Creative Commons

2006-12-21 14:53:01

Beginner's Guide to HD

If you're like me, you started your search knowing anything and everything about LCD's as they pertain to computers; however, when it came to TVs, DVDs, and video games, you're lost. I spent a couple weeks researching these topics so I could make an informed purchase of a HD LCD TV. I decided to share my findings with everyone so you don't have to spend two weeks scouring the internet (like I did) only to find conflicting opinions and a lack of detail.


So what's the first thing you look at when buying an LCD TV? Size. If you've never seen LCD TV's in person, your first step should be to go to the store and stare at them for a while. Why? A 46" regular tube TV isn't that big. But a 46" LCD TV is huge. You don't realize how different the sizes are until you see them in person. What makes the size so different? As far as we were taught, 46 = 46. The difference here is the aspect ratio.

Aspect Ratio

Normal tube TV's have an aspect ratio of 4:3. That means that for every 4 inches of width, there are 3 inches of height. This, however, has nothing to do with the size of the TV that's printed on the box. That number is the diagonal measurement (not width or height). It only makes sense if you use some simple geometry to figure things out.

Let's say you have a 46" tube TV. Since some manuals don't print the dimensions of the product, you don't know how wide or tall your TV is. Here is an off-scale drawing of the TV, with the diagonal being [poorly] marked and the sides given variables. To the right of it is a drawing of what we know, the 4:3 ratio.

---------------- ---------------- | /| | /| | / | | / | | / | | / | | 46 | b | c | 3 | / | | / | | / | | / | |/ | |/ | ---------------- ---------------- a 4

We need to have complete data of the 4" x 3" control before we can apply those proportions to our 46" TV. So, let's solve for 'c'. Using the Pythagorean theorem (keep in mind, raising to the .5 power is the same as taking the square root):

a2 + b2 = c2 42 + 32 = c2 16 + 9 = c2 25 = c2 25.5 = c 5 = c

Our diagram now looks like this:

---------------- ---------------- | /| | /| | / | | / | | / | | / | | 46 | b | 5 | 3 | / | | / | | / | | / | |/ | |/ | ---------------- ---------------- a 4

So we now know that the diagonal size in the 4" x 3" control is 5". Now we can use those numbers to figure out the unknowns of our 46" TV. To do this we just have to use proportions, comparing like sides. To figure out the value of 'a' we need to have the value of a side known for both diagrams. Since we just did the above calculation, we have concrete values for both diagonals. We use those concrete values (actually, their proportion) and apply them to the width ('a' and 4).

46 a -- = - 5 4

Now, we cross multiply and solve for 'a'.

46 · 4 = 5 · a 184 = 5a

Get the variable alone, in this case by dividing both sides by 5.

184 = 5a --- -- 5 5 36.8 = a

Let's solve for 'b' while we're at it.

46 b -- = - 5 3 46 · 3 = 5 · b 138 = 5b 27.6 = b

Ok, so we've figured out the width and height for a 46" tube TV; 36.8" wide and 27.6" high. Now let's look at a high definition LCD TV, which has a 16:9 aspect ratio.

-------------------- -------------------- | /| | /| | / | | / | | / | | / | | 46 | b | c | 9 | / | | / | | / | | / | |/ | |/ | -------------------- -------------------- a 16 a2 + b2 = c2 162 + 92 = c2 256 + 81 = c2 337 = c2 337.5 = c 18.3576 = c

So, the diagonal of the 16" x 9" control is approximately 18.3576" (I rounded to 4 places, so all calculations from here on will be slightly off).

-------------------- -------------------- | /| | /| | / | | / | | / | | / | | 46 | b | 18.3576 | 9 | / | | / | | / | | / | |/ | |/ | -------------------- -------------------- a 16

Now let's find the width and height of a 46" 16:9 LCD TV.

46 a ------- = -- 18.3576 16 46 · 16 = 18.3576 · a 736 = 18.3576a 40.0924 = a

Now for 'b':

46 b ------- = - 18.3576 9 46 · 9 = 18.3576 · b 414 = 18.3576b 22.5520 = b

So we see that buying a 46" LCD TV means that you better have a space that can hold over 40" of width and over 22.5" of height (not taking into account the housing). That might not seem that big. Go find a large cardboard box (if you have the one your desktop computer came in, that'll work) and trace out a rectangle 40.0924" x 22.5520". Cut it out and see if it'll fit where you want to put the TV. Larger than what you thought, huh? If you don't want to go through all of that, I can tell you that a 46" LCD TV is as high as my 24" tube TV and a LOT wider.


Now that you know what size will fit you best (or at least how to figure it out) your next step is to make sure you're getting the best picture for the size of TV you bought. All kinds of things come into play at this step, but the only one we can speak of in theory is resolution. What's the difference between 720p and 1080i, or 1080i and 1080p?

