ASUS PG27UCDM

Specifications

Brand:	ASUS
Model:	ROG Swift PG27UCDM
Size:	26.5"
Resolution:	3840x2160
Panel type:	QD-OLED
Max. refresh rate:	240 Hz
Panel:	? (Samsung)
Backlight type:	n/a
Price (approx.):	USD 1200
Monitor release date:	2025-01
This review's date:	2025-12

Other Reviews

TFTcentral review of the ASUS ROG Swift PG27UCDM.

RTings.com review of the ASUS ROG Swift PG27UCDM.

At a glance

This is a 4K QD-OLED monitor capable of 240 Hz refresh rate at 10 bpc without signal compression (DSC), albeit only via DisplayPort. Note that 4K doubles the resolution of 2K only in terms of the overall number of pixels, not in terms of pixel density along X and Y. Still, with a pixel density of 166 ppi, the color fringing artifacts that come with the triangular sub-pixel layout of a QD-OLED type of monitor, which were still somewhat disturbing on a 27"/2K display, have basically disappeared (which is a totally subjective assessment, of course).

OLED technology still comes with the risk of burn-in, which means that too bright content presented for too long can leave visible marks. To reduce this risk, the monitor will only be tested and later used in SDR mode (Standard Dynamic luminance Range) and with the "Uniform Brightness" setting enabled, even though the monitor supports HDR (High Dynamic luminance Range). Moreover, we intend to use the monitor at only max. 120 cd/m² and without BFI (Black Frame Insertion), all of which should help extending the monitor's usable lifetime. Nevertheless, BFI, which is used in this monitor for realizing ELMB mode (Extreme Low Motion Blur), is still a very welcome option for improving motion clarity. The monitor supports ELMB for input signals with 120 Hz, which is where the 240 Hz maximum refresh frequency is put to a use without requiring the PC to actually drive such high frequency.

In previous reviews of other OLED monitors, a rather high level of color processing noise had been observed. Moreover, the QD-OLED monitor MSI MPG 271QRX exhibited low repeatability in those measurements, even after having filtered out mid- to long-range luminance fluctuations. Because this might be caused by firmware implementation, there was some hope that a different manufacturer, in this case ASUS, would have come up with a better implementation. Unfortunately, this was not the case, which speaks for a more fundamental problem with OLED panels or how panel manufactures implement the OLED driver electronics. All of the tested panels, which is two QD-OLEDs and one W-OLED, exhibit high color processing noise, and both QD-OLED monitors (one from MSI and now this one from ASUS) exhibit poor repeatability even short-term. Although this is not a deal-breaker, it can be counted as a plus for W-OLED.
That being said, QD-OLED still might have a slight edge over W-OLED regarding color processing noise when it comes to presenting gray-scale content. This is because QD-OLED has to use all its three sub-pixels (RGB) for presenting grays, whereas W-OLED uses only its white sub-pixels, meaning that, with W-OLED, there is no averaging going on between sub-pixels, which otherwise could potentially reduce the effect of color processing noise. On the other hand, presenting gray-scale content with only white sub-pixels rather than with RGB sub-pixels has the advantage of removing all color fringing, which might be the more desirable feature. So, which one is better depends on the use case.

A QD-OLED monitor might be the wrong choice if it will be used in a bright environment. This is because it tends to back-scatter light more strongly than W-OLED monitors usually do, which reduces the effective maximal contrast in such environments. Moreover, ambient light is not back-scattered neutrally but with a purple tint. However, in only dimly lit rooms this is considered to be a non-issue.

Quirks

• The monitor arrived with firmware version MCM103, which was updated almost immediately to version MCM105 for this review. However, the update process was an ordeal, because updating via an USB stick did not work at all. The monitor just was not able to read from the stick, no matter which of several different sticks were used. The alternative update method though, which requires to run a program on a Windows PC that is connected to the monitor via a USB cable, did work without further problems.

• It is currently unknown how to make the monitor's service menu available.

• DisplayPort 1.4 (with HBR3, i.e., 25.9 Gbps) should be good enough for driving 4K at 120 Hz and 8 bpc without signal compression (DSC). However, out of the box, Windows would always switch to a lower spatial resolution when selecting 120 Hz. This could be only fixed by overwriting the display timing parameters with values following the CVT-RBv2 timing standard (according to this Online calculator), whereas the original EDID values are following the CVT-RB timing standard.

• Using ELMB mode is cumbersome. Not only does ELMB exclude other features from being used, and those other features must first be disabled for ELMB to become available, the ELMB mode setting is also not remembered when rebooting the PC. Moreover, ELMB does not only come with relatively high input lag, but the input lag might be different between different instances of ELMB activation (see section Motion_blur_reduction).

