Refresh Rate

When a monitor bombards all horizontal lines with electrons in a single pass, each vertical pass presents one frame and is called a screen refresh. European television screens refresh at 50 Hz while U.S. television screens refresh at 60 Hz. However, users have sometimes found these refresh rates too slow, particularly in high ambient intensity environments, and detect visual flicker at such speeds.

For monitors, there are typically between 60 and 100+ vertical passes per second, depending on the design of the monitor. More expensive monitors use higher performance components that permit higher refresh rates. For example, assume a monitor refreshes 1,000 lines, each having 1,000 points 50 times per second (50 Hz refresh rate). Thus, 1,000,000 points are refreshed every 20 ms. If each horizontal retrace requires 10 [mu]s, then ignoring time lost to vertical retrace as well as vertical/horizontal blanking time:

20 ms per refresh – 1000 * 10 [mu]s = 1/100th sec for 1,000,000 pixels

So, in this example, the pixel rate is 100 MHz which requires a video amplifier with a high performance slew rate. Higher refresh rates require far better video amplifiers which were not economical in early monitor and television development, causing a search for alternatives. In one approach, called non-interlaced, the monitor refreshes all horizontal lines on the screen. In a lower-cost alternative approach, the interlaced approach, the monitor traces only half the lines. In one screen refresh only the even-numbered lines are traced. In the next pass, only the odd-numbered lines are refreshed. The set of even-numbered lines and the set of odd-numbered lines are respectively called fields.

The display controller produces an interlaced display by decreasing the horizontal retrace frequency (making horizontal passes take twice as long) and producing alternating signals for the alternating fields. This requires DACs to access frame buffer memory differently than when producing non-interlaced displays and beginning the vertical retrace operation halfway through a blanked horizontal line at the bottom of the screen. Since every line is refreshed every other pass, long-persistence phosphors help reduce flicker but have different chromaticities and visual artifacts than the shorter persistence phosphors non-interlaced displays use.

Phosphors

Phosphors are chemical compounds that emit light when bombarded with electrons. They excite rapidly and decay more slowly. There are many types, and the various types differ in their chromaticities, granularity, and persistence. Phosphors with finer granularity can provide more detail than coarser phosphors. A phosphor’s persistence is usually defined as the time it takes to decay to 10 percent of its initial fluorescence level.

The human visual system’s eye persistence and phosphor decay combine to extend an illumination sensation long after an electron bombardment is removed. However, to avoid flicker, refresh rates and phosphor decay must not descend below a user’s critical fusion frequency which varies by environment and individual. To combat flicker, some monitor manufacturers use long-persistence phosphors which can cause smearing and streaking when viewing moving images. Finally, phosphor performance decays with time, particularly with blue phosphors.

Aspect Ratio

Most popular monitor faces are one-third wider than they are tall. So, if a monitor is three units high, it is four units wide—referred to as a 4:3 aspect ratio. This becomes important when displaying images after setting a display controller to a certain mode which causes the monitor to generate a certain number of horizontal lines, each composed of a fixed number of dots.

Suppose a display controller is in a mode that generates 640 dots on each of 480 lines—called 640x480. In this instance, the pixel shape is considered square because a 400x400 dot area appears as a square area on the screen. However, if the display controller is in a mode that only displays 640 dots on each of 400 lines, called 640x400, the previously square area now appears as a rectangle—i.e., the previous square is “stretched downward.”

This distortion occurs because monitor electronics fill the display area by varying the dot size. In this example, the monitor conceptually pulls each dot 480/400 or 1.2 times farther down the screen than it would in 640x480 mode. So the primary point is display controller modes distort images if programs do not compensate for the display controller mode’s pixel shape.