Let's start first with what the numbers mean. Here is a table of resolutions and their corresponding size in pixels:

-------------------------- | Name | Pixels | -------------------------- | 1080p | 1920x1080 | -------------------------- | 1080i | 1920x1080 | -------------------------- | 768p | 1366x768 | -------------------------- | 720p | 1280x720 | -------------------------- | 480p | 852x480 | -------------------------- | Std. TV | <= 480 lines | --------------------------

A tube TV, which has an electron gun firing rays at the screen, can be any size and the picture will look just as good. An LCD TV, on the other hand, has a set field on which the image can be displayed (native resolution), a finite number of pixels horizontally and vertically. Think of your computer's wallpaper. You find a picture on the internet that you want to make your wallpaper, so you right click on it and make it so. You minimize your web browser and the picture looks horrible. It's taking up the whole screen, which is cool, but it's all blurry and boxy. That is what happens when you take an image of a smaller size than the native resolution and display it stretching to fit the screen. The same will happen with your LCD TV. What this means is that if you buy a 1080p TV (native resolution of 1920x1080) and you watch a DVD (480p) or HD TV channels (720p) the quality will be slightly degraded. This is because the image processor in the TV has to take an image that is 1280x720 (in the case of HD TV) and enlarge it to fit the 1920x1080 screen (remember the wallpaper on your computer?). Now, because most of these resolutions are proportionate, you won't notice any bad blur or super boxiness like you did with your wallpaper, in fact, you might not even notice it at all. I am just making the point that the picture would be better if you had a source that could send an image to your TV at the same size your TV expects (such as a BluRay or HD-DVD player outputting a 1080p signal to a 1080p TV).

Screen Drawing/Refreshing

Ok, now what about those letters after the numbers; 'p' and 'i'? Those letters have to do with how the image is drawn on the TV screen. Let's look at 1080i and 1080p. They have the same pixel size, but they are drawn differently. The 'i' stands for "interlaced" and the 'p' stands for "progressive."

An interlaced signal draws the image on your screen in 2 steps. First, it draws the odd numbered lines (1, 3, 5, etc.), then it draws the even numbered lines (2, 4, 6, etc.). Each step (odd and even) take 1/60th of a second, effectively drawing each frame in 2/60th of a second. This corresponds with a 30Hz refresh rate.

A progressive signal draws the image on your screen in one swoop. This type of signal is 60Hz. Since a Hz is defined as 1 cycle per second, we can deduce that a 60Hz signal is twice as fast as a 30Hz signal at drawing images on the screen. The 1080p signal can thus draw the complete frame in 1/60th of a second where the 1080i signal took twice as long, at 2/60th of a second.

Does that mean that you should choose a 1080p TV over a 1080i TV? Technically speaking, yes. Most good LCD TV's calculate their pixel response time at 8ms. That means that each individual pixel can change its state in 8ms. Let's apply this knowledge to the above refresh rates. An interlaced signal can change the state all pixels in 2/60th of a second (33.3ms) and a progressive signal can change the state of all pixels in 1/60th of a second (16.6ms). You can see that the TV's pixels are capable of performing faster than each signal can possibly make it perform. But, the progressive signal is closer to the theoretical performance of the pixels. For us humans, we really can't tell the difference. Whether the image gets drawn in 33.3ms or 16.6ms, we really won't see much of a difference in day-to-day watching. However, in movies/games with super fast paced action, the faster refreshing means less ghosting (remnants of the previous picture left on the screen because it can't update fast enough).


"Wow, LCDs have a contrast ratio of 7000:1. That must mean they're better than plasmas with their ratio of 5000:1."

That, however, isn't true. What you are seeing there are different types of contrast ratios. LCDs are usually spec'd with their dynamic contrast ratio, whereas plasmas are usually spec'd with their static contrast ratio. A contrast ratio is the ratio of maximum to minimum luminosity. A static contrast ratio is that ratio at a given instant while a dynamic contrast ratio is that ratio over a period of time. It is because of this that the two ratios cannot be directly compared. A plasma with a static 5000:1 contrast ratio will show superior colors and deeper blacks than an LCD with a 5000:1 dynamic and 1000:1 static contrast ratio. Now that you know what the different types are, you should realize that this is not something that a number alone can explain. You need to get to a store and compare LCD TVs to see which has the better picture.

Viewing Angles

This is one of the easier concepts to grasp. Imagine you're hovering over the top of your TV, looking down at it. Now picture a half circle, extending from the face of your TV. That half circle consists of 180º. Now put yourself on the ground, centered in front of the TV. You now have 90º of rotation to your left and 90º to your right until you are directly beside your TV. Most good LCD TV's have a viewing angle of 178º. That means that if you are standind directly in front of your TV, you could move to either side 89º and still be able to see the picture. That is near perfection. A 180º viewing angle is impossible because you would be directly in line with the face of the TV. With tube TVs a 180º viewing angle was possible (though not recommended) because the face was convex (bulging outwards), so you could see some of the picture. However with LCD TVs, the face is completely flat, so sitting along the 180º line would mean you couldn't see the face of the TV.

Aspect Ratio Calculator

Now that you know how to do the math, here is an easy way to find the dimensions of your TV. This javascript calculator was written by Anthony Marguccio.

Aspect Ratio:       Diagonal:
Height:     Width:



2007-07-27 13:04:17

Steven Rothwell says...
Nice guide.

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