By David Gibbons
Revised July 2010
Perceived picture quality on televisions and video displays today is the result of a combination of technology factors, business decisions, human visual perception, and the choices you make as a consumer and user of the technology.
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1ST POINT:BIG SCREENS & LITTLE IMAGES
Imagine you have have a spoonful of peanut butter. Now you want to spread it on a piece of bread. On a normal piece of bread, that spoonful of peanut butter provides a nice thick tasty coating. However, if you go to Costco and buy a giant loaf of bread where each piece of bread is six times the size of a normal piece of bread, you're going to have to spread that peanut butter pretty thin to try to cover the bread. Back during the days when television was new, screen sizes were REALLY small. That is, the piece of bread wasn't too large and so you could get a good coat of peanut butter on it. In fact, the original National Television Standards Commitee (NTSC) never dreamed that somebody would try to put the NTSC Standard television picture on a great big screen and then sit right up close to it.
Now imagine that in addition to having that giant piece of bread, you decide to stretch it sideways to make it even bigger! (Wide-screen high-definition TV's allow you to stretch a regular picture sideways to fill the screen.) Now the peanut butter has to be spread even further. At this point, you are probably not getting much peanut butter flavor...
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2ND POINT: VIEWING DISTANCE, IMAGE QUALITY, AND FIELD OF VIEW
How close you sit to the screen has a lot to do with how you percieve the picture quality. There was a French painter named Georges Seurat who explored this effect with his "Pointillist" paintings. These paintings were composed ENTIRELY of tiny dots of color. (The similarity to digital technology was entirely coincidental, I think...and Seurat had way too much time on his hands.) If you stand with your nose right up to the painting, you can hardly make sense of it. However, if you stand back 10 or 15 ft., the dots all blend together to form a beautiful picture. Pictures in newspapers work the same way. If you get up really close to them, they break up into patterns of lighter and darker areas. Get too close, and any video image breaks up also.
Your eyes (and the world-class signal processing behind them in your brain) are the other factor in viewing distance. You have a digital imaging system (Remember the rods and cones in your retinas?) which has its own limit of resolution. If you start with your nose on the TV screen, you will see the pixels or scan lines that make up the TV image. Back away slowly, and hopefully you will reach a distance where the pixel or scan structure disappears, and the picture looks good, with hopefully all of the image detail visible. Back away further yet, and you start to lose the details of the picture due to the natural limitations in your own vision. If you sit far enough away from a high-definition picture, you will reduce the detail you can make out to that of an NTSC image or less. The better the image quality, the closer in you can get, and still perceive a good image.
This matters because image resoution determines how much of your field of view the picture will take up when you back off to the "can see all of the picture detail well" distance. Supposedly the goal is to have a viewing angle of 33-36 degrees (The screen takes up 33-36 degrees of your field of view.), but NTSC was never intended to provide enough picture resolution to do this. Supposedly the highest quality levels of the modern Advanced Television Systems Committee (ATSC) Standard will allow this.
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3RD POINT : LOW IMAGE QUALITY = LONGER VIEWING DISTANCE
Screen size BY ITSELF does not determine picture quality! Picture quality (as perceived by the user) is the result of the combined effects of screen size, fineness or resolution of the image on that screen, AND the viewing distance. Even if you have a low-resolution image on a screen, the perceived picture quality can still be quite good if the screen is very small (Think 4 in. portable TV.) or if the viewing distance is very large. (Think of those video displays at the sports stadiums, where you are seated hundreds of feet away.)
Because today you can end up viewing image resolutions ranging from tremendous (top quality high-definition signals from satellite or high-definition disk players) to miserable, (the kids' half-worn out VHS tape of some awful cartoon) you have a problem. There is no one proper viewing distance. Given a television of a certain size, to avoid seeing the defects of the low-quality sources , and to be able to see the detail provided by the high-quality sources, you will need to put your viewing chair on rails. That way you can slide up close for the good stuff, and slide way back for the bad stuff.
Some companies sell "up-converting" DVD players that promise to turn a standard definition video source into a high-definition video source. Sorry folks, but you can't make two scoops of peanut butter out of one. On high-definition TV's, there is an improvement, but the picture is not high-definition. Test-drive before buying...
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4TH POINT: NEW TV TECH AND OLD TV TECH
Imagine there is a tiny painter inside your television. You know, palette, beret, smock, easel, the works. Now imagine that what is going through the wire to the television is paint. The older NTSC technology was set up so that enough paint was provided so that the painter could paint an ENTIRE picture (frame) on the TV screen every 30th of a second. If the signal coming down the cable or through the air was not up to the standard, the picture quality suffered accordingly, but a complete new picture got painted every 30th of a second.
