What Is Automated Ball-Strike Judging
The Automated Ball-Strike System (ABS) uses high-precision cameras or radar to measure ball trajectories in real time and mechanically determine whether each pitch is a strike or ball. MLB first tested ABS in the independent Atlantic League in 2019 and expanded it to all AAA minor-league parks by 2023. Two main technologies are used: TrackMan, a Doppler-radar system tracking ball position at 2,000 frames per second with a stated margin of error within 0.5 inches (about 1.27 cm), and Hawk-Eye, an optical system using 12 or more high-speed cameras for triangulation with 3.6 mm accuracy. NPB conducted an unofficial TrackMan test during the 2022 Phoenix League, reporting roughly 94 percent agreement with human umpire calls.
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Defining the Strike Zone and Why Machines Struggle
The Official Baseball Rules define the strike zone as the area over home plate between the midpoint of the batter's shoulders and the top of the uniform pants (upper limit) and the hollow beneath the kneecap (lower limit). In practice, the zone shifts with every pitch depending on the batter's height, stance, and swing motion. MLB research found human umpires tend to call low pitches as strikes, with roughly 12 percent incorrect calls at the bottom edge of the zone. ABS eliminates this bias but must calculate the upper and lower bounds in real time for each batter, making skeletal-estimation AI accuracy a key challenge. During the 2023 AAA trial, zone settings became unstable for batters with extremely low stances, requiring two mid-season calibration corrections.
Technical and Institutional Hurdles for NPB Adoption
Introducing ABS to NPB would require installing standardized measurement equipment across all 12 stadiums. A single TrackMan unit costs roughly 30 million yen, while a full Hawk-Eye system runs about 100 million yen per venue, putting total initial investment between 360 million and 1.2 billion yen for all parks. Outdoor stadiums face additional problems: rain droplets on camera lenses degrade accuracy, posing reliability concerns at roofless venues such as Koshien and Jingu Stadium. On the regulatory side, the current NPB Agreement's provisions on umpire authority would need amendment, and negotiations with the Japan Professional Baseball Umpires Association would be essential. The 2024 Owners' Meeting set a goal of trial deployment in the minor leagues by 2027, but a detailed roadmap has not been published.
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Future Outlook and Global Trends
MLB is considering phased ABS introduction at the major-league level from the 2025 season, with a "challenge system" allowing batters and catchers to request a machine review up to three times per game as the leading proposal. The Korean KBO also demonstrated ABS at its 2024 All-Star Game. NPB will need to study operational results from MLB and KBO while addressing Japan-specific issues such as outdoor-stadium reliability and umpire career-path design. Former umpire Natsuo Yamazaki has stated that "machine judging is not the enemy of umpires but can be a partner for improving call accuracy," and a human-machine hybrid model is attracting attention as the most realistic landing point.