625/50 refers to a PAL or SECAM analog TV signal. Although PAL only has 576 horizontal lines in its Active Area, a PAL 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 49 vertical sync lines, resulting in a total of 625 lines (576 + 49).
50 is a reference to PAL's Fieldrate of 50 fields per second, resulting in 25 Interlaced frames per second.
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