On the court at the U.S. Open, player challenges have now become as common as the close calls that invite them. Thanks to the Chase Review system, which utilizes Hawk-Eye technology, those challenges are quickly resolved, match after match. Just how does the review system work, and work so quickly? The secret lies in the upper broadcast booth at Arthur Ashe stadium, where the main Hawk-Eye system is located.
Ten JAI 1405 cameras placed around the center court at Ashe record every movement the ball makes at 52 frames per second. With all of the high-speed, super slo mo cameras available today, why just 52?
“There’s a tradeoff between having a bigger image with a lower frame rate and a smaller image with a higher frame rate,” explains Luke Aggis, tennis operations manager for Hawk-Eye. “In order for the field of view to be great enough to justify seeing the whole court, that’s a number that we’ve come to agree on.”
The black-and-white cameras compare each frame to the proceeding frame, down to the pixel level. A noise threshold filters out the movement of non-tennis ball items in the shot (a player’s shirt, or a shadow on the court, for example).
“Our software looks at every single pixel within that image, and if it’s changed on the grayscale level, it will compare on different parameters whether the pixel next to it has also changed,” Aggis explains. If the adjacent pixel did not change, chances are the object is a reflection, not a tennis ball; likewise if 1000 adjacent pixels changed, chances are the object is too large to be a tennis ball.
At 52 frames per second, the probability of the Hawk-Eye system actually capturing the moment the ball hits the ground is quite slim. To increase the odds and draw a better picture, the system takes the feed from all five cameras positioned on the relevant side of the court and draws a best-fit line of the ball trajectory, utilizing all five available readings. If the system is unhappy with the answer it provides – if the match is doubles and both players happen to have blocked the path of the ball – the system automatically tells the operator, and a graphic is sent to the scoreboard saying that the original call stands. Of the 500-plus player challenges that have taken place thus far at the U.S. Open, only one was unavailable for review.
The system is accurate up to 3.6 mm. “Coming down from a 150-mile-per-hour serve, that’s pretty good going,” Aggis says, “though we’re always trying to improve that.”
The company is also trying to improve the setup time of the equipment. While the six-hour calibration time from two years ago has been reduced to an hour or two, Aggis would prefer to reduce that time even further.
All 10 cameras are connected to their own rack-mounted PC that travels thousands of miles around the world every year (the U.S. Open system will be on its way to Moscow following the conclusion of Sunday’s final). The system is reinstalled annually at each Grand Slam event – this is the fourth year it has been used at the U.S. Open – but the cabling for the cameras is now a permanent fixture in the venue.