Maybe a tone in the umpire's ear if the ball is out of the zone (or a tone if it's in and a buzzer noise if it's not). Then the ump can decide if he thinks it caught a part of the zone or something.
But no reviews. I like the review system we have now - although it takes WAY to long - but more reviews would interrupt the pace of play too much.
The players are the human element in the game.
No it wouldn't. If you do it like tennis, it would take 15 seconds, tops. There's virtually zero interruption into the flow of the game in tennis.
If you're buzzing the umps on every pitch, then just make the complete jump. Don't tell them that "technology says it's a ball or strike but in the end it's up to you". That's a bad combo.
Poll
