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Some basic terminology:
Resolution: This refers to how much detail your TV can display. The higher the number, the better the picture.
p/i – Usually you will see “p" or “i" following the resolution number (1080p). P = Progressive. I = Interlaced. We will talk more about this later.
Update Oct 2010: "i" is almost no longer relative to TV purchases. If you are buying an old rear projection TV on craigslist, you may come across a resolution like 1080i, but all new TVs are either 720p or 1080p.
Refresh Rate: Refers to how many times the image is re-drawn on the TV. This will generally only be a number that movie buffs or gamers are concerned about.
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What resolution do I want?
The standard middle of the pack HD TV will generally have an advertised resolution of 1080i, or 720p. Here is where the “i" or “p" tag is important to understand. Even though one has a higher number, it is an interlaced number. 720p shows more lines of resolution per refresh (720), where 1080i only shows 540 lines of resolution per refresh (two refreshes to make the whole picture).
Which is better? It depends on what you watch most. 1080i is showing you the full picture (assuming your source is 1080p), but it only shows you half the picture per refresh. So it takes two refreshes to display the full picture. Your eye can’t see this, but if the image on screen has fast movement (sports), you will see artifacts or blocky splotches. 720p will instead show the full image in a single refresh, but scaled down to its lower resolution. While some of the finer detail is lost, no artifacts or blocks will distort the image when there are fast moving objects on screen.
So both 1080i and 720p make compromises, what about 1080p? 1080p is the highest resolution that consumer TV’s offer and the highest resolution produced for consumer media, whether that be a Blu-Ray or an HD broadcast.
The reason why 1080p might not be the most viable option now is because it is more expensive than similarly sized 720p televisions, and aside from expensive Blu-Ray media, most TV is broadcast at a sub 1080p resolution, or is compressed so heavily that there is no noticeable advantage to owning a 1080p television. However, the quality of both television broadcasts and internet broadcasts continues to improve, making a 1080p television more and more relevant going forward.
Regardless of the media quality pumping into the television, most people agree that in screens under 50", there is no discernible difference between 1080p and 720p. Ergo, if buying a 42" television, you might be paying more for something you will never see.
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What refresh rate do I want?
If you live in the United States, 60hz is the standard, and has been for years. If you live outside the states, 50hz is the standard. We are only going to discuss the US standard. The reason why this is an issue at all is because TV programming is shot at 30 frames per second (fps). Therefore on a 60hz TV you get one interlaced frame per refresh, or two progressive frames.
Films are shot at 24 fps which doesn’t divide correctly into 60hz. Since 24 doesn’t go into 60 properly, your TV has to implement a technique called “3:2 pulldown". While the math behind the technique isn’t important to you or me, it is important to note that while it allows for films to be displayed on a 60hz television, as it has done reliably for the past 60 years, it leaves an undesirable effect in the shape of a micro stutter. This is most noticeable on pans.
What all this means is that if you will be watching movies on Blu-Ray, and you want the most theater like experience, then you will want a TV or projector that natively supports “24p"(1080p @ 24fps). Again, while a television might support this, it needs to natively do so, which means that the TV supports a refresh rate that divides evenly by 24. A TV advertising a 120hz refresh rate or higher will generally support 24p natively. More common on projectors is 24hz, or 72hz.
This feature is generally only found more on high end TV’s. You can expect to pay a pretty premium for the feature, and so if you don’t plan on watching movies on a format that supports 24p (Blu-Ray), or don’t care for the ultimate home theater experience, then this is a feature that will likely not be missed.
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A Few Reminders
You can have the highest resolution TV on earth and still have a bad image on your television set. Before purchasing a TV, decide what resolution and refresh rate you want, and then do your research. Read reviews on TV’s or projectors in your price range. Color and shades can vary from very accurate and desirable, to washed out or overblown. Just because you spend a lot of money on a TV, doesn’t mean it is going to be good. Finally, you can have the best TV on the planet, but if you don’t use the proper cables, and have HD content piping into the TV, then the image will look terrible.