In a normal NTSC video signal there are 525 "lines" of video signal. These are split into two half-images, known as "fields", sent every 60th of a second. These images merge on-screen, and in-eye, to form a single frame of video updated every 30th of a second. Each line of each field takes 63.5 μs to send; 50.3 μs of video and 13.2 μs amount of "dead time" on each end used to signal the television that the line is complete, known as the horizontal blanking interval (HBI). When the scanning process reaches the end of the screen it returns to the top during the vertical blanking interval (VBI), which, like the HBI requires some "dead time" to properly frame the signal on the screen. In this case the dead time is represented by unused lines of the picture signal, normally the top 22 lines of the frame.