120/240 Hz LCD Problems Exposed
120/240 Hz LCD Problems Exposed
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All display technologies combine a variety of strengths and weaknesses. Whether its excess power consumption, limited viewing angle or washed out black levels, over time, engineers work to address the weaknesses while reinforcing the strengths. For example, liquid crystal displays once suffered from blurring when reproducing rapidly moving objects or people. The problem seemed inherent to the technology.
Tests conducted by HD GURU demonstrated how, in the presence of motion, 1080 lines of resolution (per frame height) dropped to 320 lines or less on displays featuring 60 Hz refresh rates, which was long the standard.
By speeding up the refresh rate from 60 Hz to either 120 Hz or 240 Hz, set makers reduced or virtually eliminated motion blur. LCD set manufacturers accomplished this using a new circuit called ME/MC (motion estimation/motion compensation) that activates when the display kicks up the refresh rate to 120 or 240 Hz.
Few technological improvements come without associated costs and as new 120/240Hz LCDs appeared, many reviewers complained that while the activation of the ME/MC circuit effectively reduced motion blur, it was accompanied by noticeable image degradation that made movies appear like videos, caused in part by a diminution or elimination of film grain. Reviewers invented colorful descriptors like “soap opera effect” or “video effect” to describe what they saw.
Using test patterns on a new test disc that can, for the first time, clearly demonstrate the ME/MC circuit’s image degradation, the HD Guru tested five leading LCD HDTV brands (Vizio, Samsung, Sony, Sharp and LG). Each manufacturer employs a trademarked name such as Motion Flo (Sony) and Auto Motion Plus (Samsung) for its respective ME/ME processing circuit.
The test disc is the Spears & Munsil Blu-ray . The “moving wedge” test pattern, recorded at 1080p/24 Hz and played back via a Blu-ray player, consists of a vertical and horizontal wedge pattern made up of slightly converging alternating black and white lines. We tested ten models from the five brands, first shutting off the ME/MC circuit while playing the content at 60 Hz and again after engaging the circuit.
We also tested a pair of plasma sets: Pioneer’s Elite Kuro (PRO-141FD) and Panasonic’s TC-P54Z1, engaging the 60 Hz and 72 Hz modes on the Pioneer as well as the 60 Hz and 96 Hz modes on the Panasonic. Regardless of setting, neither produced the picture degrading artifacts seen on all of the 120/240Hz LCDs with ME/MC engaged. See the Panasonic’s image on the accompanying video.
What We See
Minimal amounts of moire should appear on the moving Wedge Patterns, with the individual black and white lines viewable from one end of the wedge to the other. Some displays did a better job than others (with the ME/MC circuit “Off”) due to signal processing and the given display’s bandwidth. Activating the ME/MC circuit created some nasty artifacts. The wedge distorted, showing flashing blobs that momentarily obliterated the black and white lines as the wedge moved up and down or right and left. This translates to a loss of fine detail such as film grain and overall image alteration.
Why This Occurs
Why does ME/MC produce these deleterious results? Using movie content, for example, the circuit takes the actual film frames that occur 24 times per second and creates synthesized artificial frames between the real ones. The ME/MC chip guesses and reproduces what the created frames should look like. Unfortunately, sometimes the circuit guesses wrong and mis-interpolates the motion present.
In addition, it appears that none of the ME/MC chips available to set makers are capable of processing all of the data within the original film frames needed to produce artifact-free synthesized ones. Therefore, to achieve image consistency, the LCD panel must degrade the actual frame’s image quality to match that of the synthesized frame, which is why the test pattern shows the degradation.
Artifacts aside, 120 Hz and 240 Hz LCDs produce noticeably smoother motion than do 60Hz sets, or plasma displays for that matter. You will easily notice the smoother wedge movement. Some LCD set owners consider the loss of fine detail a worthwhile tradeoff to get the ME/MC circuit’s smooth presentation.
Currently, prospective flat screen buyers have two alternatives. If LCD is your preference but you wish to avoid the ME/MC circuit’s artifacts, most LCD sets offer a turn off option. The trade-off is lower motion resolution. If you want a feature like LED edge or back lighting, 120 Hz or higher is your only choice as these features are only present on the higher refresh rate sets.The second alternative is to opt for a plasma set. The Panasonic G, V and Z models tested by HD GURU all reproduce full motion resolution HD without the use of artifact-producing ME/MC circuits.
A Final Note
We made the video contained within this article on a Kodak Zx1 at 720p/60 resolution. The footage was edited and uploaded to the servers. Due to limitations of the camera, the need for compression and the upload/download process, the images seen should be taken as a demonstration of the presence (or lack) of image artifacts and not as an exact reproduction of what would be observed in person. We invite and encourage readers to obtain a copy of the Spears & Munsil test disc and perform these tests on your own display or one that you are contemplating purchasing at a willing local retailer.
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