Posted on Jan 6, 2007 8:32:00 AM
In short, high-definition video (HD) refers to any video that has a higher resolution than that of standard-definition video (SD).
A Brief History of HD
In 1953 the National Television System Committee (NTSC) adopted a video standard using 525 horizontal lines of resolution for the normal television broadcast. In the 1980's the standard was revisited by the Advanced Television Systems Committee (ATSC) to include Digital SD at 480 horizontal lines, and a range of HD standards from interlaced 1080-line video to 720-line video progressively scanned. The FCC officially adopted the new standards and the first broadcast took place in 1998.
Vertical Display Resolution of HD Projection
Today in the church presentation world we typically deal with video projectors with variations of three basic resolutions which are based on the number of horizontal lines that make up the image: 480, 720, 1080. The relative resolutions of the different formats are represented on the diagram below.
Now, simply put, the higher the resolution, the more quality and detail will be present in the projected image. Therefore, 720-line displays are capable of delivering higher-quality images than 480-line displays, and 1080-line displays are capable of achieving higher-quality images than their 720-line counterparts.
As a simple demonstration, the images below reveal the difference in quality that can be achieved with a 720-line projector compared with a 480-line display. The image that is shown first a really magnified sample of a 720-line image.
The second image represents a comparable 480-line image blown up to the same screen size.
As demonstrated, the loss of detail is quite noticeable. Most notable is the difference in the relative size of the individual pixels that make up the image. Based on line-height, each pixel in the 480-line image is 1.5 times larger than that of the comparable 720-line image. This is where HD video gets its primary advantage over SD video.
Progressive Scan and Interlaced Video
There is one other factor that affects perceived image quality that warrants a brief mention. It relates to how the pixels are "drawn" on the display.
Moving video images are made up of a series of still frames played in rapid succession-- often as fast as 60 frames per second. On a video display, each of these still images is made up of horizontal lines. In a progressive scan image, each horizontal line is drawn one pixel at a time from left-to-right, top-to-bottom.
In an interlaced image the odd lines are drawn one pixel at a time from left-to-right, top-to-bottom, and then the even lines are drawn in a similar fashion. Because each frame is drawn very quickly, the result appears as a continuous video image. The two images below demonstrate how each pixel is drawn in concept. The top example represents how a display draws progressive scan video, while the bottom image represents interlaced video.
The primary advantage of progressive scan video is that every pixel in every line of the screen resolution is used in each frame. With interlaced video, each frame only utilizes every other line, so the vertical display resolution is significantly reduced. Given two displays with the same display resolution, an interlaced video image will appear to have 40% less detail as it relates to vertical resolution than its progressively scanned counterpart.
Another advantage of progressively scanned video relates to unwanted artifacts in the video image. In interlaced systems smooth lines that are nearly vertical or horizontal can appear to be very jagged, or sometimes even flicker, due to the movement that takes place in the image between frames. These artifacts can be very distracting.
Key Points: The two primary factors that give HD its advantage over SD displays are:
1. Superior vertical resolution
2. Progressive Scan boasts higher detail related to vertical resolution
In this article we demonstrated the primary technical differences between standard-definition and high-definition video formats. These differences clearly illustrate the superior image quality that can be achieved with HD video systems over the older format.
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