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The LG 24GM77 has the same scaler (processor) and panel as, for example, the BenQ XL2411T and the ASUS VG248QE. Under the hood, however, there are notable differences. First, the LG 24GM77 offers sophisticated color control, which allows for counteracting the typically washed-out looking colors of these TN displays. Second, the LG 24GM77 appears to refrain from dithering at the scaler level, which can reduce color noise, besides alleviating artifacts that come with dithering in general. Third, the LG 24GM77"s motion blur reduction, dubbed Motion240, uses an accelerated screen update process and a well adjusted overdrive, both of which provides better settling behavior than 2D Lightboost or BenQ"s Motion Blur Reduction (MBR). In flicker-free mode, the settling performance of the LG 24GM77 is on par with the ASUS VG248QE.

But there are also downsides. First, the color controls are difficult to handle (12 parameters) – nothing simple like the Vibrance parameter provided by the BenQ XL2730Z. Second, having no dithering at the scaler is only of advantage if the factory preset is in use. As soon as the Contrast, Gamma, Black equalizer, or color channel gains (white point) are changed, color noise is actually higher than with other monitors that have the same panel. Third,

LG"s motion blur reduction comes with an input lag that is about 5ms higher than with, for example, ASUS" Lightboost (at 120Hz), for no technical reason. Moreover, Motion240 works only for 100Hz and 120Hz but neither for 60Hz nor 144Hz.

In the LG 24GM77 OSD, the motion blur reduction feature is called Motion 240 and works only with 100Hz or 120Hz. At a first glance (but see below), Motion240 works in the same way as BenQ"s MBR, without having any control over the backlight pulse timing though. Pulse width is 3.4ms for 100Hz and 2.8ms for 120Hz. The momentary pulse luminance (which roughly reflects the LED current) is increased by 150% over the luminance in normal mode (i.e., when Motion240=Off). This factor is relatively low and the pulse width is relatively high as compared to other monitors capable of strobed backlight. The pulse onset comes shortly before VSYNC, i.e. shortly before the screen is updated with the next frame, and the pulse lasts until shortly after VSYNC. Interestingly, this is one of the few pulse positions which cannot be chosen for BenQ"s MBR. Despite having no control over the pulse timing and despite the relative high pulse width, the settling performance of the LG 24GM77 is way better than the settling performance of the BenQ XL2411Z (MBR) or the ASUS VG248QE (2D Lightboost).

The settling performance being even better than that of an ASUS VG248QE in LightBoost mode cannot be explained by just a more optimal overdrive setting. Measurements confirm that the LG 24GM77 also uses accelerated panel update when in motion blur reduction mode, which normally is a Lightboost feature or achieved by tweaking the video timing (VT tweak). The panel update is about 140% faster in Motion240=On mode than it is in Motion240=Off mode, which gains about 2.0ms of additional time for the bottom pixels to settle (at 120Hz). This is faster than what could be done with the normal VT tweak (1350 / 1144=118%). On some monitors, a more aggressive VT tweak might work as well, e.g. VT1502 (1502 / 1144=131%). By the way, a VT tweaked video timing, although being accepted by the monitor, does not make a difference (not really surprising given that there is the internal tweak active already). Besides all the good, there is one technically unnecessary drawback, which is the high input lag. The LG 24GM77 appears to first buffer the entire frame before starting the accelerated screen update reading from that buffer. What could be done instead, and what is done in other Lightboost monitors, is to buffer only part of the frame so as to provide just enough data for the delayed but accelerated update process until the update of the last pixel line, at which the update process would have caught up with the real-time signal input again. The simplified buffer management used in the LG 24GM77, however, costs about 6ms of input lag which otherwise could have been cut. (Technically, the update process would need to be delayed by just the 2.0ms by which the update is faster than the normal update; instead of the 2ms,however, the update is delayed by a full refresh cycle, i.e., by 8.33ms at 120Hz, which makes a difference of about 8.33-2≈6ms.) This is partly compensated by an earlier backlight pulse onset with respect to VSYNC, as compared, e.g., to the ASUS VG248QE, causing a net difference in input lag between these monitors of about 5ms at 120Hz.

Part of the normalized luminance curves (green channel only) of the LG 24GM77 and, for comparison, the ASUS VG248QE. The top panel shows the measurements for Contrast=50, in which case the scaler needs to do some calculations on the pixel values. One can clearly see how some pixel values are rounded to the same output value (missing steps in the red curve). The bottom panel shows how it looks when the monitor is left at the factory preset: no round-off errors and less residual noise as compared to the ASUS.

