Technical Articles and Editorials
- Written by Brian Florian
- Published on 30 November 2007
Even though we've discussed video resolution in the pages of Secrets more than once, a real understanding of it still eludes a large number of people that I've encountered recently. What struck me as particularly curious is that some of the computer professionals I've talked to at my office have a firm understanding of resolution in computer terms but don't know where to begin when it comes to home theater video. Making matters worse, we have manufacturers and sales people throwing all sorts of figures at us: TV sets claiming 600 - 800 "lines of resolution". DVD stats saying 480, 500, 640, and 720. In the following article I would like to give you the tools necessary not only to understand our current NTSC video system, but also gain the ability to intelligently approach the new and upcoming video formats.
First the basics . . . the very basics
Video is made up of static pictures being flashed before us in rapid succession. When we talk about picture resolution we are referring to the elements that make up the pictures. Think of a good old fashioned sheet of squared graph paper. If you were to color in each square with a certain color you could come up with a picture that is a close resemblance to a digital image. Have a look at the close-up of my eye (Fig.1 shown at right). When we zoom into the picture, we can start to see the elements (in this case squares) that make up the picture. Resolution in this case means how many of those discrete, and not further divisible, elements that make up the picture. For the purposes of this essay, we can safely say that the more elements in either dimension, the better the picture. There will be more information to the picture and a sharper image as the result.
Since vertical resolution is the easiest to understand, that's where we'll start. In simplest terms it is the number of picture elements the image has in a vertical measurement (from top to bottom). People can easily get confused as the wording "number of horizontal lines" is often used. Have a look at Figure 2 on the left. Images in a picture tube are formed by an electron beam lighting up the phosphorus face. It does so by "drawing" each line, left to right, top to bottom, line by line. The number of distinct horizontal lines from top to bottom is our vertical resolution. In our NTSC television system, that number is 525, and it's 525 regardless of the source, such as VHS, laserdisc, DVD, etc. (The number is 625 for the PAL system in Europe.) This is fixed. We should note that not all of the 525 lines are used to paint the picture that we view. In actual fact, 43 of those lines get pinched and used to carry other data such as the closed captioning text, and video test/monitoring signals. Also, a few are lost during the time that the electron beam moves from the bottom back to the top to begin scanning another picture. That leaves 482 lines of information that make up the final picture we see, and this number is the limit for vertical resolution in our NTSC system.
Of these 482 lines, only half get painted at a time. That is because our NTSC system is interlaced. Every other line (1,3,5, etc.) is painted first in 1/60 of a second, then the remainder of the lines (2,4,6, etc.), one between each of the first set, get painted in the next 1/60 of a second. So a half-frame of information is delivered every 1/60th of a second. For more information on interlaced display, check out our article on progressive DVD.
Horizontal resolution . . . where the confusion begins.
Horizontal resolution should be the number of elements the make up the picture as measured across or horizontally (side to side). Like its counterpart, the horizontal resolution is sometimes called the "number of vertical lines". Personally, since we've already got horizontal lines stacked one on top the other in our minds, I'd prefer to think of horizontal resolution as the number of unique picture element which each of those horizontal lines contains. See Figure 3, below, right. This number, unlike vertical resolution, can vary and does so primarily in terms of the source's capability. The problem that I have found is that horizontal resolution can be expressed a couple of different ways. One method is used in computer circles where it is expressed as the absolute number of elements that make up the picture on your computer monitor from left to right. This is not how it is traditionally expressed in the video world. For consumer television we express horizontal resolution as the number of picture elements across per picture height. If you have a quick look at your TV (or computer monitor), you can see it's not square but rectangular, slightly wider than it is tall. When talking about horizontal resolution, we want to express it in a way that can relate to the vertical (so we can know if it is more or less sharp in one axis than the other and by how much). So when you hear that a particular video source has so many lines of horizontal resolution, it is not the total number of picture elements drawn on each line from left to right, but the number drawn within the distance of the height of the picture. This is where the confusion sets in. We've been told that VHS has 220 lines, broadcast TV has about 330, laserdisc weighs in at 400, and our beloved DVD boast 540. 540? If that is the best that is available at this moment, then my TV with a horizontal resolution of 600 ought to be plenty. Unfortunately, not so. In just about every case, the TV manufacturer is talking about the total number of picture elements its tube can show from left to right, not the "per-picture-height" that all the above figures are expressing. To put it into perspective, consider the following: the ratio of our TV's width to height is fixed at 1.33:1 (that is, the screen is 1.33 times wider than it is tall). This is called the "aspect ratio". If we want to know what the total number of picture elements in the horizontal comes from say, DVD, we take its per picture height figure (540) and multiply by 1.33 for an answer of 720.
