分析：LED大屏如何征服「镜头焦虑」？

In the era of visualization, LED displays are key to social media engagement. However, there are persistent issues, such as flickering, moiré, and low contrast. Hangjia Talk Dispaly is researching the 'flicker-free' ratio of these displays using smartphone cameras and aims to answer the following questions:

ü What kind of LED displays can perfectly meet the camera shooting requirements?

ü Why is it so challenging to photograph LED displays, even though they appear perfect to the human eye?

ü How do LED displays satisfy shooting requirements?

 Case Studies：What Kind of LED Displays Can Perfectly Meet the Camera Shooting Requirements?

First, we tested our smartphone's slow-motion mode on 20 LED displays, capturing close-up (1m-2m) footage. We then reviewed the videos on monitors to identify flicker and ghosting issues.

ü XR/XR Displays: Overall, there are no noticeable flicker issues, and 25% of the products can achieve “flicker-free and no grayscale loss” simultaneously.

ü Rental Displays: Compared to professional displays, this category generally exhibits flicker issues. Only 14% of the products can achieve “flicker-free and no grayscale loss” simultaneously. However, from a distance, around 30% can meet the standard.

ü LED Cinema: There is no “perfect display” in this category due to flicker issues. However, cinema displays are not intended for photography; color performance is much more important.

 Experimental Verifications: How High Refresh Rate Affects Shooting Performance

Based on the 20 recorded video footage, flicker-free LED displays typically feature a high refresh rate. Additionally, some LED displays use optical films to minimize issues during video recording.

To explore the impact of refresh rate, we created a control group to compare two LED cabinets—one with and one without the “Low-Gray Refresh Rate Function.”

We employed two different shooting methods:

First, we used the smartphone’s slow-motion mode to record the displays.

We observed that the cabinet without the “Low-Gray Refresh Rate Function” enabled exhibited noticeable flickering and grayscale loss.

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Second, we switched to the smartphone’s professional mode to manually adjust the shutter speed.

We found that as the shutter speed increased, the LED cabinet without the “Low-Gray Refresh Rate Function” displayed more visual artifacts. At a shutter speed of 1/60, bands became visible; at speeds above 1/120, both flicker and grayscale loss were apparent.

Therefore, enabling the “Low-Gray Refresh Rate Function” improves visual performance during filming. A high refresh rate in LED displays ensures compatibility with various filming conditions.

 Conclusion: A High Refresh Rate Combined with Low-Gray Performance Ensures Outstanding Color Performance

The visual defects observed by cameras result from data loss in the electro-optical transfer process. Most LED displays use scanning technology, refreshing images line by line. The human eye has a visual persistence of approximately 0.1 second when perceiving images.

This automatically compensates for the intermittent refresh of the LED display. However, due to the camera's slicing sampling method, it may still capture the refresh gaps of the screen.

Band occurs when the camera’s short exposure time does not cover the full refresh cycle of the LED display.

For example, consider a 2-scan LED display using PWM dimming. The display alternates between showing the upper and lower parts of the image.

Flicker primarily occurs when the signal from the photographic equipment is not synchronized with that from the display equipment. Let us assume that the LED display's frame rate and the photographic equipment's exposure time do not match for every frame. Consequently, the camera fails to capture a complete image. This leads to flicker due to the uncertainty associated with each frame captured by the camera.

Therefore, a high refresh rate entails more refresh actions, enabling the camera to capture continuous images more effectively. The higher the refresh rate, the greater the tolerance for deviation in the camera's shutter speed.

However, it is important to understand that a high refresh rate also implies shorter frame durations. If the PWM duty cycle is synchronized with the refresh rate, it may limit grayscale performance. This limitation arises because grayscale performance is dependent on the lit duration of each refresh cycle. When the lit duration is reduced, it results in lower grayscale levels. This is particularly evident under low brightness conditions, where the duty cycle is decreased, leading to reduced lit duration. 

We now introduce the LED driver IC that integrates PAM and PWM technology to achieve enhanced grayscale performance. For instance, Macroblock’s DaVinci series can achieve 19-bit grayscale while maintaining a high refresh rate of 520,000 levels of brightness.

The figure below illustrates how different low-gray refresh settings result in varying refresh rates at distinct grayscale levels. The higher the low-gray refresh setting, the greater the refresh rate for a specific grayscale level. For example, at grayscale level 25, a driver IC utilizing 16-bit PWM with 32x low-gray refresh can achieve a refresh rate of 7,680 Hz. This feature effectively minimizes scan lines during filming.

The measured data on low grayscale refresh rates of the LED board is presented below.

Therefore, assuming that high refresh rates are not the ultimate goal is essential. In shooting applications, achieving a high refresh rate should not compromise grayscale performance. In addition to flicker issues, color accuracy must also be flawless. In conclusion, both high refresh rates and low-gray performance should be achieved concurrently.