27-10-2019, 05:21 PM
Hi Peter,
I think I can spot the cause of the confusion. You are thinking in a digital world but what you are discussing belongs firmly in the analogue world.
Only one spot on the CRT is illuminated at a time. The light output of that spot of phosphor starts to diminish rapidly as the electron beam continues to sweep across the screen and we rely on persistence of vision to give our brains the mental illusion that the whole screen is illuminated evenly - but it isn't.
Take a look at your drawing in a different way:
This is the instant that the last line of the red field is written. There is enough filter area above that line to ensure that the light from the decaying earlier lines stays red.
There is then a pause for the field blanking interval when nothing is written but the filter continues to rotate so that, by the time that the first line of the green field is written, the entire length of the line will be behind the green filter.
Does that help?
I think I can spot the cause of the confusion. You are thinking in a digital world but what you are discussing belongs firmly in the analogue world.
Only one spot on the CRT is illuminated at a time. The light output of that spot of phosphor starts to diminish rapidly as the electron beam continues to sweep across the screen and we rely on persistence of vision to give our brains the mental illusion that the whole screen is illuminated evenly - but it isn't.
Take a look at your drawing in a different way:
This is the instant that the last line of the red field is written. There is enough filter area above that line to ensure that the light from the decaying earlier lines stays red.
There is then a pause for the field blanking interval when nothing is written but the filter continues to rotate so that, by the time that the first line of the green field is written, the entire length of the line will be behind the green filter.
Does that help?
