The Vertical Blanking Interval (VBI)
Even though there are 525 scan lines in a television frame, not all of those lines are visible. The electron beam that travels left to right all the way down the screen must have time to return to the top of the screen after drawing each field. The time allocated to travel from the bottom to the top of the screen is called the vertical blanking interval (VBI), and is equivalent to the time required to draw 42 lines.
Of course, the television broadcaster does not stop transmission while the electron beam travels back to the top of the screen. This means that the television signal sent for the top 42 lines of each television frame cannot carry information to be used for the picture. This leaves 483 (525 minus 42) visible lines of vertical resolution. Since TVs (like computer monitors) are wider than they are high, this yields roughly 640 pixels of horizontal resolution, or a 640 x 480 screen resolution
I also found this short:
Vertical Blanking Interval (VBI), a must-have if monitoring involves any information transmitted during vertical blanking
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