Bars, Bars and More Bars
By Richard Binckley
May 9th, 2005
The first thing I get ask when the new 16:9 HDTV is fired up is, â€œHow do I get rid of those gray bars?â€ Every new owner of a HDTV has considerable confusion about the pillar boxes that show up when SD 4:3 material is tuned in.
Edit: I have been asked to supply picture examples and I decided to do this in a separate essay. See: HDTV Pictures
The original television tubes were round and it would have been very difficult to be able to show a wide screen format on them. Square would have been the best as far as the tube goes, but there have been studies that when we humans watch a square for any length of time we get very tired, so the 4:3 format was adopted by the movies prior to the 1950s and the TV manufacturers decided to use the same aspect ratio. So now along comes HDTV and in addition to greatly improving the picture, they decided to make it 16:9 widescreen as well to emulate the movie experience. Obviously, if you take a 4:3 SD picture and try to show it on a 16:9 widescreen, it will not fit. Enter pillar boxes, or vertical bars on the left and right of the screen.
Why Gray Bars and Sometime Black Bars?
Plain and simple the gray bars are used by the TV manufacturers to reduce the phosphor aging difference between the SD picture and the pillar box areas of the screen. If the pillar box areas of the screen were left black, the phosphors would not age as fast in those areas as the picture area would. Using the gray pillar box areas will cause the phosphors to age roughly the same as the picture area.
So why sometimes are there black bars? The black bars are added by the broadcasters for the 16:9 sub-channels. Once a sub-channel is formatted for 16:9 HD material, they will not switch the format. They just merge the 4:3 SD video into the black 16:9 frame. They could switch formats between the SD format of 480p and the HD format of 720p or 1080i, but that will cause most TV to blink and stutter while they resync to the new format, so they donâ€™t switch formats back and forth. So why donâ€™t the broadcasters use gray pillar boxes, you ask. Because black is the natural no video state and to use gray pillar boxes would require a video generator and quite frankly, they are not worried about your phosphor aging. Some broadcasters, like ESPN and our local FOX station, have added video generators to put their logo in place of the black bars.
How Do I Get Rid Of The Bars?
When I was looking at this set in the store, I didnâ€™t see any bars, how did they do that? Another study made early on in the adoption of TV was that the vast majority of people do not like to see blank spaces on a TV screen. This is why overscan was adopted by the TV manufacturers from the very first sets. Early on the problem was two fold. Not only were the tubes round, but they were not very accurate out at the edges of the CRTs. By starting the scan a ways beyond the edges of the screen, the edge of the screen that was visible would have less distortion. Bottom line is for decades, we have been conditioned for a TV picture to fill up the screen and we find it annoying if it does not. So enter the zoom and stretch modes to allow the picture to be manipulated to fill the screen. Just a note on this subject, the need for the picture to fill our screens is somewhat unique to TVs as we all seem ok with our computer monitors being adjusted just shy of filling the screen.
To accommodate this need the TV manufacturers have incorporated various methods to manipulate the picture to fill the screen. The electronics to manipulate the video was developed before the HDTV even existed for the early SD widescreen TVs that were for viewing widescreen DVDs, so they will only manipulate SD video. Only a very few HDTVs have any ability to manipulate HD video.
There is the 16:9 mode where the 4:3 SD picture is just stretched horizontally to fill the screen. If the original video is 4:3 material, then everybody on the screen looks very fat. This mode is best suited for widescreen video that was recorded on a VCR from a 16:9 source or viewing an older DVD player through s-video or composite inputs.
There is the Cinema mode where the picture is stretched vertically a small amount and the horizontal stretch is more near the edges than it is in the center of the screen. Tuning into the news channels where they have the crawl at the bottom of the screen will demonstrate this effect very well as the letters of a word will start fat on the right of the screen, gradually shrink to normal size in the middle of the screen and then get fat again as they disappear off the left of the screen. Different manufacturers have different names for this mode, but most all brands have some version of this mode as it is the favored mode for the stores to show SD material, because until you watch for a while it is hard to detect the stretch unless there is a crawl on the screen. Actually, they try their best to only show you HD or Finding Nemo because it shows the TV the best. I have said many times, it is best to audition your prospective HDTV on SD material and get the one that looks the best on SD. The HD will just be all that much better.
The last video mode many manufacturers provide is the zooms. Some provide a zoom that will stretch the screen horizontally to fill the screen and the vertical not quite as much, so while not keeping circles exactly round, the screen is filled without the Cinema effect. Sometimes the TV will have the ability to shift the visible part of the picture up and down. Full zoom where the screen is stretched equally in both directions and the top and bottom of the screen is cropped.
As I said before, the video processors for manipulating the video was developed for SD video. Since most HD video is 16:9 widescreen and will therefore fill the screen, most manufacturers do not provide any video manipulation for the HD video as it would require another video processor. So if you are viewing SD video that the broadcaster has inserted in a black 16:9 frame, you will not be able to stretch it like you could if you were viewing a SD input.
Letterboxed Movies and Shows
Another source for black bars is the letterboxed movies and commercials. More and more commercials are being shot in widescreen and even some shows are shown letterboxed when broadcast in SD. An example is American Chopper on the Discovery Channel. It is shot in HD widescreen for their HD channel, but is shown in letterbox on their SD channel. On a 4:3 TV the image will have black bars at the top and the bottom. If this image is merged into a 16:9 frame, you will end up with black bars all around the picture.
In the case of a letterboxed movie on DVD any number of strange occurrences can occur. If the DVD is formatted for widescreen then there will be letterbox bars top and bottom, but people will look correct. If however the DVD is formatted for full screen (4:3) there will still be letterbox bars top and bottom, but people will look fat.
Tip for viewing SD video
If you want to view stretched video when the broadcaster has opted for black pillar boxes, you might consider hooking up a SD connection, either s-video, composite video or even the RF output to your TV in addition to the HD connections. Then you can select the SD input on the TV when you want to stretch the picture and return to the HD input when HD is being viewed. I know this is a bit of pain, but if you had bought a more expensive TV that allowed HD zoomingâ€¦. Well thatâ€™s the trade off. Some cable and satellite boxes allow for stretching and zooming of the video and that might work for you as well.
Copyright 2005 - Richard Binckley