OLED care

OLED monitors are potentially susceptible to burn-in, meaning that static image content can alter pixel luminance permanently when presented for a prolonged period of time. Manufacturers implement a number of counter-measures, but, luckily – in the case of the ASUS PG27UCDM –, all of these can be turned off in the on-screen menu. Another counter-measure can be adopted by the user by not presenting bright image content in the first place, where "bright" is relative to the maximum luminance in HDR mode, which is around 1'000 cd/m² for this monitor.

Figure 1: Time course of the screen content's X,Y screen position (relative to its average position) when the OLED care feature "Screen move" is set to "Middle" (the default). The position does change once per 2 minutes and by one pixel at a time in mostly diagonal directions.

Time will tell whether burn-in is still an issue when operating the monitor under more moderate conditions.
One of the counter-measures implemented by ASUS is called "Screen move". If activated, the entire screen content will be shifted in regular time intervals by one pixel at a time. Besides "Off", there are three more "Screen move" options available: "Light", "Middle", and "Strong", where "Middle" is the default. These levels differ in how often the screen content is moved (once per 3 minutes, 2 minutes, or 30 seconds). The motion pattern has some randomness to it, but the motion direction is mostly diagonal and the motion pattern extends over 16x16 pixels (see Figure 1).

Figure 2: Message that appears on the monitor after ELMB has been turned on.

Motion blur reduction

Motion blur reduction, which is called ELMB in the monitor settings (= Extreme Low Motion Blur), is only available at a 120 Hz refresh rate and is implemented through BFI (Black Frame Insertion). The BFI on/off ratio is fixed to 50:50. Neither VRR (Variable Refresh Rate) nor HDR (High Dynamic Range) are available while ELMB is active, and other settings are affected too, without obvious technical reason (see Figure 2).

For the ASUS PG27AQDP, two issues regarding ELMB had been observed. The input latency (aka input lag) changed gradually over time by as much as one refresh cycle (~8 ms), and ELMB was disabled whenever the refresh rate was changed back and forth, or the screen resolution was changed, or the PC was rebooted. Unfortunately, not much has changed for the ASUS PG27UCDM, except that the latency is kind of stable now. "Kind of", because the latency can assume one of two possible values, depending on the exact timing conditions when ELMB was activated. Theoretically, the minimal possible latency for the screen center is about 6 ms, whereas the observed latency was either 10 or 14 ms. So, there is still room for improvement!

Figure 3: ASUS PG27UCDM settling curve examples measured after having activated ELMB at two different occasions but without having changed anything else with the setup in between. Still, different signal latencies (aka input lags) were measured.
The vertical gray lines mark the times when the OpenGL command sequence SwapBuffers();glFinish(); returns control to the PC program, which is about when the 1st line of a screen content is sent out to the monitor. This is also when, for the black to white switch, a hardware trigger is updated (pink trace), which is recorded along with the photodiode signal. For this recording, the photodiode was placed at the vertical center of the screen.

Figure 4: Normalized white luminance error Δe=(L_RGB−L_W) / L_RGB over programmed color values for the ASUS PG27UCDM (in bold red) and, for comparison, the MSI MPG 271QRX (QD-OLED monitor), the Razer Raptor 27 165Hz (IPS monitor), and the BenQ XL2540 (TN monitor).

Saturation, White

When it comes to color accuracy, one potential problem lies in the interactions between the primary color channels (red, green, blue) when rendering arbitrary colors, including white. Specifically for white, if everything was perfectly accurate, the luminances of the primary colors would add up to L_W=L_R+L_G+L_B(=L_RGB for short). Deviations from this perfect relationship can be quantified by the (normalized) error Δe=(L_RGB−L_W) / L_RGB (see Figure 4). We can interpret this error also as a saturation error, a de-saturation coefficient, or a cross-talk coefficient. Note that, normally, the according measurements are also sensitive to de-saturation effects caused by the residual background illumination, which becomes overly dominant for dark shades. This is why very dark shades have been excluded from the graphs shown in Figure 4, besides dark shades being more difficult to measure accurately in the first place. Obviously, residual background illumination is not an issue with OLED monitors, because they don't have a backlight that could cause residual background illumination.

Normally, the Δe presented here does somewhat correlate with the more familiar dE color accuracy value known from other review websites. The dE₂₀₀₀-value for this monitor is, with dE₂₀₀₀ = 0.323 (averaged over 400 colors), higher than the 0.182 of the MSI MPG 271QRX (QD-OLED panel) and the 0.152 of the Razer Raptor 27 165Hz (IPS panel), and only a little better than the 0.428 of the BenQ XL2540 (TN panel).

