Explaining an HDTV's Refresh Rate
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By Adrienne Maxwell
A 120-hertz refresh rate has become the buzz feature for many new LCD HDTVs, and we're beginning to see the arrival of models with even higher refresh rates. Sony's KDL-52XBR7 has a 240Hz refresh rate, and several new 240Hz TVs were on display at CES 2009, as was LG's 480Hz LCD. You've been told that you should want a higher refresh rate and be willing to pay extra to get it, but what does the technology bring to the table?
The traditional television that we've all come to know and love has a 60Hz refresh rate. That means the picture refreshes itself 60 times per second. As you can probably deduce, in a 120Hz TV, the picture refreshes itself 120 times per second. (When we move to higher numbers like 240 and 480, the process can vary amongst manufacturers, so that's a topic for another day.) The more complicated part of the answer involves the how. How does the TV double its refresh rate? To do so, the TV must add frames, and different LCD manufacturers employ different methods of frame addition. Early methods involved the insertion of a black frame between each of the 60 frames. Another option is to simply repeat each frame once to equal 120. Finally, the TV can employ a frame interpolation method that allows it to pull information from the existing frames to create an entirely new frame in between. Many newer 120Hz LCDs include both frame repetition and frame interpolation modes, as each option creates a different look or quality in moving images.
Then there's the question of why. Why do we need 120Hz? The original goal of 120Hz was to help reduce motion blur, a common issue with LCD TVs. (Motion blur is not a problem for plasma TVs, which is why 120Hz is not a selling point in that category.) This has proven successful in most cases: motion blur is often less evident on LCD TVs with higher refresh rates, thus keeping fine details intact, especially with faster-moving sports programming and action films. A second and perhaps even more compelling benefit has emerged: frame interpolation can help reduce the appearance of judder in film sources. Film judder is a jerky, stuttering effect created when 24-frames-per-second film sources are converted to 60fps for output on a 60Hz TV, via a process called 3:2 pulldown in which the first film frame is doubled, the second tripled, the third doubled, and so on. Many 120Hz LCDs offer special frame interpolation modes (like Sony's Motionflow or Samsung's Auto Motion Plus) that minimize the appearance of judder and render super-smooth movement. This technology has become popular enough that even some plasma manufacturers have added frame interpolation modes.
A higher refresh rate can be especially beneficial when you mate the TV with a Blu-ray player that can output the native 1080p/24 resolution off of a Blu-ray disc (see our article "Explaining 1080p/24" for more information). Many of the newest 120Hz TVs will accept the 24fps Blu-ray source and simply repeat each frame five times to create 120 frames (called 5:5 pulldown). The resulting motion is smoother than you see with 3:2 pulldown, but is not as artificial or video-like as you get with frame interpolation. It's the film purist's choice.
Higher refresh rates are clearly the future for HDTV and will likely become ubiquitous. For now, though, they are usually considered a premium feature that adds to a TV's bottom line. If you're not bothered by motion blur or film judder, this feature is not a necessity, but it can be an indicator of higher-quality LCD performance for the more discerning video fan.
Keywords
120-hertz, 120Hz, 60Hz, 240Hz, 480Hz, refresh rate, frame addition, frame repetition, frame interpolation, film judder, 3:2 pulldown, Blu-ray, 1080p/24, 5:5 pulldown
