Contrary to popular belief, a high TV screen refresh rate isn’t a universal standard that can be used to compare all TV displays and TV technologies. In addition, these days TV manufacturers do not always list true refresh rates for their products, opting instead for a refresh rate “equivalent” achieved by proprietary technology. So the first thing to ask is, how much does refresh rate matter for the type of TV I am considering?
The best way to explain is to discuss why refresh rates are important in the first place. Video sources either have a frame rate of 24 frames per second (fps) or 60 fps; 24 fps for film, and 60 fps for most televised content. As long as a TV has at least a 60 Hz refresh rate, it can display incoming content at the full rate it was recorded at. So why have higher refresh rates at all?
Refresh Rate: LCD vs Plasma
TV display technologies differ in the way that they create images on the screen. Each technology has its strong points and drawbacks. It is a proven fact that LCD screens suffer from the effect of motion blur more than plasma TV’s. This effect has two causes: the “sample and hold” method by which LCD pixels operate, and pixel persistence issues caused by pixel response time, or pixel latency. Persistent light signals reach the eye throughout the duration of each frame due to the sample and hold method, which accentuates the discrete (not smooth/analog) nature of a moving edge made up by pixels when the eye doesn’t have time to refresh between signals. In addition, the pixel transition is visible, which contributes to blur during especially fast movement. For plasmas, images are created by successive pulses of light throughout the duration of the frame, so since the signal isn’t constant, no transition is actually visible. The other cause, pixel persistence, can cause “ghosting” if the transition between pixel values is not complete by the time the frame has changed. In plasmas, pixel response time is about 1000 faster than in liquid crystal pixels, so this is not an issue.
Counteracting Motion Blur and Judder
LCD manufacturers have used various techniques to combat these issues. Listing refresh rate for LCD’s is a bit misleading, because what is truly important is the pixel response time (though they are obviously related). On old LCD’s, pixels were not able to make full transitions between successive frames of incoming video. Today, refresh rates of 120 Hz and higher on LCD’s simply indicate that the liquid crystal pixels are able to make quicker transitions; in other words, they are able to transition fast enough to mostly keep up with 120 incoming frames per second (or more). This speedier pixel transition does help combat some aspects of motion blur, like pixel persistence. In addition, new techniques for backlighting in LCD’s have been introduced to mimic the strobing that is part of normal functioning in plasmas, which allows the eye to reset between signals and obscures the transition of the liquid crystal pixels. Examples of these technologies include scanning backlights, strobing backlights, or black frame insertion. These technologies bring LCD’s and plasma TV’s even closer in terms of motion blur, though in practice most plasmas still handle motion blur better than most LCD’s.
Another benefit of fast pixel transitions that allow for more frames to be shown per second is the ability to counteract judder. There are more than one kind of judder, but this refers to the temporal judder resulting from incoming sources with only 24 fps, so fast pans across the screen look jerky (since there simply aren’t enough frames to make the movement look smooth). One solution is motion interpolation, where additional frames are actually created by the TV and inserted between each original frame by averaging the frame before and after, which artificially adds more detail during movement. This interpolation is what creates the “soap opera effect”, however, which is very bothersome to many people when watching movies and TV shows. The picture looks too smooth, or too lifelike- no longer having the feel of classic film. Image interpolation can be turned on and off with most TV’s, though if this is an important feature, research should be done on specific TV models to make sure. When image interpolation is turned off, frames are still inserted into the original content, but these frames are repeats of each original frame. On the other hand, it should also be noted that image interpolation is thought to significantly improve content like sports, where this smoothed out motion can significantly positively affect the viewing experience and add a desirable level of detail.
Today, both plasmas and LCD’s have the capability to manage judder using methods like these that take advantage of an ability to display more frames per second, and all can display 24 fps and 60 fps video content from broadcast television, movies, and gaming consoles. Plasma technology is still naturally better at limiting motion blur, though the gap is decreasing.
Choosing the Best Refresh Rate
If you are shopping for plasma TV's, you’ll see something like 600 Hz (or more), or 600 Hz sub frames, which actually refers to the number of bursts of light per frame (10 bursts of light per 60 frames per second = “600Hz”). If you want to make sure the plasma TV is equipped to handle judder, look for a feature called something like “smooth motion”, “motion smoother”, or “frame repeat”. For example, Panasonic has a feature called “Cinema Smoother”. If your mind is set on an LCD and you are bothered by motion blur (which you may not be, as some people truly don’t notice it), then look for an LCD in your budget with the highest refresh rate. While at the store, you will want to make sure that you look at the displays while viewing different types of content. For example, motion blur can be noticed more easily while playing a video game or watching sports. If an LCD with a high refresh rate doesn’t seem to fix the problem, you may need to consider a plasma TV instead. An important thing to watch out for is numbers that look like refresh rates, but aren’t. If you don’t actually see “# Hz Refresh Rate” (with a number for the #, of course), what you’re looking at might not actually be the actual native refresh rate, so don’t get tricked into spending more for a TV than you should. If you see terms like “SPS”, “Motionflow XR”, “AquoMotion”, “CMR”, “Scenes Per Second”, or “Clear Motion Rate”, know that these are terms made up by the TV manufacturer and numbers associated with them do not necessarily correspond with the actual refresh rate.
Note: The TV types covered in this article were plasma and LCD displays, but DLP’s do not suffer from motion blur, in the same way that plasmas do not. LCOS projectors are closer to LCD in regards to motion blur.
