User`s guide

E-Prime User’s Guide
Appendix D: Display Presentation
Page A-54
Step 1 - Writing to Video Memory
When a display command is executed, it writes data into a video controller card. This card has a
certain amount of memory that can be thought of as a direct representation of the computer
monitor (i.e., each memory location corresponds to a screen location). While the technical details
of writing to video memory are not essential to this discussion, it is important to remember that
this is a distinct event in the process. In the case of one or a few letters, the step is nearly
instantaneous – on the order of tens of microseconds (0.00001 seconds).
If the stimulus involves extensive graphics (e.g., filling a circle), the process of writing to video
memory can take a significant amount of time (milliseconds). The amount of time necessary for
this step is dependent not only on the nature of the stimulus, but also on the computer and the
display card.
Step 2 - Scanning the Screen
The second step is the physical activation of the stimulus on the screen. Most computer monitors
today are based on raster technology, which works in the following fashion: The monitor has
three raster guns, which emit a beam of electrons to activate red, green, and blue dots of color.
The position of this beam can be adjusted to accommodate several different pixel resolutions. A
pixel is the smallest unit of drawing on the monitor, and it is defined by the pixel resolution of the
current graphics mode. For instance, if you are using the standard VGA graphics mode, the pixel
resolution is 1024x768, which means that there are 1024 pixels per row and 768 pixels per
column. The raster guns are deflected to draw each pixel on the screen sequentially, from left to
right, top to bottom (see Figure 1, lines in Monitor), and the guns can only draw one set of pixels
(red, green, blue) at a time. The raster guns shoot electrons at the screen, or front of the monitor.
The electrons activate the phosphors to emit light. As soon as the guns are moved to the next
pixel, the phosphors in the previous pixel begin to dim, or decay (see Figure 2 below). The decay
period can vary from one monitor to the next, but for most color monitors, the intensity of the pixel
will be 10% of its maximum within 2-5 milliseconds. The video display card sequentially scans
each memory location in order, turning on the gun's intensity based on the contents of the video
memory set during Step 1.
The raster guns are continuously scanning the screen at a constant rate. This means that any
given pixel on the screen is drawn with some constant frequency, known as the vertical refresh
frequency. The vertical refresh frequency is the number of times per second that the entire screen
will be drawn. The inverse of this measure, the refresh rate, is the amount of time necessary to
draw one full screen. So, if a monitor has a 70Hz vertical refresh frequency, the refresh rate of
that monitor is 1/70 of a second (= 14.2857 milliseconds). Basically, once the scanning begins, it
sweeps at 70Hz, independent of any writing to video memory operations.
Associated with the refresh rate is the vertical retrace, or top of screen event. This event occurs
whenever the raster guns are moving from the bottom to the top of the screen to begin a new
scan of the monitor. This event can be detected by the computer and can be used to synchronize
the drawing of stimuli. By sensing the top of screen and only writing to video memory in a short
period of time, the display will always occur within a fixed period of time after the top of screen
event. For example, if text is displayed at the center of the screen and can be written in less time
than half the refresh period (7ms), the data will be in video memory before the display locations
are scanned for the next cycle (7.14ms after the top of screen event).
The effect of Liquid Crystal Display (LCD) monitors is basically identical to CRT monitors.
Current active matrix LCD monitors work by having an electrical signal change the angle of the
crystal of the LCD for each colored dot on the display. In contrast to a CRT display, where each