Quick Guide: HDTV Resolution Explained

Introduction

Resolution is the main reason why HDTV looks so much better than standard television. On a high-definition TV displaying a high-definition source, a million or more pixels combine to create images that appear sharper and more realistic than TV ever has before. Resolution isn't the be-all and end-all of picture quality, however, and its numerous, well, numbers, can be incredibly intimidating at first. In this article we'll try to demystify HDTV resolution and help you cut through the hype that surrounds all of those numbers.

How important is resolution?

Although resolution separates HDTV from standard-definition TV, it's not as important to overall picture quality as other factors. According to the Imaging Science Foundation, a group that consults for home-theater manufacturers and trains professional video calibrators, the most important aspect of picture quality is contrast ratio the second most important is color saturation, and the third is color accuracy. Resolution comes in fourth, despite being the most-cited HDTV specification.

The point is, once you get to high-definition, it's difficult to discern further improvements in the sharpness of the picture. All other things being equal--namely contrast and color--HDTV looks more or less spectacular on just about any high-definition television regardless of its size, native resolution, or the HDTV signal's resolution itself. The leap from normal TV to HDTV is so big that additional leaps in resolution--from high-definition to higher-definition, let's say--are tiny by comparison.

Nonetheless the HDTV landscape is littered with resolution discussions, in regard to both sources and displays, so a little knowledge of how they interact is a good thing.

Native resolution: The fix is in

Nearly every HDTV sold today is a fixed-pixel display. A fixed-pixel display is any HDTV or monitor that uses discrete pixels to produce an image, including flat-panel LCD and plasma screens as well as rear-projection microdisplays and front projectors that use DLP, LCD, or LCoS technology. We'll ignore non-fixed-pixel displays, namely direct-view CRTs, because they treat incoming resolutions differently than their fixed-pixel cousins do--since they don't use discrete pixels, their specifications are much more difficult to pin down. They're also basically extinct as a product category.

All fixed-pixel displays have a native resolution specification that tells you how many pixels the display actually has. Native resolution is the absolute limit on the amount of detail you'll see. Typical native resolutions include 1080p and 720p.

Fixed-pixel displays follow a few basic rules:

No matter the resolution of the source material, whether VHS, DVD, or HDTV, a fixed-pixel display will always convert, or scale, it to fit its native resolution.

If the incoming source has more pixels than the display's native resolution, you will lose some visible detail and sharpness, though often what you're left with still looks great. If the incoming source has fewer pixels than the native resolution, you're not getting any extra sharpness from the television's pixels.

If the incoming signal matches the native resolution of the display exactly,such as when a 1080p HDTV displays a 1080i HDTV channel or a 1080p signalfrom a Blu-ray player, no scaling occurs as long as the TV is set to the proper aspect ratio mode, typically called "dot-by-dot," "native," or "1:1." Ideally, you always want to match the source resolution to the display's native resolution, to minimize picture anomalies that can be caused by scaling.

HDTV source resolutions

If you read those three axioms closely, you'll see that source is everything with HDTV. Or, as George Fueschel first said, "Garbage in, garbage out." High-definition sources today come in one of three different resolutions: 1080p, 1080i, and 720p. Comparing the latter two, 1080i has more lines and pixels than 720p, but 720p is a progressive-scan format that should deliver a smoother image that stays sharper during motion. 1080p combines the superior resolution of 1080i with the progressive-scan smoothness of 720p. True 1080p is restricted to Blu-ray, some video-on-demand sources and the latest video games, however, and none of the major networks has announced 1080p broadcasts.

Source resolution name	Resolution in pixels	HDTV?	Progressive- scan?	Wide- screen?	Networks / sources
1080p	1,920x1,080	Yes	Yes	Yes	Blu-ray players; PlayStation 3 and Xbox 360; some video-on-demand sources like Vudu
1080i	1,920x1,080	Yes	No	Yes	Includes CBS, NBC, PBS, DiscoveryHD Theater
720p	1,280x720	Yes	Yes	Yes	Includes ABC, Fox, ESPNHD
480p	852x480	No	Yes	Yes	Progressive-scan DVD players
480i	Up to 480 lines	No	No	No	All standard-definition TV broadcasts

Despite the obvious difference in pixel count, 720p and 1080i both look great. In fact, unless you have a very large television and excellent source material, you'll have a hard time telling the difference between any of the HDTV resolutions. It's especially difficult to tell the difference between 1080i and 1080p sources. The difference between DVD and HDTV should be visible on most HDTVs, but especially on smaller sets, it's not nearly as drastic as the difference between standard TV and HDTV.

HDTV display resolution

Now that we've considered the source, let's look at the televisions. As we mentioned above, all fixed-pixel HDTVs scale the incoming resolutions to fit the available pixels, throwing away information if they have fewer pixels and interpolating information if they have more pixels than the source. The table below shows some of the more common native resolutions; rarer ones like 1,600x1,200 are commonly found on smaller-screen LCDs derived from computer monitors.

