HP ProLiant 3000 Video Streaming Technology - Page 3

ECG068/0798 WHITE PAPER (cont.) 3... Video Technology The Human Eye An understanding of video technology starts with an understanding of the properties of the human eye. This is because the electronic eye of the video camera tries to mimic what the human eye sees. Basically, the human eye detects, or "sees", electromagnetic energy in the visible light frequency spectrum ranging from a wavelength of about 400 nanometers (nm) to 700 nm. The eye cannot detect electromagnetic radiation outside this spectrum. 8000nm TV Radio Waves Radar 700nm 400nm mm Infra Ultra Red Violet X-Rays Gamma Rays .0001nm Cosmic Rays Visible Light The human eye detects this light energy through the use of photoreceptors known as "rods" and "cones". There are approximately 120 million rods distributed across the spherical surface called the "retina" at the back of the eye. These rods are sensitive to light intensity. By contrast, there are only about 8 million cones that are sensitive to color on the surface of the retina. These cones tend to be centrally located at the back of the eye. The large number of light-sensing rods, compared to color-sensing cones, makes the eye much more sensitive to changes in brightness than to changes in color. This fact is taken advantage of in the video and image compression schemes discussed below which sample color at a lower rate than that of brightness. This is also why night vision, which relies on the low light intensity sensing capability of rods, is devoid of color, and why peripheral vision, which is not directed at the center of the retina is not as color sensitive. Brightness changes detected by 120M rods Color changes detected by 8M cones For color sensing there are three types of cones capable of detecting visible light wavelengths. It has been determined that a minimum of three color components-e.g., Red, Green, and Blue- corresponding to the three types of cones, when properly filtered, can simulate the human sensation of color. Since color does not exist in nature-it is literally in the eye and brain of the beholder-these cones sense light in the visible spectrum and our brain processes the result to provide us with the sensation of color. This process is additive in that the brain can create colors-e.g., red + blue = purple-that don't exist in the pure spectrum. These properties of human vision are used in video compression schemes, as well as in display systems to provide efficient methods for storing, transmitting, and displaying video data. Another characteristic of human vision important to video technology is that of "image persistence". This is where an image remains on the retina, even though the original object has been physically removed or replaced by another image. This persistence tends to be around .1 second. This causes the eye to perceive motion if the image is changing at a rate of greater than 10 frames per second (fps). It has been determined that smooth motion requires a frame rate > 15 fps.