Hummm…..recall for a second what we said the vertical resolution in our system was: 482. If DVD is delivering 720 elements horizontally, the resulting picture is 720 x 482. If the total number of picture elements horizontally delivered by DVD is 720 then this so called "high resolution" television of mine with a horizontal resolution of 600 is already incapable of showing all the information that DVD has to offer!
In reality, there are a number of other very technical factors that limit the actual resolution you physically get, but these are beyond the scope of this introductory paper. There is one I'd like to touch on though: Overscan.
Overscan: A thief
Ever notice how the image on your TV goes right to the very edges? Do you really think that your set is so precise as to fit the picture perfectly to its glass? The answer is of course no. All televisions have what we call "overscan". Portions of the actual picture area fall outside of the physical viewable area of our TVs, sort of like a photograph that has the matting covering the rough edges of the picture. How much of the picture that gets lost to overscan varies from one set to the next, but it's safe to say that about 5% of the picture area is missing from each edge with some sets cutting off as much as 15% on one edge or another. Before you panic about missing some really important detail on "Seinfeld" tonight, take heart in the fact the program creators are aware of the situation and are careful not to put something critical too close to the edge of the picture. They have what is called a "safe title zone", somewhat in from the edges where it is considered safe to put, say, a title (duh!) They also have a safe action zone closer to the edge within which it is considered safe to put action that you really need to see. This does not mean we should all be happy with this state of affairs. We are losing, wasting if you will, real picture detail at the edges. So, instead of physically seeing 720 pieces of the picture along each horizontal line, I might only be seeing 620 - 650 (assuming all other hardware can display that resolution). So why overscan? Simply stated, it's our own fault. Consumers hate seeing any fraction of an inch of their picture tube going to waste. They want the image to fill the screen (never mind the fact that you end up missing part of the picture). The funny thing is, in the computer world, we don't mind a small black border around our windows desktop at all. Go on, have a look right now. If you've ever played with the sizing controls on your monitor, go ahead and stretch the picture vertically until your Windows "Start" button disappears below the bottom of the screen. Now do the same in the horizontal. How would you feel about working Windows like that all day? You can't even see your task bar (or menu bar for the Mac users). If you've managed to make either disappear off screen, you've now overscanned your monitor. Got an idea what you might be missing on your TV?
Looking to the future: DTV, HDTV
So far we've been talking about our current NTSC system. Having a fixed vertical resolution of 525 interlaced lines, it has come to be called our "525 line interlaced system". DTV and HDTV incorporate many different resolution possibilities that are coming to be referred to by the following syntax: X,XXX(y) where X is the vertical resolution of the picture area and y marks whether it is interlaced like our current system or "progressively scanned" (all lines drawn one after the other). Following that syntax, our current system could be called 480(i). New standards include 480(p), 720(p), and 1080(i), just to name a few. This is actually some comfort to us DVD collectors, as the information on most discs, though stored for the benefit of today's 480(i) TVs, can be reassembled into 480(p) by a good progressive display system. With the right future DVD player and DTV/HTDV screen, we should be able to see our current collections in a better light than currently possible.
I hope by now you have an understanding of what picture resolution figures represent and the potential for things to be less than they seem. The examples used here represent ideals, and as I've said, there are other technical reasons that these are not absolutes. Nevertheless, the next time a salesperson tells you that his DVD player does 480, and the next one says his is better at 640, or even 720, you'll know the reality of both . . . marketing.
P.S. Although some people choose to size their monitor's image close to the edges, most graphic artists and high end visual professionals traditionally choose to size the image further in from the edges than most people would like. Although it produces a smaller image, these professionals know that a picture tube, even very expensive ones, distorts the image as they approach the edge. Even with today's superior image control electronics, for the ultimate in color fidelity and alignment, they keep the image away from the edge.