Native resolution ¹	Commonly called ²	Meets definition of high-def? ³	Frequency	TV types and typical sizes
1,920x1,080	1080p	Yes	Common among all display types	LCD, plasma, DLP of all sizes
1,366x768	768p, 720p	Yes	Relatively rare among even entry-level LCDs	Entry-level LCD 40 inches and lower, entry-level 50-inch plasma
1,024x768	768p, 720p	Yes	Restricted to entry-level plasmas	Entry-level 42-inch plasma
852x480	EDTV plasma	No	Rare	42-inch plasma
640x480	VGA	No	Rare	Small LCD TVs

Technically speaking, all of these numbers are accurate and useful, but don't put too much stock in them. In the real world, it's difficult to tell the difference between native resolutions once you get into high-definition. For example, despite the fact that a 37-inch LCD with "only" 1,366x768 pixels has to throw away a good deal of information to display a 1080i football game on CBS, you'd be hard-pressed to see more detail on a similar 37-inch LCD with 1,920x1,080 resolution.

The truth about 1080p

In the last couple of years, HDTVs with 1080p native resolution have taken over the market at nearly every price and size point. But as we've been saying all along, once you get to high-definition, the difference between resolutions becomes much more difficult to appreciate. We've done numerous side-by-side tests between two same-size HDTVs, one with 1080p resolution and another with lower resolution, and every time it's been almost impossible to see the difference with regular program material, especially when that material is moving. The difference becomes even more difficult to see at smaller screen sizes or farther seating distances--say, more than 1.5 times the diagonal measurement of the screen. For example, to see the benefits of stationary 1080p content on a 50-inch screen, you'll generally need to sit about 6.5 feet or closer. Few viewers want to sit that close, especially when low-quality content seen at that distance (remember the "garbage" maxim?) looks so bad. The main visible benefit of 1080p native resolution comes when the display is asked to show computer sources. With a PC set to output 1080p resolution and a 1080p display that can accept it, computer desktops and text generally look superb, and quite a bit better than when displayed on a TV with lower native resolution. But for movies, games, and other standard video material, the benefits of 1080p are negligible unless you're sitting quite close. That doesn't matter much anymore though. 1080p native resolution is so common among HDTVs, and has so little impact, that you shouldn't even consider it as a factor in your purchasing decision. As we mentioned at the top, factors like contrast and color are more important to image quality, and unfortunately, you can't depend on a specification sheet for an accurate representation of those factors.

More mixed signals: 1080p/60 versus 1080p/24

1080p HDTVs are a dime a dozen, but not all 1080p HDTVs are created equal. First off, some older HDTVs with 1080p resolution couldn't accept 1080p sources at all. More recently, the advent of Blu-ray has delivered another video format variation to worry about: 1080p/24.

The numbers 24 and 60 refer to frame rate. Moving video is composed of a certain number of frames transmitted every second that combine in the viewer's mind to create the illusion of movement. The nominal rate for film is 24 frames per second, while the rate for video is 30 frames per second. In standard 1080p video, which is technically 1080p/60, each frame is repeated twice. Every 1080p HDTV sold today can accept and display 1080p/60 sources via its HDMI inputs.

Not every 1080p HDTV properly displays 1080p/24 sources, however. Most Blu-ray players, as well as the PlayStation 3, have a setting that lets the player transmit 1080p/24 video directly. Blu-ray Discs with movies that originate on film are encoded at 1080p/24 to preserve the proper cadence of film--that characteristic motion that's smooth but not too smooth. If your player is set to output 1080p/24 directly, and your TV can properly display it, you're seeing the image as close as possible to what the director intended--how it looks when displayed on a cinema screen from a film projector at your local movie theater.

Generally, for an HDTV to properly display 1080p/24 it needs to have a refresh rate at some multiple of 24. The standard refresh rate for HDTVs of all varieties is 60Hz, which is not a multiple of 24. There's no benefit to sending these displays 1080p/24 instead of 1080p/60. If the HDTV can actually show the signal (some cannot), the result usually looks the same regardless of the setting on your Blu-ray player.

On the other hand, increasing numbers of LCD TVs have refresh rates of 120Hz or 240Hz, for example, while a few plasmas refresh at 48Hz, 72Hz, or 96Hz. All are exact multiples of 24. Some of these HDTVs come closer to preserving the cadence of film than others, and some can introduce extra dejudder video processing (usually user defeatable) that also affects cadence. Unlike with resolution, there's no easy way to tell from the spec sheet if a display with a multiple of 24 as its refresh rate handles 1080p/24 correctly, although most such displays that we've tested do.

For most viewers the visible benefits of 1080p/24 are slight. Displays that cannot show it correctly can nonetheless produce a viable semblance of film's cadence, one that to experienced viewers appears to stutter slightly, especially in pans or camera movement, instead of move more smoothly like true film cadence. But for purists interested in seeing every last benefit of film, 1080p/24 signals mated to a 1080p/24-compatible display are worth the investment.

Cnet

¹This is the number of physical pixels the television uses to produce a picture. You may notice that some of the resolutions in the table do not match the HDTV source resolutions exactly. That's mainly because TV makers find it more cost-efficient to make panels with the pixel resolutions in the table, namely 1,366x768 and 1,025x768, and then scale the incoming sources to fit the screen. It's true that ideally you'd like to exactly match the incoming source with the display's native resolution, but it's much less important in HDTV than in, say, computer monitors. That's because scalers in HDTVs generally do a good job of converting the signals, and because most HDTV is in motion and seen from a distance, as opposed to static text seen up close.
²All fixed-pixel displays are natively progressive-scan, meaning that even if the source is interlaced, they'll convert it to progressive-scan for display. That's why, for example, you'll hear about a "1080p LCD" but never a "1080i LCD."
³According to the CEA's DTV definitions, which, for obscure marketing reasons, actually include televisions that have fewer pixels than HDTV source resolutions in the section above.

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