Color processing noise

Ideally, the monitor processes incoming pixel values so that the according output luminance follows a smooth transfer function. This processing takes place in the digital domain and aims for some favorable Gamma characteristic while taking other parameters into account, like the Contrast setting and the RGB channel gains. One way of quantifying how consistently this is done across the pixel value range, without needing to know the intended Gamma characteristic, is to measure deviations from a smooth transfer function. This means that we do not focus on how well the transfer function is described by a simple Gamma function (which would be gamma tracking) but how well the measured data points can be described by a reasonably smooth transfer function. We only measure the Green channel here, because it is the brightest of the color channels and, therewith, provides the best signal-to-noise ratio. The lowest (x<16) and the highest pixel values (x>240) are not taken into account here for several technical reasons related to the measurement method and data analysis.

Figure 5: (Unsigned) deviations of measured luminances from a smooth transfer function, normalized to the local 8bit step size, for the ASUS PG27UCDM (red) and the BenQ XL2540 (blue). The histogram on the right collapses the data shown on the left. For more information on the method, see Measuring color resolution.

Figure 5 shows the results for the ASUS PG27UCDM and, for comparison, the BenQ XL2540. For this comparison, the ASUS was operated at 8 bpc in order to match the capabilities of the BenQ which accepts only 8 bpc inputs. However, operating the ASUS at 10 bpc does not make a difference here (data not shown). Clearly, the ASUS is doing much worse in this test than the BenQ (SD=19.3% vs. SD=6.3%; smaller standard deviations are better). Unfortunately, the measurements are somewhat contaminated by high measurement repetition errors which are caused by low luminance stability over time (see the curve below the x-axis in Figure 5). This seems to be a particular issue with QD-OLED monitors (see also MSI MPG 271QRX). Note that these repetition errors are not even reflecting absolute luminance differences between the two measurement runs but residual luminance differences after having removed medium-term luminance fluctuations. That being said, we are talking here about repetition errors that are big in comparison to other monitors, which does not necessarily mean they are of practical relevance. Further investigation is need for characterizing the spatio-temporal nature of these luminance fluctuations in order to better assess their potentially negative impact on image quality.

By the way, these measurements should closely correspond to the Gradient score given in the RTings.com review, which is 9.8 of the possible 10.0 for this monitor. Obviously, this is not at all in agreement with our findings, which is

Figure 6: Upper part of the 10 bpc transfer function for the Green channel of the ASUS PG27UCDM, which shows artifacts that go beyond simple round-off noise.

probably because the RTings.com gradient scores are not only subjective but also very coarse. The vast majority of the monitors tested by RTings.com score between 9.5 and 9.9, which is only 5 steps for basically the entire relevant range and, therefore, does not allow for a very refined evaluation.

Although the ASUS, being an HDR-capable monitor, allows input signals with 10 bpc color depth, switching to 10 bpc does not seem to have an effect on color processing noise (not shown here), but the high measurement repetition errors make it difficult to conclude that the measured differences were only reflecting measurement noise. There is no improvement through 10 bpc to be expected anyway, which was also shown in previous reviews that compared 8 bpc to 10 bpc. To be clear, for these measurements and comparisons, color processing noise was always sampled at 8bit pixel value steps, i.e., the software was not made aware of whether the graphics card was transmitting these values with a color resolution of 8 bpc or 10 bpc. This is somewhat different from looking at the transfer function at the full 10 bpc resolution, i.e., where the application is 10 bpc-capable and steps through the pixel values with 10bit resolution steps. Figure 6 shows such measurement for the very upper part of the transfer function (for just the Green channel). The regular bumps in the curve are reproducible and show systematic errors as big as a full 10bit resolution step, which is difficult to explain by just assuming some regular round-off noise.

Settling behavior

Photodiode with rubber sleeve put directly on the screen for measuring the stripe.

The following measurements were made with a photo diode PDA36A (Thorlabs), the gain of which was set to 60 dB resulting in a bandwidth of 37.5 kHz and a minimal rise/fall time (10%-90%) of about 9 µs. The photodiode was placed at 4.5 cm from the screen surface at a straight angle. Ambient light was kept from the measured area by a rubber sleeve of 3 cm diameter which also limited the maximal incident angle to about ±20°.

The vertical extent of the measured screen area was not only limited by the rubber sleeve but also by the stimulus being a horizontal stripe covering just 5% of the screen height. Note that the OLED pixels are updated sequentially 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 is limited to a well defined value. For example, at a refresh frequency of 120 Hz, the screen is updated within around 8 ms, so that a theoretically instant luminance onset on a single-pixel level would result in a measured luminance curve with a 0% to 100% transition ramp over a duration of 5%·8 ms = 0.4 ms. This smear effect is what limits the bandwidth of the measurement and determines, for example, the shortest rise/fall times that can be accurately measured with this method. Normally, i.e., when measuring LCDs, this is not the overall-limiting factor, because LCD pixels switch considerably slower anyway. But with OLEDs, where the luminance onset – on the single-pixel level – can indeed be assumed to be instantaneous, this smear effect is the overall-limiting factor.
For more detailed information on the measurement method and the presentation of the results, see Flicker-free settling.

