PO BOX 13305 · PORTLAND, OR 97213 · FAX: (503) 229-8064 · (800) 837-8428 · [email protected] · WWW.VESTIBULAR.ORG Computer Monitors and Digital Televisions Visual Sensitivity from Vestibular Disorders Affects Choice of Display Visual sensitivity from vestibular television’s screen size. A very large disorders can be exaggerated when a television placed in a small room can be person uses a computer monitor or problematic for a person with a vestibular watches television. Certain types of disorder because the screen will dominate displays—and certain ways a person the visual field so much that any might use them—can be more movement on the screen may create the problematic than others. These are illusion that it is actually the viewer who discussed in the following introduction to is moving. terminology associated with three common display technologies: cathode- Display-aspect ratio (not to be confused ray tube (CRT), liquid crystal display with “screen-aspect ratio,” see pg. 3) (LCD), and plasma. To help readers characterizes a display’s width relative navigate the technical jargon, key points to height. Traditional computer and aimed at assisting people with vestibular television displays have a display-aspect disorders are flagged with the symbol ratio of 4:3 (sometimes expressed as the at left. fractional amount of 4/3 = 1.33). Wide- screen displays have a 16:9 aspect ratio Basic terminology (or 16/9 = 1.77), and thus provide added Televisions and computer monitors share viewing area (see Figure 1). similar technologies, but they apply them in different ways. These differences are A wide-screen computer monitor with a based on the brain’s ability to process screen size ranging between 19 and 22 information from a distance (televisions) inches is especially helpful for a person as opposed to up close (computer with a vestibular disorder who writes and monitors). edits documents. This screen allows two full-size pages to be displayed side by Display size is expressed by two types side, thus reducing the amount of of measurement: screen size and display- scrolling required. It also allows two aspect ratio. Screen size is measured separate documents to be compared side diagonally across the viewable screen (for by side—a helpful feature when a person LCD and plasma displays) or across the is moving or copying a block of text or viewable screen plus the outside casing data from one document to the other. (for CRT displays). Viewing distance is an The ability to view both documents important consideration in selecting a simultaneously eliminates the steps © Vestibular Disorders Association ◦ www.vestibular.org ◦ Page 1 of 9 involved in minimizing and maximizing (HDTV) signals. SDTV signals are more windows in order to jump between compressed and picture definition is not documents. This helps a person avoid as sharp as HDTV signals. potential errors caused by the concentra- tion problems that sometimes accompany Pixels: The human brain has the ability vestibular disorders. to compile thousands of individual visual elements into an image. On this basis, Flat screen and flat panel: A flat screen televisions and computer monitors pro- monitor is a modified CRT monitor duce images by fusing many small dots flattened to reduce the distortion called pixels (from “picture-elements”). associated with traditional CRT screens. Each pixel contains a tiny red, green, However, flat screen monitors do have and blue triad. some distortion, unlike flat panel monitors used for LCD and plasma Resolution and dot pitch: Image displays, which generally have no sharpness is determined by the resolution distortion. (the number of pixels contained in a display) and the dot pitch (the distance Digital and HDTV signals: Digital between the pixels). The smaller and television (DTV) signals are transmitted closer the pixels are to one another, the as a stream of digital data represented by more realistic and detailed the picture patterns of ones and zeros. Guidelines appears. An image will become grainy have been established for defining the and uncomfortable for a person with a quality of DTV signal transmissions. vestibular disorder to view if the These range from the lowest level, pixels are too far apart because each standard definition digital (SDTV), to the one becomes noticeable as an individual highest level, high definition digital element. Standard Display Wide Screen Display Figure 1: Aspect ratio for standard and wide screen displays. © Vestibular Disorders Association ◦ www.vestibular.org ◦ Page 2 of 9 Screen-aspect ratio (as opposed to Screen size and viewing distance are “display-aspect ratio,” defined above) is factors that affect the brain’s ability to an optional computer setting that allows interpret specific resolutions without a user to adjust the operating system graininess or blurring. For example, the properties so that they impose a different low-resolution picture in Figure 2 isn’t computer-screen resolution from the clear when seen at reading distance, but native resolution (the resolution