockey purists hated the intrusion into their game, and casual fans didnt flock to hockey just because the puck was suddenly easier to follow. However, the system inspired producers to think of new ways to insert computerized images into live sports broadcasts. The idea of using a line to mark the first down in football was a natural extension, and Sportvision debuted its during ESPNs broadcast of a Bengals-Ravens tilt on September 27, 1998.

A couple of months later, rival company Princeton Video Image unveiled its Yellow Down Line system during a Steelers-Lions broadcast on CBS. (Sportvision is still kicking, and ESPN acquired all of PVIs intellectual property in December 2010. ) BUT HOW DOES IT WORK? It takes lots of computers, sensors, and smart technicians to make this little yellow line happen. Long before the game begins, technicians make a digital 3D model of the field, including all of the yard lines. While a football field may look flat to the naked eye, its actually subtly curved with a crown in the middle to help rainwater flow away. Each field has its own unique contours, so before the season begins, broadcasters need to get a 3D model of each stadiums field. These models of the field help sidestep the rest of the technological challenges inherent to putting a line on the field. On game day, each camera used in the broadcast contains sensors that record its location, tilt, pan, and zoom and transmit this data to the networks graphics truck in the stadiums parking lot. These readings allow the computers in the truck to process exactly where each camera is within the 3D model and the perspective of each camera. (According to, the computers recalculate the perspective 30 times per second as the camera moves. ) After they get their hands on all of this information, the folks in the graphics truck know where to put the first-down line, but thats only part of the task.

When you watch a football game on television, youll notice that the first-down line appears to actually be painted on the field; if a player or official crosses the line, he doesnt turn yellow. Instead, it looks like the players cleat is positioned on top of an actual painted line. This effect is fairly straightforward, but its difficult to achieve. To integrate the line onto the field of play, the technicians and their computers put together two separate color palettes before each game. One palette contains the colorsusually greens and brownsthat naturally occur on the fields turf. These colors will automatically be converted into yellow when the line is drawn on to the field. All of the other colors that could show up on the fieldthings like uniforms, shoes, footballs, and penalty flagsgo into a separate palette. Colors that appear on this second palette are never converted into yellow when the first-down line is drawn. Thus, if a players foot is situated on the line, everything around his cleat will turn yellow, but the cleat itself will remain black. According to How Stuff Works, this drawing/colorizing process refreshes 60 times per second. All this technologyand the people needed to run itwasnt cheap at first.

It could cost broadcasters anywhere from $25,000 to $30,000 per game to put the yellow line on the field. Sportvision had to deploy a truck and a four-man crew with five racks of equipment. The cost has come down since then, and the process is now less labor-intensive. One technician using one or two computers can run the system, according to Sportvision, and some games can even be done without anyone actually at the venue. Now you can explain it to everyone at your Super Bowl party during one of the less-exciting. Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at. This post originally appeared in 2011. Asked by: Charlie Gregson, by email Sound travels faster through a lighter gas than a heavier one because the individual molecules have less mass and so can move more quickly in response to the pressure changes of the sound wave. The speed of sound in helium is almost three times faster than in air and this changes the resonant frequency of your throat so that high frequencies sound louder than low ones. If you inhale a gas that is denser than air, such as sulphur hexafluoride, the sound travels at just 39 per cent of its speed in air and your voice sounds deeper. P Subscribe to for fascinating new Q As every month and follow on Twitter for your daily dose of fun facts.

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