The debate over screen resolution is one that has greatly diminished over the past few years, since most TVs now boast 1080p resolution. The only remaining 720p sets are LCD TVs under 40-inches and entry-level plasmas. If you're in the market for one of these sets and your budget is limited then read on because you may be able to save a few dollars by going with 720p rather than opting for a more expensive 1080p HDTV.
In simple terms, a 1080p HDTV will display more detail than one that's only 720p. The problem is that depending on the size of the television, you may not see any difference unless you sit abnormally close to it. On a 50-inch screen this distance may be as short as 6 feet, thus if your regular viewing distance is 7 feet or more, your eyes will be unable to discern any difference in sharpness between a 50-inch 1080p TV and a 720p unit. On TVs less than 40-inches, the difference between 1080p and 720p is virtually indistinguishable at any reasonable viewing distance.
In addition, there is more to a good picture than just screen resolution. Black level, contrast ratio and color rendition are also important factors in picture quality. A 720p set that does everything well is a better choice than a mediocre 1080p set. According to the Imaging Science Foundation (ISF), a group that consults for home theater manufacturers and trains the professionals who calibrate video equipment, the four most important aspects of picture quality in order of importance are contrast ratio, color saturation, color accuracy and screen resolution. That’s right, screen resolution is fourth down the list.
In evaluating different HDTVs, you are more likely to notice differences in black level detail and color accuracy than any difference in screen resolution. Since 1080p units are more expensive, this can translate into a worthwhile saving. Unless your screen is larger than 65 inches, your eyes probably will not be able to resolve the greater detail found in a 1080p picture from the normal seating distances most people use. If you don’t mind spending extra for 1080p then go for it, but if cost is an issue, pass it up and don’t fret since you’re unlikely to notice the difference.
These two factors are critical to a good picture. Black level refers to the ability of an HDTV to display an inky, deep black while still maintaining detail in dark scenes. Black levels are a particular problem with fixed pixel displays, especially LCDs. Recent improvements in LCD technology such as local-dimming LED backlights have improved the black levels on these sets, but as general rule, plasma TVs still tend to outperform LCDs with regard to black levels.
The contrast ratio is the difference in light intensity between the brightest white and the darkest black an HDTV can produce. Generally, the higher the contrast ratio the better. The problem with comparing contrast ratios between different television makers is that there is no industry standard of measurement. For this reason, only use contrast ratio specs for comparison of TVs from the same manufacturer.
Further complicating the matter, usually TV manufacturers provide two separate contrast ratios - static (sometimes referred to as native) and dynamic. Without getting into the nitty-gritty of how the two different contrast ratios are measured, most TV experts consider static to be the more reliable of the two since the measurement technique employed for it better represents real world conditions.
However, keep in mind the eye can only discern a contrast ratio of up to 1200:1, so specs that claim 3000:1 and more become meaningless even if they were true.
Refresh rate refers to the number of times an image is redrawn every second. The refresh rate for a standard TV is 60 Hz, or 60 frames per second. Even though each time the screen refreshes a new image is drawn, this rate is fast enough that our brain interprets them as a continuous picture. In a typical LCD panel, this means that every 1/60th of a second the panel displays a pixel and holds it for approximately 16.67 milliseconds.
A problem unique to LCD TVs is the blurring of fast moving objects. There are two reasons for the problem of motion blur on LCDs that are inherent in the technology:
1. LCD TVs create continuous light and the liquid-crystal pixels twist and turn to block or allow light as needed. The first problem is the speed at which these liquid-crystals pixels can twist or change its state, which is known as its response time. This response time, quoted in milliseconds (ms), is usually between 2 and 10ms. The slower the response time the more blur you will see. Unfortunately, 2ms is still slow enough for some people to perceive smearing from one frame to another.
However, even if the pixels responded instantly, our brain would still perceive motion blur. This phenomenon is known as sample and hold and results in the second problem.
2. When viewing motion, our brain expects a moving object to continue to move in the same direction. The second problem is again related to the fact that LCD TVs create continuous light. Unlike other display technologies such as plasma or DLP, LCDs don’t flash or have any downtime between frames. This means that the LCD image is stationary on screen for the entire 1/60th of a second. When displaying fast movement, this is long enough to confuse our brain into thinking the image is stationary. When the next frame is displayed and the image has suddenly moved, our brain blurs this movement.
It is this second problem that LCD makers are attempting to solve by doubling, and in some case quadrupling the refresh rate to 120Hz or 240Hz. The TV interpolates (creates) a new frame in between the two existing frames that attempts to estimate where moving objects would be in that frame based on the previous and subsequent frame. This way, no frame is held more than 1/120th or 1/240th of a second, which helps reduce the blur effect created by our brains. Most mid to high level LCD TVs employ this technology to help reduce motion blur.
There is the potential for this technology to make motion look too smooth, particularly with film, resulting in an unnatural appearance to motion reproduction. Some people find that it makes film look too much like video. If this is the case, you usually have the ability to turn off or control the amount refresh and interpolation the LCD TV performs.
An added benefit of this feature is the compatibility with 24 frame-per-second (fps) sources such as Blu-ray discs. When a movie is transferred to Blu-ray, it is done at the original 24 fps of film. Since our television system operates at 30 fps, the player has to add new frames to make up the difference for every second of video. This process is referred to as 3:2 pulldown and is done by breaking the frame into fields and reorganizing those fields so that you end up with 30 frames-per-second. The problem with this process is that 24 does not go evenly into 30 (or 60), so the TV must create an uneven number of new frames for each existing frame which results in a stuttering artifact with motion called “judder”. Since 24 goes into 120 evenly (120/24 = 5), a TV with 120 Hz refresh rates simply quintuples the original 24 fps, eliminating the need for 3:2 pulldown and thus making panning and motion look that much smoother and more film-like.
Next: More HDTV Buying Considerations
