Scanning Line and Bright/Dark Lines of LED screen

Scanning Line

The Application of Time-division Scanning for LED screen

If you were to use a traditional static scanning screen design, one square meter P16 LED display would need 768 LED driver ICs. But for a P4 LED display of the same size, you'd need a whopping 12,288 LED driver ICs!(How to calculate?) It's pretty much impossible to fit that many Semiconductor Components onto the same circuit board area. That's why we use something called time-division scanning, especially for high-density, small pixel pitch LED screen. Take a look at Figure below: it shows how this works. The LEDs light up one row at a time, starting from row 1 to row n. When it's time to light up row 1, transistor Q1 turns on, and the LED driver chip activates the LED based on the grayscale value. Meanwhile, transistors Q2 to Qn are turned off.

The principle of time-division scanning in LED displays is similar to that of a cathode-ray tube (CRT). When we take a photo or record video of an LED display with a shutter speed higher than the refresh rate of the LED display, black lines appear on the resulting image, as shown in below Figure . These are referred to as Scanning Lines.

Below Figure is a 4-scan design as an example, it takes time "A" to complete the scanning of 4 rows. If we take a photo of this display with a shutter speed faster than time "A", we call it time "B", only rows 1 to 3 will be displayed, so a black line will appear every 4 rows on the screen. However, if we use a slower shutter speed than time "A", Let's say time "C", the LED screen will be able to complete the display of rows 1 to 4, and we will get a complete image. We call the refresh rate of this display 1/A Hz.

Bright/Dark Lines

10 Times Refresh Rate

As LED screens have become popular for conference backdrops, photographers often face issues. For instance, when capturing someone in front of an LED screen, the bright background can cause the person to appear too dark in photos. To counteract this, extra lighting is used on the subject, brightening the overall scene. However, when taking a picture, the camera adjusts by narrowing the aperture and shortening the shutter speed(S ≒ 200). Unfortunately, this leads to another problem: bright and dark lines appearing on the LED screen in the background, as seen in Figure below.

Why does the bright/dark line appear instead of solving the scanning line ? It will be explained with the following simulation schematic.

A. Shooting a 1/16 scan LED screen with Refresh Rate of 960Hz by using 1/200 shutter speed:

If LED screen "A" is designed for 1/16 scan with a refresh rate of 960Hz, when taking a photo of this LED screen with shutter speed of 1/200, it will get a similar result to Figure above↑: where Bright Line and dark line in LED screen. The reason is that for an LED screen with a refresh rate of 960Hz, it takes at least 1042us to complete one full display. (1/960=1042us).While 1/200 shutter speed means exposure time is 1/200=5000us.

For a 1/16 scan, each Scanning Row/Line requires 66us(1042us/16≈66us). Suppose we press the shutter when the 4th row is displayed at the time interval of Time 1, then rows 4 to 15 will be repeatedly exposed 5 times on the camera's photosensitive component, but rows 1 to 3 and row 16 will only be repeatedly exposed 4 times. Because 5000us is only sufficient for 16 Scanning Lines/Rows to be exposed 4 times, but the remaining 832us (5000us - 1042us * 4 = 832us) is only enough for approximately 12 Scan Lines/Rows to be exposed once (832us / 66us ≈ 12.6 Scan Lines/Rows). The remaining 4 Scan lines/Rows (16 - 12=4) do not have enough time to complete another exposure. Therefore, rows 4 to 15 on this photo will be 25% brighter than other Lines/Rows, which is the origin of the bright lines appearing on the LED screen.

B. Shooting a 1/16 scan LED screen with a refresh rate of 1920Hz by using a 200 shutter speed:

Assume that LED screen "B" is also designed for 1/16 scan, but the refresh rate is doubled to 1920Hz, and when taking a photo of this LED screen with the same shutter speed of 1/200. An LED screen with a refresh rate of 1920Hz needs at least 521us to complete one full display.(1/1920≈521us)

For a 1/16 scan, each Scanning Line/Row requires 33us(521us/16≈33us). Suppose we press the shutter when the 4th row is displayed at the time interval of Time 1, then rows 4 to 10 will be repeatedly exposed 10 times on the camera's photosensitive component, but rows 1 to 3 and rows 11 to 16 will be repeatedly exposed 9 times. Because for an LED screen with a refresh rate of 1920Hz, it takes at least 521us to complete one full display(1/1920=521us).While 1/200 shutter speed means exposure time is 1/200=5000us.Because 5000us is only sufficient for 16 Scanning Lines/Rows to be exposed 9 times, but the remaining us (5000us - 521us * 9 = 311us) is only enough for approximately 9 Scan Lines/Rows to be exposed once (311us / 33us ≈ 9.4 Scan Lines/Rows, rows 4 to 10). The remaining 7 Scan lines/Rows (16 - 9=7, rows 1 to 3 and rows 11 to 16 ) do not have enough time to complete another exposure.Compared to LED display screen "A" with a brightness difference of 25%, the brightness difference for LED display screen "B" will be reduced to 11%. As shown in below Figure , the bright line problem on the LED display screen has been significantly reduced.

To solve the common troublesome issues of "bright/dark lines" or "scanning lines" during shooting, adopting a LED screen with a higher refresh rate is the fundamental solution. It is believed that a 200 shutter speed is very common in high-brightness environments. According to the 10 Times Refresh Rate theory, the refresh rate of a general LED screen should at least meet 200 shutter speed * 10 Times Refresh Rate = 2000Hz to avoid the above problems.

To get or discuss more technical information, please contact:

Name: Tony Tong

E-mail: [email protected]

Whatsapp/Phone: (+86) 186 8001 7552