An instant replay system is a set of servers that records every camera at a sports ground all the time, and lets an operator play back any moment within seconds, often in slow motion. It shows the wicket, the close run-out or the goal again, from the best angle, before the next ball is bowled. It is also called a replay server or a slow-motion system.
How can it play back something that happened a few seconds ago? The system never stops recording. Each camera's feed goes into the replay server, which writes it to fast disks while it plays back other moments at the same time. Each frame is stored with its exact time, called a timecode. So the operator can jump straight to any moment, on any camera. Some cameras record at a very high frame rate, such as 150 or more frames per second instead of the normal 50. When those frames are played at normal speed, the action appears several times slower, but stays smooth.
Say a T20 match is being played at a stadium in Indore, and the broadcast uses 16 cameras. A batter is run out by a direct hit at the bowler's end. The replay operator has a control panel with a jog wheel, a dial that moves the recording frame by frame. Within two seconds of the throw, she has picked the camera square to the stumps. She marks an "in" point just before the ball hits the stumps, and an "out" point just after. She does the same on a high-frame-rate camera behind the bowler. When the director asks for the replay, she plays the first clip at full speed and the second at one-quarter speed. The two clips are on air about eight seconds after the run-out. The same frames are then made available to the third umpire, who studies them frame by frame to decide whether the batter is out.
The operators also save every important moment as a clip, with a label such as "run-out, over 14". Information from sports data, which records every ball, can be attached to each clip. At the end of the innings, the highlights package is made from these clips in minutes. The clips are also sent to the channel's app and social media pages. AI tools now find many of these moments automatically, for example by recognising every boundary in a match; see AI-generated content.
The replay system receives its camera feeds through the broadcast video router. In remote production, the replay servers and their operators often sit in the control room in another city, not at the ground. The replays then go into the programme that playout sends out. Replays matter so much because live sport is what viewers and advertisers pay most for, as described in sports media rights.