Refresh rate is a common concept in display industry, frequently used in LCD or OLED display. But what does refresh rate mean to a LED screen?
“You can think of the refresh rate as the number of times the screen’s image is refreshed per second. When watching a movie, what we are seeing is actually a series of still images, like a slideshow. We perceive movement because of the *persistence of vision* effect. The image from the previous frame remains in our brain for a short time, and the next frame follows closely. Since the differences between frames are minimal, an action is displayed through many frames, which creates the illusion of motion. This sequence of image changes is called refresh. If an action takes 20 frames to complete, it looks somewhat like an animation, but if the action takes 30 frames, it appears smoother—this is due to the refresh rate.”
This explains the concept of refresh. For LED screens, “the refresh rate is the number of times an LED on the screen turns on and off per second.” In other words, each time an LED turns on and off counts as one refresh. If an LED turns on and off 60 times in one second, the refresh rate is 60Hz.
However, the LEDs on an LED screen do not turn on and off simultaneously. They light up row by row. The second row only lights up after the first row is fully lit and this procedure continues until the last row in the data group is illuminated. Therefore, the reduction of scan number on the hardware enables the increase of refresh rate. Moreover, this method can also increase the number of greyscale and decrease the effects of coupling. However, reducing the scan rate causes a significant increase of the required number of driver ICs, which raises costs. Thus, a balance between refresh rate and cost needs to be considered based on actual needs.
Apart from the costly approach mentioned above, is there a more economical and practical method? At this point, today, chip manufacturers provide the most commonly used solution. In the diagram below, you can see the lighting time of an LED.
If we divide the entire lighting time into multiple intervals of turning on and off, the refresh rate can be significantly increased. Assuming that the maximum brightness is divided according to grayscale levels, for a 13-bit screen, there are 8,192 levels of brightness. Yet, a problem also appears here. At 8,192 grayscale levels, the refresh rate reaches 8,192 * 60 (frame rate), which is far more than we actually need. Excessively high refresh rates also lead to diminishing returns. Consequently, LED screens today typically have a refresh rate fixed at either 3,840Hz or 7,680Hz.
However, there’s a problem: at high grayscale levels, the grayscale can be divided into many shorter wavelength, resulting in a very high refresh rate. But at low grayscale levels, the wavelength cannot be further divided so the refresh rate is hard to maintain. Besides, the refresh rate at the first grayscale level is only 60Hz, which is visibly flickering and uncomfortable for the human eye. This is a major pain point in the industry, as low-grayscale flickering is difficult to resolve.
Aside from the methods mentioned above, another way to improve the refresh rate is to increase the frame rate. If we raise the conventional 60Hz frame rate to 120Hz, the refresh rate can be doubled. This approach can effectively improve low-grayscale refresh, but increasing the frame rate places certain demands on the system’s hardware architecture.”
