TIMING: Stimulus Preparation Challenges [22849]

We’ll continue now with the rotating checkerboard example. The first timing issue, median display times in excess of the intended display duration of 200 ms, in large part reflects the time that is required to prepare the stimulus for display. Recall that the images stored on the disk are to be displayed for 200 ms each. In order to present these files, the following process must occur:

• The image data is read by the computer from the disk
• The image is prepared for presentation
• The image is copied to the display screen
• The computer waits for the stated display time (200 ms)

This cycle is then repeated for the next image presentation. The time to read the image from disk, prepare it for presentation and copy it to the screen required a median time of 36 ms. Thus, the median total time for the image display is 36 (setup time), plus the specified display duration of 200 ms, for a total of 236 ms. This is an illustration of the problem: It is not possible for the computer to put up a display without preparation, which accounts for the “setup time”.

Similar issues occur with other types of stimuli. Any graphic file, regardless of format, requires time to load. Graphic files that are in compressed formats (e.g. .gif and .jpeg) must be interpreted (“uncompressed”). Similarly, visual stimuli that are created with E-Basic’s graphic’s commands (e.g. circle, line), require setup time as well.

Lastly, sound and movie files also require preparation time. Clearly, as is the case with image files, sound and movie files must be read by the computer from the disk. Sound and movie files are also compressed when they are initially stored. It is not feasible to store and/or transmit audio and movie files in an uncompressed format. For example, uncompressed stereo audio files that are CD quality, meaning 44 KHz sampling rate, 16 bits per sample, requires about 150 KB of hard disk space for each second of sound.

The compression and decompression process for audio and video are more complex than for static image files. There are hundreds of codecs, and they can be packaged in different ways. For example, “containers” are often used to store both audio and video streams for video files. AVI is an example of a popular container. Containers required an extra processing step, because the AVI must first split the incoming signal to be presented into the audio and then video components, and then hand these components of to a codec that knows how to process them.

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