evolution of the raster CRT primarily revolved around one
thing, the speed at which the horizontal magnetic field
could be manipulated. As you'll recall, the horizontal magnetic
field deflects the electron beam from left to right. The
faster a CRT can manipulate this field, the faster it can
scan a horizontal line, the more lines it can display per
second (remember more lines = larger resolution). To better
understand why this is, let us return to our theoretical
CRT (pictured above).
If our theoretical CRT takes 4 seconds
to draw each line of data, and it draws 12 lines of data,
then we know that it takes exactly 48 seconds to draw the
entire resolution (12 lines x 4 seconds/line = 48 seconds).
Do notice, though, that it takes more than 48 seconds for
our CRT to refresh its entire screen. This is because of
the blank space between the last line drawn and the first
line drawn. In the digital world this blank space is measured
in lines. On our theoretical CRT there are 3 blank lines.
Blanking lines give a CRT the time it needs
to position the electron beam in the correct spot. You will
find that older CRTs, like arcade monitors, require a large
number of "blanking" lines, while newer CRTs don't.
This is because modern CRTs use faster, more accurate components
that require less buffering.
Because video cards think in terms of pixels
(and not seconds) they separate displays into active pixels
(the resolution that's displayed) and blanking pixels (buffering
time for a CRT). So from our video card's perspective (i.e.
the digital world) there are actually 15 lines of data,
12 lines are active and 3 lines are blank. If we include
the vertical blanking time in our calculations we find that
it takes 60 seconds for our CRT to refresh its entire screen
(48 seconds to draw 12 lines of data + 12 seconds for 3
blanking lines = 60 seconds per screen).
Now suppose we want to display more than
12 lines of data on our CRT (we want to increase its resolution).
Logically, we could deduce that it would take longer for
our CRT to refresh its screen. For example, if we wanted
to add 4 lines of data to our resolution, we could deduce
that it would take 16 extra seconds for our CRT to draw
the entire screen (4 lines x 4 seconds/line = 16 seconds).
By the same token, we could deduce that it would take twice
as long for our CRT to refresh its entire screen if we doubled
the number of lines it displayed (24 lines x 4 seconds/line
+ 24 seconds for 6 blanking lines = 120 seconds). This basic
relationship illustrates a CRT fact. If you increase the
number of lines a CRT draws at a given speed, you increase
the time it takes for a CRT to refresh its screen.
From the image below, we can observe this
basic truth in operation. Here the same 4 second per line
CRT draws 24 lines instead of 12. Take note of the unusual
pattern used to draw the lines. Half the lines are drawn
on the first pass, then the other half are drawn on a second
pass. This interlacing between even and odd lines is common
on displays running at very low refresh rates. At very low
refresh rates, interlacing produces a brighter, less "flickery"