What is Refresh Rate?
Author:Soomal
Source:eng.soomal.com

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In 2017, display technologies for mobile devices, such as mobile phones and tablet PCs, are constantly being updated. HDR10 and P3 color gamut have gradually become the standard for high-end mobile phones. In the near future, Soomal also conducted a series of introductions to HDR display related technologies.

However, the world martial arts, but not too fast, Apple's second-generation iPad Pro and Razer Phone in the two major mobile system camps came to provide the highest 120Hz refresh rate. What the hell is high refresh rate? What can fast screen speed bring to me? Mobile phone user who never opens a computer is bound to be confused. After talking about color and brightness enough, today we will talk about the basics and importance of the speed indicator for mobile display.

Refresh rate [Hz]

The refresh rate refers to the image switching frequency of the display screen, and the unit is Hz, which is the basis for the modern display device to display the motion picture. In the era of CRT [Cathode Ray Tube], the refresh rate is the vertical scanning frequency of the screen. If the refresh rate is set lower than 72 Hz, the vertical flicker of the screen will be clearly felt, and it is generally recommended to increase to 75-90 Hz or above. Keeping distance while watching to avoid long-term flickering affecting vision is also recommended.

However, in today's LCD screens, the motion on the screen does not need to be achieved by scanning. Therefore, the refresh rate is the rate at which the screen draws graphics. The refresh rate of mainstream liquid crystal display devices can meet the normal use of most people at 60 Hz. The "flicker" of the LCD screen is the PWM switching frequency from the backlight tube to control the brightness of the LCD screen. It is independent of the motion of the screen. This frequency is at least 200Hz, but if the user adjusts the screen brightness to the lowest, the lamp unit time duration of the shutdown is longer than the duration of the work. It will also cause the flicker and cause discomfort. The simplest verification method is to use the camera of the mobile phone to face the liquid crystal display or the TV screen. The blinking you see is caused by frequent PWM switching. The solution is to increase the PWM frequency as much as possible or adjust the light and darkness of the LED by changing the current mode.

Response time

Here is another concept. In the LCD display, the movement of the picture is achieved by the change of the pixel. Then the pixel molecule needs to respond to the change of the picture within 1/60 second [about 16.6 milliseconds] if the response If the time is higher than 16 milliseconds, and cannot keep up with the refresh rate, then the user will see the trajectory of the movement staying in the screen change. This negative effect is often referred to as "smear."

10 years ago, the gray-scale response time of mainstream LCD devices, including mobile phones and tablet PCs, could not keep up with the refresh rate. Even in 2017, there were many mid to high-priced mobile phones with gray-scale response times exceeding 40-50ms. Since the Nokia Lumia 920 began to adopt technology such as drive acceleration and shutter refresh, and the emergence of mobile OLED panels have gradually achieved response synchronization. Another enemy of the response time is the "General Winter ". At low temperatures, the electron-migration speed of LED backlights is lower than at room temperature, so the colder the phone screen is, the more sluggish it is, even if the color and brightness of the phone meet the demand. , ghosting and blurring can also make users feel uncomfortable.

Frame Rate per Second [FPS]

Since the higher the refresh rate and response speed the better, why is the current mainstream screen refresh rate remaining only 60Hz rather than higher? The limiting factor of the display refresh rate is the display bandwidth and hardware performance. In the high resolution of 3840x2160, single picture needs to handle 8 million pixels, although the bandwidth can be upgraded with the interface and protocol versions, such as upgrading from HDMI 1.4 to 2.0, and the internal display bandwidth of the mobile phone is also high enough, but the graphics hardware has limited computing and output performance. Determining factors the speed of the screen is not only the refresh rate of the monitor, but also the picture rate that can be generated by the hardware and the operating system itself. People usually refer to the speed of the screen display as FPS, ie, the number of frames per second that can be generated [1 frame is a complete picture].

