Before 2010, the umpire was stationed at all times behind the defensive line, allowing the league to put a microphone on the umpire so that the sounds of the trenches could be added to the audio of the broadcast.
Starting this year -- and specifically this week -- every team will be required to put microphones on at least two offensive linemen.
The memo to all teams, a copy of which PFT has obtained, explains that as of Week Four all teams must allow the starting center and backup center, or both starting guards, to be wired for sound by NFL Films technicians. The reason for wiring both guards is to ensure that, if one leaves the game, the other one will be wearing a microphone. (At any given time, only one microphone will be open.)
The microphones will be active only when the players are on the field of play, and not when they are in the locker room. Per the memo, microphones will open after the offense breaks the huddle, or as the offense approaches the line of scrimmage in the case of a no-huddle offense. Then, microphones will close “a few seconds” after the impact of the offensive and defensive lines following the snap of the ball.
Microphones will not be open when the players are in the huddle or in the team bench areas, and sound captured will be used only in the “live ambient audio mix” during the game. As a result, the TV networks will be prohibited from isolating and recording (and in turn playing back) the audio at any time.