People new to the world of HDTV sometimes have a difficult time knowing what they are watching. Truth is the picture quality can vary all over the map depending on many factors all along the way from the source to your TV set. In the following article I will delve into the various issues that create the pictures on your HDTV screen. In order to illustrate some occurrences on what is anticipated to be a small computer monitor, some artifacts have been exaggerated. Also due to limited space the illustrations in the body of the text are small by necessity. If you click on the illustrations you will be given a full screen version which will show up the artifacts much clearer. Maximize the window and use the enlargement tool to get a full size picture.
Standard Definition (SD) and High Definition (HD) Differences
The first two photos are illustrating the picture first as would be shown on a 4:3 standard definition TV at a resolution of 640×480 and then followed by a picture as would be shown on a high definition TV at a resolution of 1280×720. The relative size using the same pixel spacing is used. Notice the clarity of the two photos does not seem all that much different due to the size difference. This is what happens when you go from a 27" SDTV to a 50" HDTV. The clarity of the two does not seem all that much different. But bear in mind to get the same clarity in the 50" HDTV that was perceived in the 27" SDTV there were 307,200 pixels in the 27" SDTV vs. 921,600 pixels in the 50" HDTV.
Now there are a couple of things to notice. Although your monitor may not show much of a difference, if you download the HD picture and the merged SD picture and examine them in a photo editor, you would find there is quite a bit more detail in the HD picture even though they are both the same size. Also there are black pillar boxes on the left and right of the 4:3 picture to fill out the width of the 16:9 frame. This is necessary to prevent distortion of the original 4:3 picture.
Stretching and Zooming
The following picture is what will be shown on a HDTV that is set to full screen displaying a 4:3 picture without the pillar boxes. The picture has to be stretched horizontally to fill up the wider HDTV frame.
Notice the result is to cause the objects in the picture to look fat. There are other stretch modes that will cause less distortion of the picture, but will cause some of the top and the bottom of the picture to be lost. Many HDTV sets have a couple of zoom modes where the top and bottom of the picture is cropped. Zoom modes will stretch the picture equally for the width and height or some zoom modes will stretch the width more than the height, still making fat objects, just not quite as fat. Still another popular stretch mode is to leave the center portion of the picture unstretched and begin to stretch more and more outward toward the edges. Since the focus of many programs is in the center of the screen, this mode will work very well for many programs.
Here is an example of a zoomed picture where both the width and the height is stretched equally.
Notice how the top and the bottom is cropped due to the equal stretching. Sometimes this is not objectionable, but on news channels the crawl shown at the bottom of the picture is usually gone as is the score line at the top of many sports programs when operated in this mode. Some of the other stretch mode may work better.
Many HDTVs will handle the SD inputs differently. SD inputs can be the internal NTSC (analog) tuner, or any of the SD A/V inputs either through a composite video input or a s-video input. These inputs are 640×480 pixels and scaled up to the native resolution of the display. Most CRT displays have a native resolution of 1080i, or 1080 lines interlaced, and fixed pixel displays can have varying resolutions from 1280×720 to 1920×1080. Some displays have resolutions like 1366×768 due to pixel sizes and other manufacturing requirements, but the picture will be scaled to fit.
Why Sometimes I Get Gray Pillarboxes?
CRT and plasma displays have a possibility of burn in where the phosphors decay at different rates depending on the amount of light they output. Due to this phenomenon the use of black pillarboxes will cause the edges of the display to age less than the center 4:3 portion of the screen. Over time the center portion will dim compared to the edges and the result will be a noticeable difference when 16:9 full screen material is viewed. To combat this occurrence many sets will use gray pillarboxes as shown in the next photo.
When you see gray pillarboxes you should know it was your TV that supplied them not the broadcaster and they will only be used when you are viewing a SD source. Black pillarboxes that are present while you are viewing a HD source will be provided either by your set top box, satellite, cable or over the air receiver or the TV station. Quite often on many HDTVs the HD sources will not allow stretching of the picture.
Sometimes I Have Black Bars All The Around The Picture
Many shows and commercials are being shot in 16:9 widescreen for broadcast in high definition. More and more these 16:9 shows and commercials are being shown in letterbox on the analog SD channel to preserve the image the director wanted in the original widescreen filming. Saturday Night Live is one such show in that this season the SD broadcast is letterboxed. here is an example of what this will look like on a 4:3 SDTV.
When one of these widescreen shows or commercials (mostly commercials) is shown letterboxed and then merged into the widescreen frame of the HD channel, you end up with black bars all around the picture .
This essay has gotten to be pretty long and although some of the artifacts are visable with careful study of the images furnished, there is probably a need for a separate essay covering the various artifacts involved with HDTV pictures. We will investigate this in the essay HDTV Artifacts.