Vertical sync

Refresh rate and frame rate must be displayed synchronously. Once there is a difference between the two, for example, if the picture generated is faster or slower than the refresh rate, the frames of the next frame and the previous frame will appear alternately at the same time, and the screen appear will be abnormal, such as tearing.

Although picture showing, video, and other apps, most hardware is easily in coping with under the 60Hz per second, but in complex 3D rendering, mobile phone is difficult to achieve 60 frames per second speed. In order to avoid tearing, the screen must be wait for the GPU to finish rendering before the synchronization is refreshed. Conversely, for computers with too strong performance (such as my desktop with GTX1080 SLI), it is necessary to limit the GPU's screen generation rate to achieve synchronization. These screens and graphics cards adapt to each other rely on a technique called vertical synchronization.

Why we need high refresh rate and high frame rate?

Smoother interaction: The display device is not simply used for viewing, but there are also a large number of user interaction operations. The input operation may cause delay due to the response time of various devices. No matter gamers or ordinary users, the ideal delay is always as low as possible. Because the concept of response time not only exists on the screen, but also widely exists in various types of input devices, such as the touch screen components of the network, hard disk, mouse, keyboard, and mobile phone, stylus, and even the screen itself displays the input screen signal, all these would takes time. The response speed of these input components is not ¡°synchronized¡± with the screen. If the refresh rate of the screen is 60 Hz, the response time will eventually be greatly superimposed, and the actual delay from input to display is much larger than 16 ms. For example, the 1st gen Surface and iPad Pro, stylus response speed delay is usually between 0.1-0.3 seconds, if the user feels the phone's touch operation is always slower than the finger, it is also this reason.

Therefore, to increase the screen refresh rate is not only to display a faster response, but also for a smoother interactive experience. The adoption of ProMotion's adaptive refresh rate technology in the iPad Pro not only improves the screen response speed, but also accelerates the response of the touch component. . Combined with the 240Hz screen processing capability of the A10X processor, the second-generation iPad Pro's stylus response speed can be reduced to 20-50 milliseconds, which is a very significant improvement for users who like drawing and writing.

Smoother animations: Because the speed of the movements will not only make the animation smoother, it will also make the liquid crystal smear significantly less. 120Hz means that the screen can generate 120 frames per second and 8 milliseconds of liquid crystal response delay, even when browsing the web, wiping microblogging WeChat, watching movies, ghosting and afterimages can be effectively reduce. Mobile games are currently the hottest game market. Naturally, there are users who are pursuing game quality and operation. 120Hz means lower input delay and smoother screens. It also encourages the development of aesthetics and standards of developers to a higher level. In mobile AR applications, faster response and display speed means that real-time virtual information can seamlessly interface with the real world. The harsh requirements of VR applications on the screen are not to be said. At present, the refresh rate of mainstream PCVR devices is not smooth at 90 Hz, and mobile VR devices are even more appalling.

Summary

People's demand for viewing experience is more real and natural. Therefore, like color and brightness, people¡¯s pursuit of display technology speed has never stopped. On the small screen, one of OLED's competitive advantages is speed. Sub-pixel active light emission can bring extremely fast liquid crystal response time. The gray-scale response time is up to less than 5ms, such as LG/Samsung on Google Pixel series phones and Samsung Galaxy series phones. The display refresh rate of the OLED panel has reached 75 Hz, so the picture and interaction smoothness is higher than the average level in the industry. However, the advantages of OLED are not only, TFT panels of iPad Pro and Razer Phone can also provide high refresh rate of 120Hz through technological improvement. The rival between industry and technical standards has never stopped.

Like the color gamut and brightness, outdoor use requirements and the emergence of HDR technology have made sRGB, 1000 nits, and other standards that have become nearly obsolete for nearly 30 years. The same "Billy Lynn's Intermission" and other new films have begun to try to record and playback in 60-120FPS, with consumers spending more on mobile movies, games and entertainment, mobile applications to AR, VR, etc. As the field expands, people's demand for display speed will increase.

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