Settling curves

Figure 7 shows the luminance signal over time for the horizontal stripe being switched from black to white and back. Since the pixels in OLED monitors respond virtually instantaneously, the exact shape of the luminance transition is of minor interest here. Nevertheless, Figure 7 still provides some noteworthy insights.

1. The signal latency (or input lag) is as short as can be expected, namely about half the duration of a refresh cycle. Note that these measurements were taken at the screen center; therefore, and assuming that the pixels are updated on the screen at the very time they are received, half of the frame had to be received when the update process arrives at the vertical screen center where the photodiode was placed, which is why latency was measured to be half a refresh cycle.
2. The amplitude for the first refresh cycle after the switch is higher than for later refresh cycles, as if the monitor was using overdrive. This is only the case for specific luminance values though, which will be shown in the Settling matrix measurements section. Note that this effect is not there (or is obfuscated) when ELMB mode is active (see Figure 3), because, then, the settling process is always interrupted by inserted black frame and therefore never continues beyond the first frame.
3. Each refresh cycle ends (or – depending on how you look at it – starts) with a very brief low-pulse. This is typical for OLED monitors and reflects a pixel reset phase that is part of the pixel refresh procedure. We cannot infer any precise per-pixel pulse timing from our simple measurements here, because the photodiode lumps together several pixel lines into one signal, thereby smearing out such short time events (as explained above). But the smearing is why the relative pulse amplitude is seemingly higher for 240 Hz than for 120 Hz; at double the refresh rate, the recorded pixel lines are simply updated twice as fast, which results in less smearing and, thus, in a less washed-out pulse.

Figure 7: ASUS PG27UCDM settling curves for switching from black to white and back, at refresh rates of 120 Hz (left) and 240 Hz (right).
The vertical gray lines mark the times when the OpenGL command sequence SwapBuffers();glFinish(); returns control to the PC program, which is about when the 1st line of a frame is sent out to the monitor. This is also when, for the black to white frame switch, a hardware trigger is updated (pink trace), which is recorded along with the photodiode signal. For this recording, the photodiode was placed at the vertical center of the screen.

Settling matrix measurements

The metrics typically presented in this section, such as settling times, fall/rise times, and similar measures, are intended to characterize the shape of the pixel response curves. However, for OLED monitors with their ultra-fast pixel response time, most of these measures have become largely irrelevant. Those that remain useful may have to be tweaked in order to account for new signal features, such as the pixel reset pulse, which otherwise would contaminate the measurements.

Figure 8: Settling errors in terms of average luminance amplitudes, for the first refresh cycle after having switched between pixel values, measured at refresh rates 120 Hz (left) and 240 Hz (right).
Note the z-axis has been clipped at 5% to improve the scale for the values of interest.
A "positive" error corresponds to the luminance being higher in the first refresh cycle than in later refresh cycles, no matter whether the FROM pixel value was larger or smaller than the TO pixel value.

Figure 7, in the previous section, suggests that overshoot – or, alternatively, the average luminance error in the first refresh cycle after a pixel level change – might still be worth looking at. Since luminance remains mostly constant throughout a refresh cycle, the average luminance is the measurement of choice, provided that time intervals potentially affected by the limitations of the measurement method are excluded. To this end, the pixel reset pulse was excluded from averaging.

To assess the aforementioned luminance error, the methods described in Flicker-free settling were used, with some modifications: the photodiode gain was set to 60 dB (instead of 70 dB), a low-pass filter frequency of 4 kHz (instead of 70 Hz) was applied to preserve the features of the fast OLED pixel response, and the signal averaging was limited to a time interval that is free of potentially contaminated samples, as outlined above. Note that, with these procedural adjustments, the signal bandwidth is ultimately constrained by the width of the measured stripe on the screen.

Figure 8 shows the luminance errors for the first refresh cycle following a pixel level change, for 120 Hz and 240 Hz. The large negative errors (red bars, clipped at 5%) are of little relevance because they just refer to switching from black to dark. The other errors, one the other hand, are very low.

Color spectra

Figure 9: Spectra of the primary colors and white, recorded with the spectro-photometer X-rite i1Pro2 (10 nm optical resolution, 3.3 nm sample readout resolution). These spectra were recorded in Racing mode at a D65 white luminance of 120 cd/m².