Wed Aug 09, 2017 9:37 am
This is because, in an old-fashioned, standard definition, analogue TV world a frame of a TV picture was created by two interlaced scans of lines - alternatively odd lines (1,3,5,7,9 etc) and then a flyback to the top halfway point to fill in the even lines (2,4,6,8,10 etc). Each scan set is called a "field" with two interlaced fields producing a full frame with either 25 (PAL, SECAM) or 30 (NTSC) frames produced per second.
So........... the step at the top of a digitised frame is displaying the half-line scan start-point of the second field in the whole frame sequence. On an old-fashioned CRT TV screen all this is outside the normal picture display area (known as "overscan") but of course when you're creating a digitised recording from the analogue source you see everything.
I'm currently digitising to ProRes422 a load of material from a pile of professional-quality NTSC Hi-8 recordings and lots of this stuff becomes apparent (top and bottom) when viewing on a 4:3 CRT monitor in "under scan" mode.
Blackmagic Teranex 2D, Ultrastudio Express, Intensity Shuttle (Thunderbolt), Two H.264 Pro Recorders (Mac OSX) & lots of old VTRs used for digital archiving of legacy video formats for major libraries, broadcasters, universities and public archives.