Vertical Retrace Interval

The time during which the electron beam is moved from the lower right corner to the upper left corner of the screen to draw the next field. in NTSC video, 1.4 milliseconds, or enough time to draw 21 lines, has been set aside for this purpose and to insert information to synchronize the electron beam with the transmitted picture for maintaining vertical hold. Typically the electron beam draws 240 odd lines from top to bottom, then 23 lines while it returns to the top, then draws 240 even lines, then 22 lines during retrace, and so on. The exact number of lines blacked out by the TV during retrace and exactly where and how the 525'th line is drawn varies slightly depending on the program content and the make and model of the equipment. Once in awhile you see a TV picture criss crossed with several spurious diagonal lines slanting up from left to right. These are the vertical retrace scan lines mentioned above but due to a defect they were not made black enough. For digital video no pixels of data are associated with the vertical retrace interval but the monitor has to set aside drawing time for it anyway.