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 The 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" display.
 But suppose we need our CRT to refresh every 60 seconds, and that if we change this length of time all sorts of bad things happen, our game runs jerky, our display "flickers" too much, or our display becomes unstable / garbled. There is one other alternative to decreasing the refresh rate, and that's to increase the speed at which our CRT scans a horizontal line. So, for example, if we wanted to double the number of lines on our CRT from 12 to 24 and yet have our screen refresh every 60 seconds, we would need to double the speed at which our CRT scans a horizontal line. This would allow us to draw twice as many lines in the same amount of time, for if it only takes 2 seconds to draw a line instead of 4 seconds, then 24 lines x 2 seconds/line + 12 seconds of blanking lines = 60 seconds.
 Similarly, if we only wanted to add a couple of lines to our resolution, say, for example, 4 more, so that our CRT displayed 16 lines, then we would need to increase the scanning rate from 4 seconds/line to 3 seconds/line (48 seconds / 16 lines = 3 seconds/line). This would give us 16 lines x 3 seconds/line + 12 seconds for of blanking lines = 60 seconds. So we can now see that if we want to increase the number of lines our CRT displays, then we need to either increase the time it takes for our CRT to refresh its display, or increase the speed at which it scans horizontal lines. The reverse holds true if we want to display fewer lines of data on our CRT. We can either decrease the time it takes for our CRT to refresh its display or decrease the speed at which it scans horizontal lines. Its absolutely essential that you understand this relationship before you begin tinkering with modelines. Now that we have a basic understanding of how both CRTs work with images (the analog world) and how video cards work with images (the digital world), we can finally see how the two worlds collide to produce the displays we see.
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