OLED TV panels variable refresh rate
A newsletter published on the OLED Association website (link in SOURCE) takes stock of the problems encountered when using the Variable Refresh Rate ( VRR ) on LG OLED TVs. Before continuing, let’s take a small step back. What is VRR? This is one of the functions introduced by HDMI 2.1 and available on some (actually not very many) TVs on the market including, precisely, the OLED range of LG 2019 and 2020 even if with some differences: the products of last year they are not compatible with FreeSync.
What is VRR for and how does it work? VRR was created to improve the experience of using televisions during gaming sessions. Normally the images generated by PC or console are shown on the screen with a fixed refresh rate, such as 60 Hz. This system can cause problems as the GPU can take different times to render the various frames. If the frame does not appear within specific, predetermined ranges (for a 60 Hz signal, for example, the interval is approximately 16.6 ms) the GPU must maintain the on-screen frame until the next interval, or it must only partially show the next one.
What the user sees on the screen are defects that, depending on how they appear, take different names:
- Stuttering: The fluidity stumbles turn out not to be perfect as a frame is held for a longer time.
- Frame tearing: the image tends to break down due to the overlapping of information from two different frames.
- Lag: the delay in receiving commands.
The VRR intervenes precisely to eliminate these problems: the image refresh rate is not fixed but varies to ensure that the individual frames are displayed on the screen without the need to respect predetermined intervals.
In simpler words, it can be said that it is the source that indicates to the screen how often to show the frames. The latter is then synchronized with the rendering frequency. The VRR on LG OLED TVs can operate in a range between 40Hz and 120Hz. If a source (PC or console) operates below 40 Hz, what is called Low Framerate Compensation ( LFC ) comes into play. This solution, as its name indicates, works precisely when it is necessary to compensate for a lower fluidity. In this case, the GPU intervenes by repeating the frames several times(generally double) to thus fall within the refresh rate supported by the screen; in this way the VRR, initially cut out, becomes usable again.
We have therefore clarified when VRR is used and what purpose it serves. Now let’s see what are the limits found on LG OLED TVs. As evidenced also by the tests carried out by many owners, the activation of the VRR on the OLEDs causes an increase in the black level and lower stability of the images (the “flickering” or flickering if you prefer) which sometimes occurs in the darkest. The newsletter published by the OLED Association explains why using the statements released by LG Display (LGD), the branch that produces OLED TV panels.
The cause is related to the operation of the panels themselves and is therefore not of a software nature (a bug or some malfunction). LG OLEDs use a gamma curve optimized for 120Hz operation. The gamma curve is a function that, simplifying for space requirements (the topic deserves a dedicated study), can be described as the setting that determines how the light and dark colors (in addition to the mid-tones of course) are shown on the screen.
In the case of LG OLEDs, the optimized 120 Hz curve fully exploits the characteristics of the technology to exhibit absolute black and to correctly reproduce all the levels that are in the middle, progressively climbing up to white. We speak for the precision of settings specifically designed to provide the necessary charge so that the sub-pixels (the units that make up the pixels, have a WRGB structure on LG OLEDs) of the panels show the images on the screen. What is described applies to all TVs equipped with an OLED panel produced by LG Display (therefore not only LG ones).
VRR undermines the panels as the refresh rate changes inconsistently with the gamma curve. When the frame rate falls towards the lower levels, the time interval set to supply the charge to the sub-pixels is excessively long; the sub-pixels are then overloaded and this causes the black level to rising and the onset of flickering. The solution, according to the OLED Association, lies in the use of multiple gamma curves set no longer only at 120 Hz but also at other refresh rates, optimized for even the lowest frame rates.
The solution would therefore be at the software level but it may not be completely decisive. The management of sub-pixels is entrusted to a hardware component called “T-CON” (Timing Controller Board), a controller that takes care of putting into practice what we have described. It is therefore necessary to understand whether the T-CON currently in use is capable of guaranteeing the necessary room for maneuver to make all the adjustments.
As for the incidence in real conditions of use, it is always necessary to evaluate how the frame rate of the games is set. The effects become easier to notice the lower the refresh rate is. With titles with a locked frame rate or that is in any case maintained at high levels, the problem does not arise or tends to manifest itself more mildly. We will not fail to provide updates on the point as soon as news arrives in this sense from LG.