This Article was wroten by Tony Tong, the sales manager of Huasun, and initially published in Linkedin.

Problem 1: Ghosting Phenomenon

This issue with indoor scanning screens is the ghosting phenomenon, which is mainly caused by the charging and discharging of parasitic capacitance on the PCB during the operation of switching rows and columns on the display screen. This results in LEDs that should not be lit up being illuminated, especially when applied to oblique scanning, where the ghosting problem is more pronounced. Ghosting issues on LED scanning screens have both upward and downward ghosting effects.

Problem 2: Colorshift at Low Grayscale

The test pattern used at picture 2 consists of a low grayscale white pattern. When the pre-charging function is turned on, we can see that the module appears reddish. Picture on the right shows use of some better chipset to eliminate color shift when pre-charging has just been turned on.

Problem 3: Non-uniformity at Low Grayscale

Non-uniformity is particularly noticeable under low grayscale conditions, which places very strict requirements on the uniformity of driver ICs. Pre-charging is performed to raise the voltage levels in the rows to eliminate downward ghosting effects. However, this method can lead to issues of non-uniformity in certain areas. This effect is more visible to the naked eye when displayed with low grayscale images. Unevenness in medium and high grayscale images may result from differences in PCB layout and varying voltage levels due to discrepancies between driver ICs. A low grayscale monochrome test pattern was utilized in Figures 1 and 2.

There will be deviation between LEDs in the same patch. The sum of such characteristic deviation is the cause of the blurry screen effect. As shown in Figure 3 and 4, the same source materials are displayed in full screen. The brightness of individual pixels differs and is distributed randomly as shown in Figure 3, and does not show significant improvement even under increased brightness. Brightness calibration is an effective way to fix the blurry screen effect. However, the cost of brightness calibration is high and recalibration will be required for aged LEDs. In other words, recalibration will be needed at regular intervals, creating higher maintenance costs. Some better IC utilizes built-in brightness equalization to create more even, smooth screen brightness as shown in Figure 4.

Problem 4: Dim Line at The First Scanline

The way a scan type display works is to light up LEDs line by line, You will notice that the first scan line in the upper and middle parts of the picture are abnormally dark; This phenomenon is known as the dim line.If LEDs in a frame are off for longer than they are conducting, parasitic capacitance in the PCB module will lead to increased column voltage. In particular, the column voltage when Row 1 is scanned and conducting will be higher than the column voltages when the other rows are scanned.

Problem 5: Gradient Dim Lines

The test pattern in below picture consists of a white gradient pattern. The pattern increases from a 0 gray level to a 256 gray level toward the right, and is used to check the smoothness, linearity, and contrast of the display's color scale gradient, enabling whether the display can accurately express the details of images when used for video displays.

When using this test pattern, the dim line at first scan can be seen very clearly under low grayscale. The right part of the picture shows an image of smooth variation in gray level and high contrast are maintained while eliminating the gradient dim lines.

Problem 6: Cross Effect Caused by Failure LEDs

The cross pattern formed by the failure LED in an LED display can be very confusing for display end users. Especially in the case of time-multiplexing designs, a cross pattern may be formed when LEDs extending in the horizontal direction are mistakenly lit, and mistakenly-lit LEDs in the vertical direction extend in proportion to the number of scan lines. Since fine pitch displays are typically designed with more than 16 scan lines, as soon as a bad pixel occurs in the LED, a clearly-visible cross pattern will appear, creating a display defect. The test pattern used in below picture left part consists of a white monochrome oblique scan pattern. A bad pixel in the LED has led to a blue light LED malfunction; the bad pixel consequently appears yellowish, and a cross with the bad pixel at its center is visible.

The right part of below picture shows an actual test pattern in which the cross pattern has been eliminated; it can be seen that the cross phenomena has disappeared completely.

Problem 7: High Contrast Interference

The problem often occurs in areas adjacent to high-brightness and low-brightness blocks in images, causing color cast or reduced brightness in parts of the low-brightness block. This phenomenon is caused by mutual interference between channels, and since it is commonly seen at the junction of high-brightness and low-brightness areas of the screen, it is called 'high contrast interference'. High contrast interference is often found in situations where there is black background with white text on the screen or in the shadows of the subject, such as the shadows on the edges of a person's face. Below pictures shows the situation when high contrast interference occurs on the screen, with visible color cast and reduced brightness on the edges of the face.The right part of picture is the normal image.

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