How is an interlaced scanning pattern generated in a fast-scan (NTSC) television system?
Why NTSC fast-scan TV builds each 525-line frame out of two fields transmitted one after the other: the first field paints the odd-numbered lines, the second field fills in the even-numbered lines between them. Each field takes 1/60 second, so a complete frame appears 30 times per second while the screen is refreshed 60 times per second. Interlacing this way gives flicker-free motion without doubling the required video bandwidth.
Watch out Scanning two fields simultaneously is impossible in a single-beam raster system; the fields are sequential, which is exactly what makes the pattern 'interlaced'. Scanning direction is always left-to-right and top-to-bottom in both fields.
Odds first, evens second: 2 fields at 60/sec interlace into 1 frame of 525 lines at 30/sec.