LG 55LM6700 HDTV Review- When 120 Hz Isn’t 120 Hz-Updated
Note (7/7/12): We’ve added an update below to reflect changes due to a new firmware version.
All LED LCD TVs have an inherent problem with motion resolution. For this reason, HDTV makers adopted higher refresh rates, namely 120Hz and 240Hz. These faster rates reduce motion blur, but at a price premium over “standard” 60Hz models. LG claims the 55LM6700 has 120 Hz refresh. After extensive testing, we find this claim very misleading. The 6700 failed to show any improvement in motion resolution compared to any 60 Hz HDTVs we’ve tested.
So what’s the story?
First, the Features
The LG 55LM6700 is the 2012 version of its highly rated 2011 LW5600 series television. New for 2012 is an attractive thin-bezel style LG calls “Cinema Screen.” The 6700 is the entry-level for this top LED LCD series. Major features include Smart TV with streaming movies and apps, passive 3D, a Magic Motion remote and LGs so-called 120Hz refresh.
Until the arrival of the LM6700, all 2012 120 Hz refresh HDTVs employ a circuit called Motion Estimation/Motion Compensation (ME/MC). These chips analyze consecutive original frames then create and insert synthesized frames between the original frames. This results in better motion resolution. To date, using the industry accepted FPD reference disc, we’ve found every 60 Hz LCD we’ve tested has about 320 lines of resolution. With ME/MC circuit engaged on a ME/MC circuit-equipped 120 Hz HDTV, the motion resolution increases to 600 lines.
All 60 Hz HDTVs use a technique called 3-2 pulldown, which converts the 24 original frames to 60 Hz by displaying the same frame three times then the next frame twice, then the next one three times etc. During pans, this uneven cadence creates a jerkiness called judder. We use a judder test sequence on the Marvell Qdeo test disc to test whether TVs created smooth motion with 24 frame sources.
LG’s Version of 120 Hz
We asked LG if any of its 2012 HDTVs with a factory specification of 120Hz refresh rate employ an ME/MC chip. The response was no. LGs spokesperson told us they are using a technique called “Black Frame Insertion.” Their claim: in between the 60 active images, LG shuts off the LEDs sequentially from top to bottom. This shortens the time the LCD panel is illuminated in section by section thereby reducing motion blur.
60 Hz LCDs sample a frame and keep the same image about 16.67 ms. This allows little time for the pixels to move to create the next frame. The panel holds the image and then repositions the pixels quickly for the next frame. Our eyes and brains are programmed for seeing continuous motion, not sample and hold, change and repeat. The result is we see smear know as motion blur.
A proven technique to reduce motion blur is to sequentially darken the image from top to bottom, making that portion of the frame black instead of an illuminated still image. This reduces the visible hold time and the brain perceives sharper motion. This is the technique LG claimed to use in the 6700.
Our Test Results
To confirm LG’s backlight scanning claim we used a video camera recording at 480 Hz (four times the 120 Hz claimed refresh rate). We then chose a slow-motion playback of 15fps to illustrate the LED shut off sequence. As you can clearly see in the video, the 55LM6700 does not employ top to bottom scanning. Instead, it shuts off the LEDs at the sides, then the center and repeats the sequence.
Contrary to LG’s claim, this technique had no effect at all on the TVs motion resolution. We tried Cinema and Expert picture modes and observed the same results, 320 lines of motion resolution, the same motion resolution as every make and model 60 Hz LED and LCD we’ve tested to date. Furthermore, there was always judder present, again confirming 3:2 pulldown as in a 60 Hz display. We were able to shut off the blinking LEDs by placing the TV in its “Game” mode, and found the motion blur results were the same as when the lights were blinking, only 320 lines of motion resolution .
We presented our findings to LG. They confirmed the LEDs in the 55LM6700 are only mounted at the bottom of the screen, making a vertical scan impossible as they would have to be mounted under on the left and right edges). LG provided its own 24fps judder test material; however, we saw the same judder as observed using our Qdeo test disc.
Based on our findings we asked how they could call this HDTV a 120 Hz model. Their response: “With respect to these advanced testing methods, the results illustrated are an overly magnified representation of the average user experience; what really matters is what consumers can see with their own two eyes.”
LG continues to post on its website, and dealer descriptions, simulated photos showing a significant reduction of motion blur with its claimed “TruMotion 120Hz” refresh. (photo below)
We find these claims incorrect and unsupportable based on the FPD motion resolution tests used by HD Guru and other major well-respected TV reviewers. Our high-speed camera clearly shows LGs scanning claim to be incorrect as well. This TV is a 60 Hz LED LCD. All tests yield no motion blur reduction compared to any 60Hz TV.
We measured the 55LM6700’s red, green, and blue color points, minimal black level, image uniformity, signals processing and color temperature.
