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E-Prime User’s Guide
Chapter 3: Critical Timing
Intended and Actual Display Duration - E-Prime
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Display Duration (ms)
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Intended
500
Actual
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Image Number
Figure 2. Intended versus actual display time in E-Prime for a 200ms display.
E-Prime can present displays at a very high rate (e.g., new image every 12ms or less on
appropriate hardware, see section 3.4.2.3). In Figure 2, there is still a small oscillation in the
timing data that is due to the next problem area we discuss.
3.2.3
Problem 3: Displays are accomplished via
refresh cycles
Computer display devices (CRT/TV type, or LCD types) paint every dot or “pixel” sequentially,
starting from the top left, painting pixels horizontally across the row, and then moving down to the
second line and continuing horizontally to paint pixels. The process is repeated for each row of
pixels until the bottom of the screen is reached. This process of painting all the pixels
sequentially for a full screen is called the refresh cycle, and typically takes 10-18ms (the “refresh
duration”). The start of a refresh cycle is referred to as the vertical blank event, the time that the
electronic gun on the monitor moves from the bottom right of the display to the top left of the
screen restart the refresh. The computer can sense the vertical blank event in order to
synchronize the computer events to what is occurring on the display. The display card, the
monitor, and the screen settings (e.g., resolution and color depth) determine the refresh cycle or
refresh rate. When the video card has its setting changed the display monitor typically flashes
for hundreds of milliseconds as the monitor adjusts to the new settings. Since changing the
refresh rate takes a long time and causes visual artifacts, experiments generally do not change
the refresh rate during a session.
A typical display resolution includes 1024 pixels horizontally and 768 pixels vertically. Figure 3
shows time progression of the display of each pixel row in a 1024x768 display when a 71.4Hz
refresh rate is in effect.
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