LCD Refresh Rates Explained - 240HZ vs. 120HZ vs. 60HZ
Submitted by jwhitakr on Tue, 08/25/2009 - 11:30
Television technology has come a long way - starting out with the humble Cathode Ray Tube which fired light guns behind the glass down the screen to provide the picture. It fired every other gun each pass in series, which was referred to as interlacing. Here the technology stayed for almost forty years before the first consumer LCD screens became available.
Today, LCD (Liquid Crystal Displays) uses a chemical that is somewhere between liquid and solid, which is highly conductive and reflective. By passing a current through each crystal they are “turned” on and off, producing a pixel by causing the crystal to twist and reflect light in different ways. Each pixel has three subpixels in red, blue and green which react in different ways to the voltage. The TV manipulates the voltage applied to each pixel to produce the shade required to such a degree that these three colors can produce 16.8 million variations.
LCD Refresh Rates Explained
The “refresh rate” of 60, 120 or 240Hz refers to how many times a frame is drawn each second. A movie signal is broadcast at 24Hz, or 24 frames per second. A 60Hz LCD will convert the frames to 30Hz frame twice every 30th of a second and a 120Hz LCD will repeat the 24Hz signal five times within every 24th of a second to create the picture you see on the screen.
So with the math out of the way, what does it actually mean? The difference between a 60Hz picture and a 120Hz picture can be detected by most human eyes. The picture will appear sharper, clearer and with less “flicker” with 120Hz as the refresh is too quick for the average eye to detect. The real difference is seen when watching a movie or sports where the picture moves quickly and “motion blur” rears its ugly head. Motion blur is where the 60 Hz frame or image is frozen on the screen and shifts to a different position which is detectable to the eye. In effect a frame is held for 1/60 of a second then abruptly shifts to the next frame. This shows on the screen as a kind of lag in movement when there is a lot going on.
To get around this manufacturers moved from the original 60Hz to 120Hz, which doubled the amount of times the frame is repeated on the screen. This lessened the effects of motion blur a bit, but not completely. Shifting up a gear to 240Hz is another attempt to reduce motion blur and to make the moving image as smooth as possible.
Faster Speeds, Better Picture Quality?
Faster is better right? Not always. Movies and TV broadcasts are filmed at 24Hz so the TV has a lot of work to do bringing that up to 240Hz by repeating the frame 10 times every 24th of a second. The speed of the refresh rate alone doesn’t do much to improve the picture quality and reduce motion blur as it just repeats the same frame more times.
Is it worth spending the extra money to go for a 240Hz LCD over a 120Hz one? At the moment not yet as the main differences are imperceptible. Reducing motion blue is done by pre-processing using a scanning backlight or MEMC (Motion Estimation Motion Compensation) rather than speeding things up. The faster TV may score well in things like test patterns, but in real life you won’t really notice the difference. MEMC is a clever way of reducing motion blur by predicting where the image will be in between frames. For example, the TV will take frame 1 and frame 2 and repeat them 5 times per 24th second. But, it will also add its own frame in between 1 and 2 by combining the two frames and using prediction. The idea is to simulate a 48Hz signal into the TV. This is the technology that reduces motion blur.
Conclusion
So, to summarize, a 240Hz TV will produce a slightly better viewing experience than a 60 or 120Hz one. However this is because of the programming technology like MEMC, not the increased speed. The improvement at the moment may not warrant spending the extra dollars for this technology yet - it really is up to you and your wallet to decide.
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Television technology has come a long way - starting out with the humble Cathode Ray Tube which fired light guns behind the glass down the screen to provide the picture. It fired every other gun each pass in series, which was referred to as interlacing. Here the technology stayed for almost forty years before the first consumer LCD screens became available.
Today, LCD (Liquid Crystal Displays) uses a chemical that is somewhere between liquid and solid, which is highly conductive and reflective. By passing a current through each crystal they are “turned” on and off, producing a pixel by causing the crystal to twist and reflect light in different ways. Each pixel has three subpixels in red, blue and green which react in different ways to the voltage. The TV manipulates the voltage applied to each pixel to produce the shade required to such a degree that these three colors can produce 16.8 million variations.
LCD Refresh Rates Explained
The “refresh rate” of 60, 120 or 240Hz refers to how many times a frame is drawn each second. A movie signal is broadcast at 24Hz, or 24 frames per second. A 60Hz LCD will convert the frames to 30Hz frame twice every 30th of a second and a 120Hz LCD will repeat the 24Hz signal five times within every 24th of a second to create the picture you see on the screen.
So with the math out of the way, what does it actually mean? The difference between a 60Hz picture and a 120Hz picture can be detected by most human eyes. The picture will appear sharper, clearer and with less “flicker” with 120Hz as the refresh is too quick for the average eye to detect. The real difference is seen when watching a movie or sports where the picture moves quickly and “motion blur” rears its ugly head. Motion blur is where the 60 Hz frame or image is frozen on the screen and shifts to a different position which is detectable to the eye. In effect a frame is held for 1/60 of a second then abruptly shifts to the next frame. This shows on the screen as a kind of lag in movement when there is a lot going on.
To get around this manufacturers moved from the original 60Hz to 120Hz, which doubled the amount of times the frame is repeated on the screen. This lessened the effects of motion blur a bit, but not completely. Shifting up a gear to 240Hz is another attempt to reduce motion blur and to make the moving image as smooth as possible.
Faster Speeds, Better Picture Quality?
Faster is better right? Not always. Movies and TV broadcasts are filmed at 24Hz so the TV has a lot of work to do bringing that up to 240Hz by repeating the frame 10 times every 24th of a second. The speed of the refresh rate alone doesn’t do much to improve the picture quality and reduce motion blur as it just repeats the same frame more times.
Is it worth spending the extra money to go for a 240Hz LCD over a 120Hz one? At the moment not yet as the main differences are imperceptible. Reducing motion blue is done by pre-processing using a scanning backlight or MEMC (Motion Estimation Motion Compensation) rather than speeding things up. The faster TV may score well in things like test patterns, but in real life you won’t really notice the difference. MEMC is a clever way of reducing motion blur by predicting where the image will be in between frames. For example, the TV will take frame 1 and frame 2 and repeat them 5 times per 24th second. But, it will also add its own frame in between 1 and 2 by combining the two frames and using prediction. The idea is to simulate a 48Hz signal into the TV. This is the technology that reduces motion blur.
Conclusion
So, to summarize, a 240Hz TV will produce a slightly better viewing experience than a 60 or 120Hz one. However this is because of the programming technology like MEMC, not the increased speed. The improvement at the moment may not warrant spending the extra dollars for this technology yet - it really is up to you and your wallet to decide.
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