What does refresh rate on my HDTV mean?

HDTV Image Gallery A high refresh rate can smooth out fast action on an HDTV.  See more HDTV pictures.
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The era of high-definition television is in full swing. A few years ago, only early adopters owned an HDTV set and had very little access to high-definition content. Today, HDTV sets line store shelves and fill Amazon's warehouses. Prices have fallen and many families find that an HDTV fits their budget. But much about HDTV technology remains a mystery to the average consumer.

The two dominant formats on the market right now are LCD sets and plasma screens. The two technologies each had their own advantages and disadvantages when they first became available. Today, technological advances have all but erased the differences in performance between them.

When shopping for an HDTV, you'll notice there are a lot of odd terms and numbers. You may see that the television's resolution is either 720 or 1080 -- that refers to the number of horizontal lines of pixels displayed on the screen. In general, more pixels mean a sharper, clearer picture. But a 1080 resolution isn't always necessary -- for television sets 32 inches or smaller, you may not notice a difference.

To make matters more confusing, there's the issue of progressive scan versus interlacing. When HDTV sets debuted, the two options you had were a 720p (or progressive scan) set or a 1080i (interlaced) set. Later, manufacturers began to offer 1080p sets. The consumer was left to wonder which was better. Progressive scan delivers a sharper picture -- the television displays each line of pixels sequentially. Interlacing alternates the odd lines of pixels with the even lines -- changing them so quickly that the viewer sees a cohesive image.

At first, screen technology, resolution and television size were the primary factors shoppers needed to consider as they looked for a good HDTV. But now there's another element to consider: the refresh rate. A television's refresh rate refers to how often the television changes the display of pixels per second. But why is that important?

The Importance of Refresh Rate

To show moving images, a television has to change out the pixels displayed on the screen. This is what we mean when we say the television refreshes the image -- it has to draw images in pixels so quickly that the human eye can't detect the process. If televisions didn't refresh the pixels, they could only display a still image. That's not good TV.

The standard television refresh rate is 60 hertz. That means the screen displays an image 60 times every second. An interlaced television will refresh the odd and even lines 30 times a second each in an alternating pattern. Even at this rate, we don't notice the screen refreshing because it's too fast for us to detect.

Early LCD high-definition televisions had great resolution but experienced some problems when displaying fast-moving images on screen. Action movies and sporting events in particular gave early LCD sets problems. The images tended to blur as they moved across the screen. Plasma screens didn't have the same problem, giving that format the advantage when it came to high-speed television content.

The solution to the LCD problem was to increase the refresh rate. A few years ago, the first 120 hertz sets showed consumers that by doubling the refresh rate, the set could reduce the blurring effect. By early 2009, sets with a 240 hertz or higher refresh rate were either on store shelves or scheduled for release.

The higher refresh rates indicate that the televisions refresh the screen more often each second. Whether the faster rate has a noticeable effect on the viewer's experience is subjective. A viewer may not be able to tell the difference between a set refreshing at 120 hertz and one with a 240 hertz refresh rate.

Now let's take a quick look at the relationship between an HDTV set's refresh rate and film.

Film and HDTV

Refresh rates are really important for 3-D television technology.
Refresh rates are really important for 3-D television technology.
AP Photo/Panasonic Corp., HO

Have you ever used a notepad or flipbook to create simple animation? By drawing slightly different sketches on several sheets of paper and viewing them in rapid sequence, you create the illusion of movement. If you slow down the process, the animation becomes uneven and choppy. But if you move at a steady, rapid pace it can be very convincing. This illusion forms the basis for film.

The standard frame rate for film is 24 frames per second. That means every second of film you watch contains 24 separate pictures. As we mentioned earlier, standard interlaced televisions have a refresh rate of 60 hertz. In one second, the film will display 24 images, but an interlaced television set will refresh 30 times each for the odd and even lines of pixels. The difference between the film's frame rate and the television's refresh rate requires a special process called 3:2 pull-down to align the two rates. The process isn't perfect and as a result, film can look unnatural or jerky on television. Home theater enthusiasts call this effect film judder.

An HDTV with a 120 hertz or 240 hertz refresh rate is a different story. You can divide 24 frames evenly into either refresh rate. Because of this, your television doesn't need the 3:2 pull-down process to align the film's frame rate with the refresh rate. As a result, you get a smoother viewing experience while watching content originally captured on film.

Refresh rates are also important if you plan to watch 3-D content. A higher refresh rate allows the television to display the different sets of images necessary to create the illusion of depth. One method of showing 3-D content involves displaying alternating sets of images. A special pair of 3-D glasses synchronizes with the television set, allowing each eye to see only one set of images at a time. Your brain assembles the two sets of images into a single image that appears to have the dimension of depth. Several television sets on the market today are capable of using this 3-D technology.

When shopping for an LCD HDTV, keep an eye out for refresh rates. An HDTV from a trusted manufacturer should provide a smooth picture with a refresh rate of at least 120 hertz.

Learn more about high-definition television by following the links on the next page.

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Sources

  • Maxwell, Adrienne. "Explaining an HDTV's Refresh Rate." HDTVetc. (July 30, 2009) http://www.hdtvetc.com/education/explaining-an-hdtvs-refresh-rate.php
  • ProjectorPeople.com. "3:2 Pulldown Explained." (Aug 3, 2009) http://www.projectorpeople.com/resources/pulldown.asp
  • Russell, Braden and Derderian, Ara. "HDTV and Home Theater Podcast #361 - Refresh Rate and Response Time." March 9, 2009. (Aug. 1, 2009) http://www.hdtvmagazine.com/podcast/2009/03/hdtv_and_home_theater_podcast_361_refresh_rate_and_response_time.php
  • Tech-Evangelist.com. "The Relationship Between HDTV Refresh Rate and Frame Rate." (July 31, 2009) http://www.tech-evangelist.com/2008/10/15/hdtv-refresh-rate-frame-rate/