Discussion on High Grayscale and High Refresh Technology of LED Display

The main factors that determine the image quality of an LED display are: uniformity, grayscale, refresh rate, contrast, color gamut and color temperature.

There are three main audiences for LED displays: human eyes, cameras, and video cameras. For cameras and video cameras, the two indicators of "gray scale" and "refresh rate" are particularly important. Unfortunately, they are a pair of contradictory indicators.

Therefore, how to further increase the refresh rate of the display screen under a certain gray scale, or how to further increase the gray scale of the display screen under a certain refresh rate, or how to ensure a higher gray scale and refresh rate at the same time, is actually a question of same problem.

1. Gray scale of LED display

The higher the gray scale of the LED display, the more delicate the image and the more distinct the layering. As shown below:

According to the driving principle of the LED display, after the refresh rate is determined, the gray scale of the display depends on the following two factors:

1) The narrowest OE pulse width that the driver IC can respond to. At present, it is generally 70-100ns, and a few excellent driver ICs have achieved 20-50ns. The narrower the OE pulse width, the finer the gray scale that the control system can achieve.

2) The coding scheme of the control system. An excellent coding scheme can further improve the fineness of the gray scale on a given driver IC.

2. Refresh rate of LED display

The refresh rate of the LED display is strictly defined as: the number of times the display image is presented in a complete gray scale within 1 second.

When the refresh rate of the LED display, especially the scanning display, is seriously insufficient, the human eye can detect it. (The average person can feel the breathing effect and scrolling effect of the screen when the refresh rate is less than 240Hz).

For professional cameras and video cameras with high-speed shutters, it is very easy to capture the defect of insufficient refresh rate of the display screen.

The picture below takes a 4-sweep screen, 1000Hz refresh rate as an example, and the camera exposure time is set to 1/1000, 1/500, 1/800, 1/2000 seconds.

The contradiction between grayscale and refresh rate is very obvious on the scanning screen. The discussion in this article mainly focuses on the basic driver IC, and does not discuss the PWM chip.

The grayscale of the LED display is realized by weighting the subfields. The traditional control method is to realize all the subfields of one row first and then realize all the subfields of the next row. In this implementation, because the time spent on each line is too long, the visual refresh rate (equivalent to the line feed speed) is very low, and black lines are prone to appear when taking pictures with a camera. The picture below shows an 8-sweeping screen, a single box 128*128 using the traditional method to achieve the effect of 14-bit grayscale and 300Hz refresh rate, and then use a Canon 7D camera to set the exposure time to 1/1000 second to take pictures.

The line feed speed of the traditional control method is too slow, causing black lines to appear when taking pictures. A more effective method is to divide all the sub-fields to achieve a gray level into multiple parts, first achieve a partial gray level in the first line and then do a line break, and then implement this part of the gray level in the next line, and wait for a round of line change. , and then implement a grayscale part, which can improve the visual refresh rate. Apparently, the greater the number of sub-fields for a grayscale implementation, the higher the visual refresh rate can be obtained.

At present, this technology is relatively popular. There is no black line when taking pictures with a camera, but it will produce an effect of incomplete grayscale, commonly known as the "sweat spot" or "watermark" effect. The picture below shows an 8-sweep screen, a single box of 128*128 adopts 16 times of dispersal to achieve 14-bit gray scale and 3840Hz visual refresh rate effect, and the exposure time of Canon 7D camera is set to 1/1000 second to take pictures.

3. Nova's latest scanning screen control method

Nova upgraded and improved the control system in 2011, which can guarantee a high gray scale and refresh rate at the same time, and still has a very good performance under high-speed shutter shooting.

After the scanning screen achieves the effect of high refresh and high gray scale, there will be very serious afterglow (ghosting). Make a very bright oblique line on the display screen, the dark dot above the oblique line, we call it ascending afterglow, and the dark dot below the oblique line, we call it descending afterglow.

Solving afterglow needs to start from two aspects:

(1) For the uplink afterglow, it is necessary to perform discharge treatment on the row line to eliminate it.

(2) For downlink afterglow, it is necessary to use a driver IC with pre-charge to eliminate it through the cooperation of the control system;

At present, a variety of driver chips with pre-charging function have appeared on the market (such as Accumulation MBI515X, MBI5042B/MBI5041B, Mingyang MY9266, Riyuecheng SUM2017/SUM2018, etc.), combined with the timing adjustment of the control system, can effectively eliminate the downlink afterglow, as shown in the figure below It is a comparison of the afterglow effects of two 8-scan modules using a common driver chip and a chip with pre-charging function at a visual refresh rate of 3840Hz.

It can be seen that after using the driver chip with pre-charging function and discharging the row lines, combined with the timing adjustment of the control system, the afterglow can be completely eliminated even at a very high refresh rate.

The following is a set of photos taken by a professional camera with a 1/8 scanning screen controlled by the Nova control system:

(1) Control system: Nova M3 control system;

(2) Loading area of a single receiving card: 128*128 points;

(3) Driver IC: basic model, MBI5024;

(4) Scanning method: 1/8 scanning;

(5) Photo camera: Canon 7D; shutter: the fastest 1/8000 second can be manually adjusted step by step.

Summarize:

Although the grayscale and refresh rate of the LED display are contradictory, for the display of the basic driver IC, through the upgrade of the control system technology, even in the case of professional cameras and video cameras with a shutter speed of up to 1/2000, It can still ensure that the screen has no black lines, no bright lines, and the gray scale is basically complete; and, through the cooperation of the control system and the basic driver IC, it also breaks through the afterglow problem that traditional display screens have been unable to solve. The author believes that the technology described in this article will soon be popularized and applied in the industry.