http://www.nfl.com/news/story?id=09000d5d80783ca4&template=with-video&confirm=true One defensive player will wear a helmet similar to what the quarterback is allowed on offense. Should that player leave the game, another player can be designated to also have the device. But only one defender with the device can be on the field at a time. At least we won't need to worry so much about defensive signals, now.
