An anonymous reader shared this report from the Associated Press:

A computerized system that calls balls and strikes is being tested during Major League Baseball spring training exhibition games starting Thursday after four years of experiments in the minor leagues. Baseball Commissioner Rob Manfred is an advocate of the Automated Ball-Strike System, which potentially as early as 2026 could be used to aid MLB home plate umpires, but not replace them...

Stadiums are outfitted with cameras that track each pitch and judge whether it crossed home plate within the strike zone. In early testing, umpires wore ear buds and would hear "ball" or "strike," then relay that to players and fans with traditional hand signals. The challenge system adds a wrinkle. During spring training, human umps will call every pitch, but each team will have the ability to challenge two calls per game, with no additions for extra innings. A team retains its challenge if successful, similar to the regulations for big league teams with video reviews, which were first used for home run calls in August 2008 and widely expanded to many calls for the 2014 season.
Only a batter, pitcher or catcher may challenge a call, signaling with the tap of a helmet or cap; and assistance from the dugout is not allowed. A challenge must be made within 2 seconds... MLB has installed the system in 13 spring training ballparks that are home to 19 teams.

After a full season of testing in the Triple-A minor league, roughly 51% of the challenges were successful. Interestingly, the system makes its call exactly halfway across home plate> , where human umpires consider the strike zone to cover the whole 17 inches from the front to the back of home plate.

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