525/60 refers to a NTSC analog TV signal. Although NTSC only has 486 horizontal lines in its Active Area, a NTSC video signal also includes extra vertical sync lines, which are basically just filler timed to allow the electron guns in a CRT television time to reposition themselves at the top of the screen. There are 39 vertical sync lines, resulting in a total of 525 lines (486 + 39).

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