A single film frame of 1kHz used a guide to synchronize the sound and picture. The pop on the track negative creates a visual guide for the negative cutter who uses it to make a printing start mark. The pop occurs two seconds before the first frame of the picture, and thus corresponds to the “2” frame on the SMPTE Universal leader. On standard film leader, the number at the pop is “3” because it counts down in actual film footage.