Horizontal Retrace Interval

The time during which the electron beam is turned almost off (as if to draw black) and  moved from right to left to get ready to draw the next scan line on the picture tube. In NTSC broadcast video, 10.9 microseconds are set aside for this purpose; the entire line, retrace interval and all, takes 63.5 microseconds.. The time it takes to draw an entire line is adequate to allow the electron beam to draw at most 530 dots alternating black and white; the horizontal retrace interval takes away about 90 of these dots. Of the remaining 440 (give or take a few), 330 fit in the largest circle that in turn fits in the 4:3 screen, which leads to the 330 line horizontal resolution that is often mentioned when it comes to broadcast TV. The exact number of dots used for the picture material depends on the program source and the make and model of equipment.