Using the LGs “Rec. 709” picture setting, primary colors measured reasonably close to the industry standard actual (Rec. 709 reference in parenthesis):
Red x 0.635, y 0.343 (x 0.64, y 0.33);Green x 0.311, y 0.606 (x 0.30, y 0.60); Blue x 0.153, y 0.061 (x 0.15, y 0.06).
Signal processing was excellent, passing the HQV standard and high definition tests for jaggies, detail, 3:2 pulldown and deinterlacing of 1080i.
Black level uniformity was not good. Light areas (often called “flash lighting”) along the top edge and top corners were distracting in dark scenes. However, white level uniformity was good.
A backlight setting of 44 in the “Expert” picture mode produced a white level of 29.91 ft. Lamberts, a level bright enough for most homes with any type of ambient daytime light control (such as window curtains). The black (minimum luminescence) level at this setting was “fair” at 0.010 ftL. The LM6700 has a “local dimming” control. We did not observe deeper blacks and using normal source material we noticed less contrast ratio, not greater as one would expect. Given this display only has a single bottom row of LEDs we surmise this control is really a type of dynamic gamma, since true local dimming would require direct back lighting or LEDs on the vertical and horizontal edges.
The brightest image available is 76.31 ft.L reached in Vivid mode with the default setting of 100% contrast and backlight. This setting produced light blacks and is far brighter than any normal room would ever need.
The passive 3D in the 6700 uses inexpensive glasses (supplied) and produces and image of 1920 x 540 per eye rather than 1920 by 1080 as seen in active designs. The performance was typical: a visible line structure and some jaggies on edges are noticeable on diagonal objects or with white lettering against a black background. 3D does provide excellent depth with good source material.
Grayscale at the Warm factory default measured slightly too warm 6232K at 20 ft lamberts and 6113K at 80 ft lamberts.
The screen surface is smooth, highly reflective and not very effective in limiting reflections from lamps and other light sources.
LG includes Netflix and Vudu (not tested) as well as some of its own 3D shorts. We sampled four 3D travel videos and found very low 3D production values, not a way to impress your friends. This LG adds and Internet browser and apps.
The Magic Motion remote is like a Wii controller requiring tilting to move a cursor over the desired buttons. We find it slow and cumbersome. An LG spokesman says a conventional remote is available as an extra cost option.
The LG 55LM6700 offers svelte, contemporary styling with many features. We are disappointed by LG giving this TV 120 Hz specification as its motion resolution and judder is the same as every 60 Hz TV we’ve ever tested. However, compared to competitors’ offerings with LED edgelighting, thin form factor, 3D, Smart TV with browser, it’s priced quite aggressively, currently under $1450 ($2300 MSRP). HD Guru awards the 55LW6700 ♥♥½ out of five hearts rating.
When we made our review measurements, we used the latest firmware available at the time (version 3.20.31). LG pushed new firmware after we wrote the review. The latest firmware (version 3.21.40) added a TruMotion control for frame interpolation. We retested motion resolution and judder. We also evaluated the TruMotion control settings with a variety of film based (native 24 frame per second) movie content as well as video sourced HD content.
The new TruMotion menu has four main choices: User, Smooth, Clear and Off. There is a second setting for Judder control which available in the “User” mode and grayed out all other modes. Using the Qdeo test disc, we checked out all possible settings.
In “Clear” and “Smooth” mode film judder is eliminated. The “User” mode required the “Judder” control to a setting of 7 or higher take effect. We did not observe any difference in the image (other that the elimination of judder) in either the “Smooth”, “Clear” or User at “7” or higher. Because of the 4th of July holiday, we were not able to reach our LG contacts to explain performance differences of the three modes and if so tell us what content would be required to see it.
Like other HDTV we’ve tested in the past, the judder observed during panning 24Hz sources is eliminated by a motion estimation/compensation which creates new interpolated frames that are inserted in between the actual film frames. When activated the TruMotion settings also gives film based sources video-like look often referred to as the “soap opera effect”. For more on the soap opera effect artifact, read our article here.
When testing video based content such as 1080i and 720p native content (i.e. sports, news, and other unscripted programs) the TruMotion settings have no effect.
Next we retested motion resolution in all TruMotion modes. The 55LM6700 performed the same as with the prior firmware, with 320 lines of motion resolution, just like every other 60 Hz TVs tested. This confirms the 55LM6700 remains a 60 Hz refresh HDTV.
We applaud LG for upgrading the series by offering the option of anti-judder. However, we still do not consider a panel refreshing the image 60 times per second and simply flickering LEDs (which has no observed effect on motion resolution contrary to LG’s product claim) to qualify for a factory stated 120Hz refresh specification. Our ♥♥½ (2.5 out of five) heart rating is unchanged.
Disclosure: The 55LM6700 used in this review is a LG supplied loaned sample.
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