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120 Hz TV Sets
One of the newest fads for TV's for Christmas 2007 is 120 Hz (hertz; cycles per second; frames per second). "Everybody knows" that electricity in the U.S. is 60 Hz and many folks know that "regular" TV sets operate at 60 screen refreshes per second so just the words "120 Hz TV" must suggest something really new and better.
It's supposed to show smoother movement.
Some folks won't notice any difference between the 120 Hz TV and a "regular" TV. Some folks won't see any difference at first but will swear they can always tell the difference after viewing and comparing for an hour or two.
Like any feature,120 Hz will be "done better" on some makes and models compared with on others.
The Advantages and Rationale
The advertised advantages are: less flicker and smoother motion.
"Regular" TV sets refresh their screens (approximately) 60* times per second. Meanwhile most movies are shot at 24 frames per second. In order to show movies on TV, and because 60 does not divide by 24 evenly, every other film frame is flashed on the TV screen 3 times and the intervening film frames are flashed on the screen twice (a process called 3-2 pulldown). Some viewers will notice jerky motion, especially on full scene panning, because the various film frames did not all get equal screen time.
The faster the frame rate, the less flicker is noticed. A small number of viewers will notice flicker all the time, especially to the sides of what they are focusing on, even with 60 frames per second let alone 24. (Movie theater projectors actually give each frame two short flashes instead of one long flash, giving the effect of 48 frames per second and most viewers notice less flicker that way.)
Why not use 72 frames per second (72 Hz)? Actually some TV sets and many computer monitors do. Almost nobody sees flicker on a video screen when we get up around 70 Hz. Twenty four goes into seventy two evenly. This works fine for film source video where each film frame can now get equal time, three flashes on the screen (3-3 pulldown). But non-film video is still 60 refreshes per second and some frames will get more screen time than other frames on a 72 Hz TV. Still better is a TV that "changes gears" between 60 and 72 Hz depending on the source.
The 120 Hz TV can give equal screen time to each frame for both film source and non-film source without changing the refresh rate.
The Dirty Secrets
In order to give each film frame equal time, the TV has to be able to identify which video frames represent which film frames. For standard incoming video source, the 60 refreshes per second and 3-2 pulldown are already there. The TV would need to be able to recognize the 3-2 pulldown pattern using pixel analysis, then pick out the content of each of the 24 film frames per second. Not all TV sets do this analysis.
With the advent of hi-def DVD, a new "24 fps" video format standard is (actually two are) available. As of this writing (late 2007) only a few TV sets accept 24 fps. When a 120 Hz TV receives the usual 60* Hz video input and does not recognize 3-2 pulldown, the equal time per fim frame advantage will not be seen.
So far, only LCD TV's are advertised with 120 Hz. Most other kinds: CRT, plasma, etc. are still 60 Hz, or less commonly, 72 Hz.
LCD has an unusual disadvantage in that it takes longer to change from light to dark (or from dark to light) or from one color to another compared with the other kinds of displays. This is sometimes seen as additional motion blur. This blur is more noticeable with moving small bright spots. On older LCD TV sets, moving bright spots left noticeable trails behind them, streaks referred to as "comet trails".
We are led to believe that on a few models of LCD TV sets, with the usual 60 Hz refresh, every other refresh was black to make light to dark transitions become dark more quickly and make motion blur less noticeable. So there were just 30 refreshes of real material per second. Actually all 60 refresh time slices were used, the even rows of pixels forced to black on the odd refreshes and the odd rows of pixels forced to black on the even refreshes, again giving the illusion of 60 refreshes per second while each row of pixels got 30 refreshes of real material per second. For you video experts, this is the same as interlacing on CRT TV sets! If there are still some large non-high definition CRT TV sets in the store nearest you, you might be able to observe a coarse scan line effect on the screen if you move your eye focus up or down. LCD is not an interlaced display per se. When the coarse scan line effect is seen on an LCD screen, there is even a term for it: peekaboo scan lines
With the 120 Hz system, the entire screen can be forced to black after each of the 60 refreshes of video to reduce motion blur, and few if any viewers would notice flicker. Other strategies could be used, too. For example only those pixels that were bright in one frame and dark in the next might be forced to black. Alternatively a pixel that was bright in one frame and medium bright in the next might be forced all the way to black in the intervening 1/120'th second time slice to make for a faster fade.
We are unsure of the processing strategies used. It is possible that the processing strategy is optimized for the exact characteristics of the LCD panel.
8 milliseconds? 16 milliseconds?
When the refresh rate is 120 Hz, each refresh, or flash on the screen, is 8 ms. The fastest LCD's can change state in about 8 ms, perhaps a little less. LCD, unlike CRT, stays the same between refreshes, in other words, something must be pro-actively done to make a bright spot (transparent) turn dark (opaque). "Flash on the screen" could be considered a misnomer in this respect. The 120 Hz concept allows changes to be initiiated every 8 ms compared with every 16 ms in a 60 Hz system. Thus "refresh" for LCD refers to the batch of pixel change requests needed to show the next video frame (or a black screen). Now, "state change" could be a dirty secret. I'm not sure what an 8 ms state change means, for example could that mean the time to get halfway to the new desired darkness when another 8 ms is needed to go half the remaining distance? In this case, requesting a change all the way to black when the desired darkness is gray and making the request 1/120'th of a second sooner compared with a 60 Hz system would make some sense. (Then, another 1/120'th second later when the next real video frame arrives, the correct pixel intensities for that frame are requested.)
Refinements
Even with 72 Hz or 120 Hz refresh, most TV's just use the incoming 24 frames per second or 60 refreshes per second source video as-is. It would be nice to show finer gradations of motion than the 24 fps film originally had. Indeed there are video processors that "compute" the intermediate location of moving subjects for synthesizing intervening frames. So an actual (more correctly an educated guess of a) 72 fps (or 120 fps) film could be constructed. The process is known as "motion compensation". So far no inexpensive (read: less than mid four figure in dollars) processors for this have been developed. More commonly done is blending, or fading, or averaging if you wish, two frames to come up with an intervening frame. Let's imagine a shot of a thrown ball. An intervening frame, say, "1-1/2" might have a faded image of the ball on the left (as on frame 1) and also a faded image of the ball on the right (as on frame 2) while motion compensation would yield a frame with the ball shown just in the middle. Once the formulas for "computing" one "frame 1-1/2" are developed, adapting those formulas to construct four intervening frames for 120 Hz is easy.
What About DLP (tm)
These (projection) sets have been offered in 2x (120 Hz), 4x (240 Hz) and a few other similar styles, but without special source processing. The idea of equal screen time for 24 fps film source, say, five flashes per film frame for the 2x models, is equally applicable and 24 Hz processing may be offered soon. The reason for the increased frame rate is that each refresh is subdivided into red, green, and blue images flashed sequentially, and some viewers noticed annoying red, green, and blue edges. The faster the frame rate, the thinner these "rainbowy" edges would be.
* Interlaced video also has 60 (more precisely 59.94) refreshes per second but is usually referred to as 30 (29.97) Hz. For the time being we are referring only to U.S. video, as opposed to European PAL based 50 (50.00) refresh per second, 50 Hz progressive, and 25 Hz interlaced formats.
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All parts (c) copyright 2007, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.
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