# Technical & Editorial

## Video Resolution . . . Simplified

Introduction

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.

Vertical resolution.

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