What is the meaning of "DC" in "DC Dimming"?

Before the wave starts, make a statement. Our "LCD Teaching" and "Anti-OLED Alliance" can be collectively called "Anti-Low Frequency PWM Alliance". W

What is the meaning of "DC" in "DC Dimming"?

Before the wave starts, make a statement. Our "LCD Teaching" and "Anti-OLED Alliance" can be collectively called "Anti-Low Frequency PWM Alliance". We are opposed to low-frequency PWM dimming, not other display technologies.

We are not targeting at anyone, whether it is liquid crystal, organic light emitting diode or lamp tube, and any display and lighting equipment that uses low-frequency PWM dimming is our condemnation object.

DC dimming and high frequency PWM equipment are our allies. But now Samsung AMOLED, low-end notebooks and desktop monitors are running all over the street, and several LG p- OLED screens are also flashing (Pixel 2 XL: black question mark? ) are the targets of our "anti-low-frequency PWM alliance". Summary of the contents of this PWM popular science:

The first part is mainly about the harm of flicker, the definition of PWM dimming and some pits in the history of display;

The second half is mainly about how to evaluate the harm degree of low-frequency PWM, the personal detection method of PWM and the eye protection survival guide in the era of one screen.

To tell the truth, if you usually play with your mobile phone and watch the monitor less than 1 hour, and you still feel insensitive, low-frequency PWM dimming will not have much impact on you. However, the mobile phone computer is used for too long, and it is used for several hours every day, even for users of 10 hours. For heavy users, the dimming mode of the screen is a very important factor to consider.

PS: Personal works will inevitably have omissions. If you have any comments or suggestions, please feel free to put them in the comments section.

Evil flash

The starting point of this story is a little higher, starting directly before the display appeared. The core of this popular science is the word "strobe". "The history of artificial lighting is the history of flash", which is really no exaggeration. From Tesla's alternating current to Edison's direct current, it seems to determine the flashing history of human lighting. Because all light sources working under AC power supply will flash. ...

50Hz alternating current

Since 189 1 year, humans began to use multiphase alternators on a large scale, and finally 50Hz (the frequency being used in China) and 60Hz alternating current occupied the market. The frequency of alternating current determines that the flicker frequency of most household fluorescent lamps (fluorescent lamps) from antique tungsten lamp to now is 100Hz, that is, it blinks 100 times per second (50Hz sinusoidal alternating current has 50 peaks and valleys per second and passes through 0. 100 times).

Strobe of different LED products

The LED light source, which was once highly anticipated, has a very complicated stroboscopic form because of the different driver circuits of manufacturers. If the dimming function is added, the strobe type will become more and more complicated. Of course, a good product can't flash or flash slightly. However, after the price of LED products plummeted, many manufacturers adopted simple driving circuits, which led to the deterioration of stroboscopic conditions.

Various indoor and outdoor low-frequency flicker light sources

Now I understand why the slow-motion video taken indoors will flash. Because it is really flashing again ... In addition to being seen by humans intentionally or unintentionally, even the scanning gun in the supermarket will be affected by the flash, thus affecting the recognition rate.

According to the definition of the technical report of CIE TN006-20 16 of the International Lighting Association, flicker can be divided into three types: flicker, flicker effect and phantom effect, and the latter two refer to the physical effects of the observed object and the observer respectively. In order to simplify the explanation, they are collectively referred to as flicker and strobe below.

Sensitivity curve of human eyes to flicker frequency

The sensitivity of some different human beings to stroboscopic is very different, and there is even a saying of "flash sensitive constitution". The most sensitive frequency for human beings is 8.8Hz, and then it keeps dropping. Most people can't see the flicker after 80Hz, but even so, some people still feel tired and sore eyes. Ieee STD1789-20 15, a document of the institute of electrical and electronic engineers for 20 years, points out that lighting flicker will have potential adverse effects:

Photosensitive epilepsy or seizure induced by flash (0. 1% population)

Migraine or severe headache, often accompanied by nausea and visual impairment.

Increase the repetitive behavior of autistic people

Poor eyesight includes: eye fatigue, fatigue and blurred vision.

Baokemeng shock incident

Looking back at the most famous "stroboscopic accident" in modern times, there is an urban legend, and who would have thought that the protagonist was actually Poké mon ... 1997 12 16, the 38th sentence of Boké mon's Computer Warrior Multilateral Beast broadcast by Japanese TV station, in order to render the computer world. This kind of low-frequency flicker, no matter what monitor you look at, may cause related symptoms (I have never felt that human beings are so fragile and prone to rollover).

This incident was called "Baokemeng Shock", which made Baokemeng, a seemingly harmless but deeply loved animation, enjoy the treatment of being ordered to stop broadcasting. It did not continue broadcasting from the 39th sentence until April 16 of the following year. These 38 sentences are permanently banned on TV (and on the Internet). And the poor multilateral beast, the whole family never appeared on TV again. Coincidentally, in 2007, both the London 20 12 promotional trailer and 201Twilight: Breaking Dawn had the event that "the audience had a seizure due to the flashing of color blocks".

If you are interested, you can go to the video website to watch the 38th sentence of Bao Mengke, /RgY 1iPk, and the airborne position 18 minutes 5 1 second. Although the fineness, color saturation and excitement of digital video are not as good as in the past, we still suggest that you pay attention to safety and fasten your seat belt before driving (plus a scary nonsense: we didn't know we were sick before we got sick).