But now we have DVD's and the digital (ATSC) television standard. Now the "paint" coming through the cable is digital, and the little painter inside the television is digital also. This new digital painter can do some really neat tricks. The best one is that it can remember exactly what the last picture it painted looked like. If the next picture (frame) is not changed, or changes very little, the painter doesn't need to repaint the whole picture. It just leaves it alone, or just repaints only the small areas that changed. That way, the flow of paint is minimized.
In a digital television system, if the picture on the screen does not change, there is little or no information being sent to the little painter in the television other than "keep showing the same picture without changes." This is how it is possible to fit a full-length motion picture onto a DVD or Blu-ray disk. Whenever the action on the screen slows down, so does the amount of information being moved from the disk to the television screen. In the old NTSC system, the information to completely create every frame is present in the signal all the time. Using that old approach, several DVD's would be necessary to hold a full-length motion picture. Who remembers the 12" Laserdisk?
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5TH POINT: YOUR IMAGE QUALITY AND THEIR BUSINESS DECISIONS
The cable from the satellite receiver or cable company box is a pipe for carrying paint, and the DVD is a bucket for carrying paint. Here is where the business decision part comes into the mix. The fine corporations who sell you cable or satellite service or DVD's want to maximize their profits. Here is a couple of the ways they do it.
CABLE/SATELLITE - They cram as much as they can into the pipe. The more channels you receive, the more advertising and monthly fee dollars for the service provider. The more pay-per-view channels, even more dollars for the service provider. The new digital technology allows a huge number of channels to be packed onto a cable or satellite signal. Would it surprise you to know that as you pack more television channels onto a satellite or a cable that the quality of the pictures on those channels may start to suffer?
There are two reasons. First, the ATSC standard allows for many different levels of picture resolution, ranging from lower than NTSC quality up to some very impressive high-definition levels. As you might guess, it takes a lot more paint coming through the pipe for the little painter to be able to paint the highest level quality high-definition picture on the screen of your television, compared to the lowest quality level picture defined in the standard. (New sets are supposed to be able to display all of the various quality levels in the standard.) For profit reasons, many channels on modern cable systems (proudly advertised as "high quality digital cable") are being transmitted at the lower levels of quality, and so will never look that good on your big hi-def display.
The second reason is that modern digital compression technology makes it possible for the little painter in your television to "fudge" if the paint is not coming through the pipe fast enough to keep up. Remember, if the picture is not changing very fast, then the little painter doesn't need much paint coming in through the pipe to make the minor updates needed to keep the picture up-to-date. However, if the picture is changing a lot, then the painter is going to need a lot of paint to keep up. The cable and satellite companies can therefore cram channels into the pipe until the satellite or cable is just barely able to keep up with all the channels' demands for paint. Unfortunately, they size it to just keep up with the average demand. If it so happens that a lot of channels simultaneously need a lot of paint because their pictures are all changing rapidly at the same time, the pipe can start to run dry.
When that happens, the software and computer in your television (Oops, I mean the little painter) will fake it by simplifying the picture. What you will see is that parts of the picture will break up into large blocks of color, which are the best guess the little painter can make about what the picture looks like without enough paint. A giveaway is that the parts that break up into blocks are the parts of the picture where there are movement occurring. This is called "macro blocking". Don't blame your television. Blame the profit enhancing decisions of your cable or satellite company executives.
DVD's and BluRay's - Companies sometimes cram a lot of extras onto Disks these days to make them look like a better deal. (Note: I believe it costs them less than $2 to manufacture a DVD these days.) In addition, advertising for various other films and products can also be on the disk. This means a good part of the space in the "paint bucket" is now taken up with the extras and advertising. Therefore the amount of paint available in the bucket for the the little painter in your TV to actually paint the movie you want to watch is reduced. The little painter will do the best it can, but the picture quality will suffer.
Watch out for disks of television shows where many hours of programming are crammed onto one disk. The picture quality probably won't be very good. Macroblocking is a possibility, but more often the picture sharpness suffers, particularly in action scenes. So, look for disk sets where the extras are on another disk. You have a better chance that the main feature's picture quality will be decent. 'Super Bit' DVD's are one of the pleasant exceptions: the makers decided to provide the best image presentation possible, and so actually dedicate all of the available bits on the disk to the movie. (What a wild-eyed concept that is!) Hopefully we'll see a 'Superbit' Blu-Ray available also.
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I hope this essay gives you an idea of some of the factors involved in perceived picture quality beyond having a good TV well adjusted and a good viewing room.
Good Viewing,
David Gibbons
