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. While 94 percent sounds high, with approximately 300 pitches per game, that translates to about 18 pitches where human and machine disagree. If even one of those occurs in a pivotal at-bat, the impact on the game's outcome can be significant.
How TrackMan and Hawk-Eye Work
The two core ABS technologies differ fundamentally in their measurement principles. TrackMan uses Doppler radar to simultaneously measure a ball's spin rate, spin axis, velocity, and trajectory. It calculates speed from the frequency shift of radar waves reflected off the ball and determines three-dimensional position from multiple reflection points. Already installed in all 30 MLB stadiums, TrackMan also serves as the data backbone for Statcast. Hawk-Eye, originally developed for line-call assistance in tennis and cricket, is an optical tracking system. Twelve or more high-speed cameras positioned around the stadium capture the ball at over 340 frames per second, and the images are integrated to reconstruct the trajectory through triangulation. Hawk-Eye's strength lies in predicting trajectory changes caused by spin, accurately estimating the position of sharply breaking pitches like curveballs and sliders as they cross home plate. However, camera-based systems are sensitive to installation conditions, with recognition accuracy varying based on lighting and background colors. They perform reliably in domed stadiums but face challenges from changing sun angles during outdoor day games. If NPB pursues adoption, calibration tailored to each of its 12 stadiums' unique environmental characteristics will be essential.
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. Additionally, count-dependent tendencies exist: studies show umpires expand the zone on 3-0 counts and tighten it on 0-2 counts. ABS eliminates these psychological biases 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. NPB batters are said to exhibit even greater stance diversity than their MLB counterparts, and accommodating crouching styles or extreme open stances presents additional technical hurdles.
ABS Experiments Around the World
ABS experimentation extends well beyond MLB to professional leagues worldwide. The Korean KBO demonstrated ABS at its 2024 All-Star Game, where a fan poll showed 78 percent support for regular-season adoption. KBO has set a target of trial deployment in its minor leagues by 2026. Taiwan's CPBL installed TrackMan in all stadiums from 2023, using it as a post-game verification system for umpire accuracy, though real-time in-game application has not yet been implemented. Australia's ABL piloted a Hawk-Eye-based ABS at two stadiums in 2024, with umpires reporting reduced judgment pressure. In contrast, Mexico's Liga Mexicana has made little progress due to cost barriers, illustrating how economic disparities directly affect the pace of technology adoption. While these international results provide valuable reference data for NPB, factors unique to Japan, such as its high proportion of outdoor stadiums and distinctive cheering culture, mean the findings cannot be applied wholesale.
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. Annual maintenance and operating costs of 5 to 10 million yen per stadium add a non-trivial running expense. Outdoor stadiums face additional problems: rain droplets on camera lenses degrade accuracy, posing reliability concerns at roofless venues such as Koshien and Jingu Stadium. Koshien is particularly challenging because its coastal breeze deposits sand on lenses, necessitating an inter-inning cleaning protocol. 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. Employment implications for umpires are also unavoidable: full ABS adoption would fundamentally alter the home plate umpire's role, bringing discussions of staff reductions or reassignment to the table. The 2024 Owners' Meeting set a goal of trial deployment in the minor leagues by 2027, but a detailed roadmap has not been published.
Future Outlook - A Human-Machine Hybrid Model
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. Inspired by tennis's Hawk-Eye Challenge, this approach keeps human umpires as the default while deferring to machines only when a call is disputed. 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. Full machine judging also raises the question of game tempo: the human umpire's strike and ball calls are an integral part of a game's rhythm, and how the spectator experience would change if replaced by electronic tones or indicator lights remains unknown. Balancing technological progress with baseball's cultural traditions will require careful navigation from NPB.