HDTV World > Understanding HDTV > CNET's quick guide to aspect ratio

It's a familiar story: You've just invested thousands of dollars in a new TV, but all the actors on your favorite shows look unnaturally bloated and fat. Or there are black bars at the top and bottom of the screen--and on the left and right. Or everything on your favorite DVDs looks tall and skinny. All of these problems are examples of one of the most important--and least understood--issues in the home-theater world: aspect ratio. We'll take you step by step through the common aspect-ratio problems--and their solutions--on both standard and wide-screen televisions.

What is aspect ratio?

The concept is simple enough: aspect ratio is the fractional relation of the width of a video image compared to its height. The two most common aspect ratios in home video are 4:3 (also known as 4x3, 1.33:1, or standard) and 16:9 (16x9, 1.78:1, or wide-screen). All the older TVs and computer monitors you grew up with had the squarish 4:3 shape--only 33 percent wider than it was high. On the other hand, 16:9 is the native aspect ratio of most HDTV programming; it is 78 percent wider than it is tall, or fully one-third wider than 4:3.


At comparable screen sizes, the wide-screen image is a distinct improvement: it offers a larger image, and the horizontal orientation is more akin to how your eyes--next to each other, not on top of one another--view objects.

Both of these formats work perfectly well when they match the TV screen's native aspect ratio--standard programming on a 4:3 screen (any 1950s to 1990s Nick at Nite fare, for instance) and any newer, wide-screen material on a 16:9 set (HDTV programming or most DVDs). But as soon as you try to watch 4:3 content on a wide-screen monitor or 16:9 content on a 4:3 TV, you need to make some choices as to how you'll compromise.

Some critical caveats:

Before we examine the details of those aspect-ratio problems and solutions, it's important to affirm five key points:

• Make sure your video settings match your TV's aspect ratio. Most modern video sources--DVD players, game consoles, satellite and cable boxes, DVRs, and even the video iPod--have an aspect-ratio setting. Make sure you set each device to the setting that matches the TV to which it's attached: 4:3 for standard TVs, 16:9 for wide-screen monitors (nearly all HDTVs). The one exception is for 4:3 TVs that offer a feature called vertical compression or anamorphic squeeze. Video sources attached to these models should be (counterintuitively) set to 16:9, because they're designed to display the full vertical resolution of a wide-screen image within the letterboxed area.
• Make sure your new TV has aspect-ratio control. The next two pages suggest several solutions to common aspect-ratio problems. But they will work only if your TV--or the video source, be it a satellite tuner, cable box, or DVD player--has aspect-ratio control. In general, all wide-screen HDTVs, most HDTV set-top boxes, and a few new 4:3 TVs can control aspect ratio in some way. Many DVD players have Zoom functions, and all can be set to work with both 4:3 and 16:9 TVs, but few have additional aspect-ratio controls.
  
  
• Not every aspect-ratio choice will be available at every resolution. Almost every HDTV has aspect-ratio control, but most sets available today limit the number of choices you have, depending the incoming resolutions. In most cases, you'll have full aspect-ratio control with 480i and 480p sources (generally standard TV and progressive-scan DVD, respectively), but often you get fewer options, or none at all, for HDTV resolutions (720p, 1080i, or 1080p). Some HDTVs, especially older models, restrict the number of available aspect-ratio choices with 480p sources as well. CNET reviews will always indicate how aspect-ratio control is restricted, so be sure to check when making a purchasing decision.
  
  
• Don't be thrown off by other wide-screen aspect ratios. When shopping for wide-screen displays--especially flat-panel LCDs--you may see aspect ratios such as 15:9 or 16:10. They are, for all intents and purposes, close enough to 16:9 to be considered synonymous. Unless you're extremely sensitive to geometry, it's doubtful you'll notice the slight stretching or squashing they introduce.
  
  
• Understand native/dot-by-dot mode. Some HDTVs, most commonly 1080p displays, also have an aspect ratio mode that doesn't scale the incoming signal at all. Often called dot-by-dot or native, this mode simply takes the signal, whatever resolution it is, and displays it regardless of the display's native resolution. Depending on the signal, this can either fill the screen perfectly, leave black bars on the top or bottom, or leave black bars on all sides. For example, if you have an HDTV with 1080p native resolution and you're watching a 720p HDTV show, a true dot-by-dot mode will be window-boxed--the 1280x720 program will appear as a rectangle within the 1920x1080 display, surrounded by bars on all sides. (Many dot-by-dot modes only apply to 1080i and 1080p sources, however, so lower-resolution sources like 720p and DVD are still automatically scaled to fill the screen.) Regardless, the true advantage of dot-by-dot is that, with 1080p displays, every pixel of 1080i and 1080p sources is shown on the screen with no overscan and all of the detail promised by the source. The only real disadvantage is that some sources don't completely fill the screen, so you might see a solid line or interference along the extreme edges of the display. But in general, if you have a 1080p display and are watching 1080i or 1080p sources, dot-by-dot will give you the best picture quality.
  
  
• Burn-in can be caused by black bars. Filling the screen with a moving picture is the safest way to view non-wide-screen content on 16:9 plasma flat-panel and CRT-based rear-projection displays. Leaving the black bars on for an extended period of time can cause permanent damage to the display--often called burn-in or image retention--which often isn't covered by the warranty. Both plasma and rear-projection CRT sets are particularly vulnerable to burn-in during the first 100 or so hours of use. During that time, we recommend you watch without vertical letterboxing at all, and that you avoid still images, such as paused games or television shows. After this initial period, the danger of burn-in is greatly reduced. Other easy measures to avoid burn in include: find a set or a source that produces gray bars (instead of black) to either side of the 4:3 image and/or features other ways to combat burn-in; turn contrast down to 50 percent or lower; balance your 4:3 viewing with more wide-screen material; in particular, sports and animation make good candidates for stretching. Burn-in does not affect LCD, DLP, or LCoS TVs and is much less likely to affect direct-view tube TVs.
  
  

As long as your TV or video source has the proper aspect-ratio control settings, aspect-ratio problems are completely avoidable. Over the next few years, as both hardware manufacturers and broadcasters transition from the older 4:3 format to wider, HDTV-friendly 16:9 wide-screen, aspect-ratio control will be particularly important.