Used in television and film post-production, a 2-pop is a 1 kHz tone that is one frame long and placed 2 seconds before the start of program. For example, in a video program the first frame of action (FFOA) starts at one hour (typically timecode of 01:00:00:00 in the US, and 10:00:00:00 in the UK), preceding that, 1 frame (or the 2-pop) of tone would be placed at timecode 00:59:58:00 or exactly 2 seconds before first picture. However, in a 35 MM film print, the 2-pop is placed at the 01:00:06:00 mark (or nine feet of film) and then the first frame of action (FFOA) would begin at 01:00:08:00 (or 12 feet.)
Whereas laying down bars and tone prior to program start establishes video and audio calibration levels on the tape , the 2-pop is primarily used for picture and sound synchronization. Therefore, while the loudness of the 2-pop may be the same as the bars and tone audio level in use, this is not a requirement. The loudness level should be sufficient to be heard clearly.
Associated at the same point in the timeline with the audible 2-pop is a visual flash-frame. Together, the 2-pop and the flash-frame, are used to synchronize the audio and video tracks when more convenient methods are unavailable. Although not typically used in modern NLE outputs, a 2-pop may still be used on a digital output when there is an expectation that the media might be later transferred to a format where a manual sync point would be useful. This is similar in function to the way a film clapperboard is used to synchronize audio and visual footage.
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