Video by onelonedork. To see more check out his youtube channel here.
What he’s done is load a MP3 player with a 19k tone ( witch is basically inaudible ) and has it run the length of the video. The mixer is used to mix evenly one of the Left or right stereo channels with the other being his lapel mic. Scene the camera’s AGC will be the only one that can hear the 19k tone the other channel sound nice and clean as the camera adjusts the Automatic Gain Control to a set level. Nice.