High Definition Video (Part 1)by Diana Weynand and Marcus Weise
There are two subcategories that fall under the DTV (Digital Television) category of standards, SDTV (Standard Definition Television) and HDTV (High Definition Television). All high definition video, also referred to as high def or HD, falls under the HDTV category of standards. There are several different high def standards, each having their own unique combination of image criteria, including frame rate, pixel count, line count, and scanning mode. While HDTV standards currently represent the highest quality of video image, they are not new standards. In fact, the development of high definition standards began in the analog domain in the early 1970s. Once DTV was developed, HDTV became a digital standard. It currently makes use of the most advanced digital processes. One of the reasons high definition video was developed was to create a universal world standard. Analog television standards, including NTSC, PAL, SECAM, and variations of these standards, are not compatible with each other. To view images from one standard to another requires a conversion process that can degrade the image quality. HDTV standards represent an international effort to create more compatibility between world standards. While removing some of the boundaries, however, high def created an entirely new set of standards that are still evolving.
CDTV and SDTV standards use an image size with an aspect ratio of 4x3. A 4x3 image is four units wide by three units high. While rectangular in shape, it is much closer to a square than the widescreen image currently seen in cinemas. The human eye perceives a great deal of motion and depth information from the area outside of direct view. This area is known as peripheral vision. Having a wider video image area takes advantage of this characteristic and improves the sense of reality for the viewer.
As HDTV was developed, this fact was taken into consideration and all HDTV standards were widened to a 16x9 aspect ratio, or an image that is nine units high by sixteen units wide. (Figure 13-1) Since the HDTV standards all have a 16x9 ratio, this ratio is often referred to as the native aspect ratio for the standard. A native standard is the basic standard for which the piece of equipment was designed. It may be capable of handling other standards, but it’s original intent is referred to as the native standard.
Widescreen video can be played back on a 4x3 monitor in different ways. To see the entire widescreen image, the image must be reduced in size so the width of the image fits in the 4x3 monitor. This creates black above and below the widescreen image. This layout is referred to as letterbox. To make use of the full 4x3 image area, the sides of the widescreen image can be cropped to show just the middle portion, or 4x3 area. Also, a process of panning, or moving horizontally across an image, can be applied during transfer to reveal a particular portion of the widescreen image. This is often referred to as pan and scan. (Figure 13-2)
Image Resolution
With the 16x9 aspect ratio, there is a larger image area and therefore more room for additional pixels. Different high def standards have different pixel and line counts that make up that standard’s image area. The greater the number of pixels there are that make up the image, the greater the image resolution.
For example, one high def standard has an image resolution of 1920x1080. In this standard, there are 1920 pixels across one line and 1080 lines in one frame of the image. The 1920 pixel count is the horizontal resolution, and the 1080 line count is the vertical resolution. Another high def standard is 1280 x 720, which is 1280 pixels per line by 720 lines. The combined pixel and line count make up the spatial density resolution of the high def image.
Progressively Segmented Frames
HDTV standards use either one of the two scanning modes, interlace or progressive. When a high def standard uses an interlace mode, the odd fields are transmitted first, followed by the even fields, just as they are in CDTV or SDTV. In progressive scanning, the entire image is scanned as one complete frame. This data may be transmitted as a complete frame and received as a complete frame. If there is insufficient bandwidth to transmit the complete frame, the data may be segmented and transmitted in two parts. Because progressively scanned images are complete frames of video, they require more bandwidth to transmit than may be available. To transmit progressively scanned images within an interlace environment using narrower bandwidths, a process of segmenting progressively scanned frames was developed. In segmenting a progressively scanned image, the image is divided into two fields by scanning every other line as in interlace scanning. In the interlace scanning process, the two fields are from different instances in time. When a progressively scanned image is segmented, the two fields are from the same instant in time. When the separate fields are recombined, the result is a complete progressively scanned frame. (Figure 13-3) This process accommodates the need to segment the data without compromising the quality of the image. Images that are progressively scanned and then transmitted as segmented frames are referred to as PsF, for progressively segmented frames.
Frame Rate
In addition, one of the major forms of image creation has been film. Film runs at 24fps in North America and 25fps in Europe. To minimize the difficulty in incorporating film into HDTV, a set of criteria was developed to accommodate the film frame rate. A different HDTV standard was created with a specific set of criteria based on the ease with which the new standard would interface within an existing system. Therefore two of the many HDTV standards include 24fps and 25fps. Excerpted from "How Video Works" by Diana Weynand and Marcus Weise (Published by Focal Press). An easy to understand explanation of the entire world of video. A complete guide from analog video to all the new digital technologies, including HD, compression and encoding. Purchase Now
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