When measuring luminance over pixel value with a probe that averages over a few hundreds of pixels (like a patch of 3cm in diameter), it appears that the LG 24GM77 does not apply dithering in addition to what is done by the panel electronics. The panel used in the LG 24GM77 has a native color resolution of 6bit per color channel which is virtually increased by the panel driver to 8bit by means of dithering techniques (referred to as FRC = Frame Rate Control, a misleading term). Since the panel driver is a fixed component of the panel, this is probably the same for all monitors that use this panel, meaning the manufacturer of the monitor has probably little control over the pixel value processing done in the panel driver. However, in between receiving the pixel values from the PC (8bit per channel) and sending them off to the panel driver (8bit per channel as well), the scaler (here MSTAR) usually does some calculations on the pixel values in order to generate a certain gamma transfer function, apply color gain factors, etc. If there is no 1:1 mapping between input and output at the scaler level, and the pixel value resolution is the same at both ends, multiple round-off errors are inevitable and, effectively, cause a loss in color resolution, even if the calculations are done at a higher resolution (e.g. 10 or 12bit). But like the panel driver increases panel resolution from native 6bit to virtually 8bit using dithering, the scaler can increase the 8bit of the interface to the panel driver to virtually 10bit (for example), again by using dithering, so that the final round-off error becomes smaller. This is done in many monitors, even in those with a native 8bit panel, but – apparently – not in the LG 24GM77.

Dithering, although being useful, comes with a bag of problems, and applying dithering more than once in the processing chain comes with an even bigger bag of problems, like having the dithering processes badly interfere with each other. This adds to the problem of potential interference with pixel inversion. So having one dithering process less is not really a bad thing if one can live with the consequences: the potential loss of color resolution. However, in its factory preset mode (Reset in the OSD menu), the LG 24GM77 actually avoids the loss of color resolution, at least as it may be caused by the bottleneck between the scaler and the panel driver, simply by not applying any color processing whatsoever, i.e., by having the scaler forward the 8bit pixel values to the panel driver unchanged, i.e., just as they are received from the PC. Although this moves the burden of color and gamma correction over to the PC, it provides actually more options for preserving color resolution throughout the pixel processing chain. In the best case, no correction is needed at all and, as confirmed by measurements, this basically cuts the noise level in half as compared to the ASUS VG248QE (which serves as an example of a monitor that uses dithering also at the scaler level). This reduction in color noise is worth mentioning in so far as the measurement probe can be easily tricked by dithering because it averages over many pixels and for a relative long period of time. So the higher noise level observed for the ASUS is probably not due to poor dithering but due to limited color resolution in the scaler when doing the pixel value calculations. For an in-depth discussion of the measurement methods see Measuring color resolution, which also describes a case were things went horribly wrong despite having used a native 8bit panel.

The factory preset comes with a gamma value of about 3.0 which is rather high in comparison to other monitors. Since the pixel values are not changed as they are forwarded by the scaler, this must be the panel"s (or panel driver"s) native gamma value. In terms of coding efficiency, a high gamma value is actually good because it is well in line with the visual performance in the context of high-contrast monitors. The higher the contrast, the larger the dynamic range that has to be covered, and the less optimal in terms of visual sensitivity is the standard gamma value (2.2) that was designed for the smaller dynamic ranges (i.e. contrasts) found in vintage monitors. That the measured luminance curve shows a wiggle at its high end might be taken as another evidence for the scaler not changing pixel values, hence, not correcting for such wiggles.

Histograms (distributions) of local luminance errors, comparing the LG 24GM77 with the ASUS VG248QE for two different settings for the LG 24GM77. For realizing Contrast=50 (left panel), the scaler has to do some calculations on the pixel values which causes a lot of round-off noise that is not reduced by dithering (as it is in the ASUS). On the other hand, with the factory preset (implying Contrast=70), pixel values are not modified by the scaler and the effective noise is much smaller than for the ASUS.

The vertical extent of the measured screen area was not only limited by the rubber sleeve but also by the stimulus being just a small horizontal stripe covering 5% of the screen height. Note that the LC cells are sequentially updated from the top of the screen to the bottom, which results in different delays for the luminance curves depending on the vertical measurement position. By limiting the measurement to only 5% of the full vertical screen size, the smear effect caused by averaging over differently delayed luminance signals becomes close to irrelevant. For example, at a refresh frequency of 120Hz the screen is updated within around 6ms, so if the true luminance would change instantly, the measured rise time would be 5%·6ms = 0.3ms, which is negligible here.

Settling behavior measurements for RT=Low at 120Hz. Other than RT=Low, the factory preset was used, implying C=70. For further details on the graphs and the measurement method see Flicker-free settling.

Comparison chart for the some flicker-free backlight test cases (120Hz). Note that smaller bars/values are better. The colored bars refer to the maximum of 90% of all step sizes (max90) and the gray bars refer to the maximum of 100% of all step sizes (max100).

Comparison chart for the some 120Hz strobed backlight test cases. The factory preset implies C=70. Note that smaller bars/values are better. The colored bars refer to the maximum of 90% of all step sizes (max90) and the gray bars refer to the maximum of 100% of all step sizes (max100). The scaling of the bar lengths is the same within columns (i.e., for a particular refresh cycle), but different within rows.