set by it resolves when viewed from far enough the manufacturer). LCD picture quality away for the brain to assemble the dots will suffer if a person configures it to use into a recognizable image. a non-native resolution. This is because, unlike CRT monitors, LCD technology This difference in viewing distance is the scales images to accommodate any reason why computer monitors require variation from the native resolution. higher resolution levels than televisions, and it is why most televisions used as computer monitors do not produce high- quality images. The difference in resolu- tion levels also affects how on-screen movement is represented. Motion on television is created using gradual transitions in color, intensity, and pattern. When a computer display changes from one page of text to the next or reflects moving text as it is inserted, the movement is more abrupt, and thus requires a higher resolution to remain clear. Figure 2. This low-resolution picture resolves into a sharper image when viewed from a distance. Resolution is expressed as the number Photo of pediatric vestibular rehabilitation session of pixels displayed horizontally and courtesy of Atlanta Ear Clinic, Atlanta, Georgia. vertically. For example, the resolution for most CRT displays is 640 x 480. The Refresh rate and frame rate: The lowest HDTV resolution is 1280 x 720, number of times a complete image is sometimes referred to as “720” (which drawn on the screen per second is called refers to rows of pixels). For a television the refresh rate. This is not the same as screen size less than 50 inches, a person the frame rate, which signifies how often might not notice a difference between low the image being displayed is repeated per HDTV resolution and high HDTV fraction of a second before it is changed. resolution (1920 x 1080 or “1080”). © Vestibular Disorders Association ◦ www.vestibular.org ◦ Page 3 of 9 The rates that produce the most visually comfortable display depend in part on Aliasing and anti-aliasing: Interlacing whether the screen is a television or introduces a visually disturbing distortion computer monitor. This is because the called aliasing, an interline “twitter” that human eye is most sensitive to flicker in appears under certain circumstances. For its peripheral vision (the edges of the field example, a television character’s black- of view). Thus, low refresh rates in and-white-striped shirt will appear to computer monitors, which are viewed up buzz if it is viewed on an interlaced low- close, will produce a noticeable screen resolution television display. This disturb- flicker because the display fills a larger ing twitter with busy and high-contrast proportion of a person’s field of vision patterns is a familiar concept for people than televisions. with vestibular disorders—they experience similar distortions even when Progressive scans and interlaced they are not looking at a television or scans: These terms refer to how a picture computer monitor! For example, many is formed. Their related technologies take people find it uncomfortable to see advantage of the brain’s ability to sunlight streaming through partially integrate gradual transitions seen by the closed Venetian blinds. eye while an on-screen image is painted. Progressive scans form a picture by The process of anti-aliasing was devel- painting one row of pixels at a time, in oped as an attempt to minimize the sequential order. Interlaced scans form an distortion caused by interlacing. Anti- image in two phases. During the first aliasing reduces twitter by blurring details phase, even-numbered rows are drawn; and decreasing picture resolution. This during the next phase, odd lines are adaptation introduces other problems. It drawn. The eye integrates the two images is somewhat analogous to the strategies to create a single interlaced image (see used by a person with a vestibular figure 3). Interlacing is said to help disorder who dons sunglasses or dims improve problems with flickering caused room lights in an attempt to reduce visual by lower refresh rates. However, many contrast. people with vestibular disorders find interlaced scans uncomfortable. PHASE I: EVEN ROWS PHASE II: ODD ROWS INTERLACED SCAN Figure 3. An interlaced scan forms an image with alternating rows of pixels. © Vestibular Disorders Association ◦ www.vestibular.org ◦ Page 4 of 9 lines of pixels that refresh repeatedly, Contrast ratio and brightness: The one interlaced row at a time. contrast ratio is a comparison of the number of white pixels and black pixels Figure 4. The minimum horizontal viewing angle for a computer monitor should be 110 (e.g., 500:1). Brightness is a degrees. For televisions, 160 degrees is measurement of the amount of light preferred. produced by the monitor. It is expressed in units called nits (candelas per square meter). A very high brightness level is not necessarily good because it reduces the viewing angle (see below) and can be uncomfortable to watch, especially if the display is used in a darkened office or television room.