I turned down the brightness of the monitor so that the background circle was not visible. The circle you see is due to the medium-long persistence phosphor used in the CRT continuing to glow for several seconds after being lit up by the electron beam. If enough people ask, I will re-record this in 4K. I do not own any of the content in this video (except the monochrome green CRT monitor) if the original author asks me to take this video down, I will. visualisation v2: original video: