What is PWM dimming and why should you care

What is PWM dimming and why should you care

If you’ve ever looked at your phone or computer screen and started to feel a migraine, there could be several causes, and one of them is related to a sensitivity to PWM dimming. But what is PWM dimming? In other words, it is a “trick” used by device manufacturers to trick the human eye into decreasing the brightness of the screen, and with the spread of OLED panels, especially in phones, its use has expanded enormously. Let’s find out why it was introduced, what to do if you suffer from it and if there are alternatives, as well as reminding you of our in-depth study on how to reduce reflections on the computer screen. Contents What is PWM dimming What is PWM dimming in smartphones What is DC dimming What to do if you run into problems What is PWM dimming on PCs What is PWM dimming But what exactly is PWM dimming, and why is it used by device manufacturers? Let’s start with the basics: as you surely know, there are two types of signal, analog and digital. The first offers different degrees of intensity between the minimum and maximum value, whether it is the brightness of a screen and a light bulb or the speed of a fan. The latter, on the other hand, have two states, on and off. At this point there is a problem: our smartphones and PCs send a digital signal, even for the backlight. How do they regulate it? An analog controller should be used, but these devices are not only much more expensive than digital ones, but also less energy efficient. And if one technique is to vary the voltage (the so-called DC dimming), unfortunately a widely used way is to use a digital controller with a trick, and this is where PWM dimming comes into play. PWM stands for Pulse Width Modulation (Pulse Width Modulation) and PWM dimming is a technique to take advantage of the digital signal thanks to a “defect” of our eyesight. In practice, PWM is a very rapid frequency of “on” and “off” states of the signal, measured in Hz, so as to obtain a result similar to what could be obtained on a simple analog signal. And how is it modulated? Simple, to obtain a screen brightness of, for example, 60%, just keep the signal on for 60% of the time and off for the remaining 40% of the time. Even at 100% PWM could be used, but generally it isn’t, precisely because at 100% brightness it is theoretically not needed. But how come we don’t see the screen flickering? This is where the frequency comes into play, which must be very high in order not to be visible, at least 200 Hz, but there is a problem, and it is the reason why some studies have found that below 400 Hz the PWM dimming can give nuisance. A low PWM frequency can cause eye strain, because if low enough to be noticeable, it causes the pupils to contract and expand, which in turn causes nausea and headaches. With higher frequencies, the effects are less noticeable, but nevertheless some people are more sensitive than others and may still perceive flicker. This is where the fact comes into play that many screens are designed to have a high level of brightness, while we should keep values ​​between 30 and 40% to preserve our eyesight, especially if we stay in front of the screen for a long time. And it is here that the limits of PWM dimming are most evident. In fact, as brightness levels decrease, “off” times increase, but at the same time the interval between “on” times must be short enough not to give the impression of flickering, thus increasing the need for PWM frequencies higher. The variability between electronic devices is very high, with some not using PWM to dim brightness, others using it at very high frequencies and still others using it at low frequencies. There is also great variability in terms of users, with people who may have problems even with high-frequency screens, and others who, on the other hand, may not perceive anything even at low frequencies. To give some numbers, a high-frequency screen is a 2,500Hz PWM screen, while a low-frequency one is 350Hz. A final word on our brains. Why did we talk about a trick that exploits a flaw in our brain? Because if the frequency is high enough we do not notice the flicker, but we will see a less bright screen. Nonetheless, the effect on the brain is still that of a strobe light, which is not good for health. What is PWM dimming in smartphones Once upon a time a method was used to lower the brightness of panels, DC dimming, which consisted in lowering the voltage sent (and therefore did not cause screen flickering), but this happened when LCD screens they were more common. Now that AMOLED panels are the most popular, it is PWM dimming that is used in smartphones. Why these differences? Because LCD panels use a backlight behind the colored pixels, the color of which does not change depending on the amount of brightness supplied. Instead OLED panels use pixels made up of individually illuminated red, green, and blue sub-pixels. In this case, the brightness is linked to the color, and it was seen that varying the voltage of the individual LEDs also changed the perception of colors on the screen. That’s why manufacturers like Samsung use PWM, thus ensuring that the color rendering is always the same, at the cost of eyestrain and our brain, with frequencies at values ​​around 240 Hz, not very high, just like Pixels . OnePlus instead reaches up to 360 Hz, while Apple goes to 480 Hz, despite using the same type of panels as the Galaxy or Pixel. Xiaomi 13 and Nothing Phone (2) offer a PWM of 1,920 Hz at 50% brightness (at higher levels, they use a hybrid solution with DC dimming, which we will explain later). The Motorolas stop at values ​​of 240 Hz, like Samsung, while companies like OPPO, Realme, Vivo and above all Honor have raised the bar to very high levels. OPPO, Realme and Vivo arrive at 2,160 Hz, but Honor, with the Honor 90, broke all records with a 3,840 Hz PWM frequency screen. What matters is not only the frequency, but also the type of wave that is detected by an oscilloscope, and if Samsung uses very abrupt, squared waves, in which the transition between “on” and “off” is sudden, Motorola uses a sine wave, which is more comfortable for users and creates less trouble. As we said, some manufacturers, such as Motorola, Xiaomi and Nothing, instead offer hybrid solutions with DC dimming that are much gentler on the human eyes and brain, and still manage to maintain the visual quality we expect from OLED displays. In their case, moreover, the wave that defines the passage is much softer and not so sharp. But how do they do it? Let’s go find out. What is DC dimming In the previous chapters, we mentioned an “old” alternative solution to PWM dimming, DC dimming, which consists in lowering the voltage to the panel. But we also said that applying DC dimming to AMOLED panels changes the color of the panel, and therefore the color rendering. How to do it? Various manufacturers apply a hybrid solution, applying PWM dimming up to a certain percentage, for example 50%, below which they apply DC dimming. Xiaomi has introduced a technology called Flickering Protection, while OnePlus has developed a similar solution, effectively eliminating the problem of flickering below 50%, which however returns to gradually increase as 0% approaches, albeit with a more uniform trend which however brings a benefit for those who are sensitive to PWM. And all this without affecting the reaction times of the display. Left, OnePlus 7 Pro PWM at 0% brightness, left, OnePlus 7 Pro PWM with DC dimming at 50% brightness. Source: Notebookcheck And the colours? Colors do indeed change, and this is a limitation of current technology (and also the main reason why it is not widely adopted), but according to various measurements not by much, or at least within levels acceptable to the naked eye. That said, there isn’t a a perfect solution, if not the screens of devices without PWM. What to do if you experience problems But what to do if you suffer from PWM sensitivity? There are some precautions you can take to reduce or even solve the problem. The first is to do an eye exam. In some cases, astigmatism worsens PWM sensitivity and a new pair of glasses may reduce the problem. Another solution that you can adopt is to deactivate the automatic brightness, and set the brightness manually or better through an app that sets the brightness according to the time. Generally below 50% you can start having problems, but we must always keep in mind that it is better to keep the brightness between 30 and 40%, which can be difficult to manage. Another precaution you can take is not to use the phone in a dark room, as the contrast can worsen the PWM sensitivity. It is always good to keep the ambient light at the same level as that of the screen, and at the very least check if the light bulbs in the house are flickering (for example with an app like this). Other precautions you might consider to avoid PWM problems are not to use the dark theme, as AMOLED displays mostly use PWM dimming on darker colors, because the color and intensity of light are linked together in a single LED diode, while with DC dimming this problem does not exist (or is less relevant). Locking the screen refresh rate can also help, at least according to some. Finally, if you really suffer from this disorder, you can consider a phone that uses a hybrid dimming system, such as the Motorola Edge 40 Pro, Xiaomi 13 or Nothing Phone (2), or a phone with a very high PWM frequency, such as Honor 90. What is PWM on PCs So far we’ve talked about smartphones, but what about computers? Even computer screens, whether laptops or desktop monitors, suffer from PWM, but in this case you have much more choices. Here you find a list of devices tested by Notebookcheck.net (computers, tablets, smartphones and monitors), and it is a good starting point to choose a computer that does not suffer from them. You’ll find that laptops typically suffer from PWM dimming more severely than external monitors, or even smartphones. It is therefore very difficult to escape PWM even on computers, and if the only way is to opt for a device with a great screen (therefore more expensive), with a PWM higher than 3,000 Hz or without PWM, you may have to make do with home-made solutions. In case you already have a product with PWM, you can take several actions to reduce the effect. For example, adjusting contrast and color via your video card’s control suite (Nvidia Control Panel, AMD’s Catalyst Control Center, or Intel Graphics Control Panel depending on your GPU) to make a screen appear dimmer even at full brightness, however, at the cost of worsening the contrast and reducing the autonomy (of a laptop). This obviously isn’t an optimal solution, but until manufacturers are more sensitive to this problem there isn’t much that can be done.

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