What is PWM?

Back to our protagonist PWM dimming. PWM, full name of pulse width modulation, is translated as pulse width modulation. In the final analysis, it is the technology of modulating analog signals into pulse waves. For the display/light source, this has been a very widely used brightness control scheme. In addition, there is DC-DC dimming (CCR constant current dimming in the LED field, and DC dimming is used in this series of popular science for convenience of expression).

Before we get down to business, we need to talk about two kinds of signals:

Analog signals can have a series of values between 0- 100% (almost stepless speed change).

Digital signals are characterized by "0 and 1". These are both very geek numbers. After all, the whole human IT system is controlled by logic circuits "0 and 1".

Although the cost of digital signal control is low, the most embarrassing thing is that whether it is marked by high and low levels or the power-off and power-on of the light source, it only corresponds to two States: "0 and 1". Aiming at the dimming problem mentioned this time, the control of digital signal can only produce "on" and "off" states, and the brightness is either 0 or 100%. It is doomed to be unable to realize stepless brightness level like analog signals.

Then why can our screen adjust the brightness infinitely? Because intelligent human beings find that the naked eye has limited reaction speed and has the function of nourishing the brain. As long as the switching speed of the light source is fast enough, it will not be noticed by the naked eye. PWM dimming, so there is black and pink ...? By flashing to form a "carrier wave" and then controlling the time ratio (duty ratio) of "on and off", the effect of controlling brightness can be achieved:

In more than N flashes per second, if the required brightness is low, let the proportion of extinction time increase. For example, if you want to adjust the brightness to 10%, just make the time ratio of brightness reach 10%. Although it sounds a bit like a chicken thief, PWM dimming has many advantages, such as simple structure, high precision, power saving and low heat generation, besides controlling "brightness change" without causing obvious color cast. The eyes of human beings and even most creatures on the earth have evolved under continuous natural light. If blinking is used as visual deception to adjust brightness, will it roll over?

Although, as mentioned above, most people can't detect flicker exceeding 80Hz, they can actually detect flicker with higher frequency in the corner. The nervous system and cerebral cortex can detect the stimulation of 160Hz, and the retina is more sensitive and can respond to the flicker of 200Hz. These have been proved to cause headaches, migraines and fatigue.

Review the definition of "stroboscopic" given by the national standard (according to IEEE, this is actually the definition of "stroboscopic phenomenon"). Strobe means "when observing a continuous moving object under a flickering light source, the image that should be continuous appears discrete". When we read or remove our eyes from the screen, the naked eye can detect the flicker of hundreds of hertz due to the stroboscopic effect.

Extracurricular reading: The most famous example of using human physiological characteristics to "cheat" eyesight is definitely a movie with 24 frames per second. Here you can also extend why 24 frames of the movie is enough, but the game needs 60 frames to start. If you are interested, you can poke our old popular science "Classic Popular Science: Why is the movie 24 frames and the game 60 frames?" 》/RI59bAj .

Human history, all kinds of black history.

Every time the display equipment changes in history, it has gone out of the pit. If you are old enough, you will remember those big and heavy CRT TVs and monitors. In the past, except for some flicker caused by the scanning speed of cathode ray tubes, was the impression particularly transparent? Particularly gorgeous?

Later liquid crystal displays, except for size, were all goods rubbed by CRT on the ground in all directions. The poor backlight of the early desktop LCD led to poor color gamut coverage and larger viewing angle. What's even more frightening is that LCD was a little more expensive at that time, but it was small and handsome (at that time, the attitude of human beings was the same as that of full-screen mobile phones now. Handsome, really can do whatever you want).

After waiting for 5 or 6 years, the backlight of mainstream LCD caught up with the large-scale use of IPS panels, and the picture quality barely caught up. During this period, the desktop LCD screen has undergone the adjustment of screen proportion and the improvement of resolution. The screen ratio ranged from 5:4 and 4:3 to 16: 10, and finally it was 16:9. The resolution ranges from 1280* 1024 to 1400*900, and finally it is 1920* 1080.

At about 20 10, the backlight of LCD has experienced the transition from CCFL (cold cathode cathode fluorescent lamp) to LED. Due to power consumption, environmental protection and other factors, CCFL was banned from production by many countries and regions in the following two or three years. Then, for the first time, human beings felt the horror of being ruled by "LED+ low-frequency PWM dimming". A large number of users found that the new display looked more eye-hurting and eye-tiring, and even there was a stalk of hoarding CCFL displays.

Fan frequency measurement flashover method

If you are old enough, you should still remember the popular "pen-wielding fan" examination method in China at that time. The CCFL backlight display that has fallen into the historical trash can has almost all PWM dimming frequencies of 175Hz, but it has obvious afterglow effect (it still emits light when the PWM control is turned off), which greatly slows down the stroboscopic effect.

Later LED backlight displays used low-frequency PWM dimming, and the frequency was usually between 180-420Hz. However, because the response speed of LED is very fast, the afterglow effect is close to zero, and its stroboscopic effect is so serious that it can produce "stroboscopic effect" (that is, it can freeze the picture of a moving object like high-speed photography).

Some display manufacturers seized the business opportunity and started to use DC dimming or DC+ high frequency PWM dimming on a large scale with "no flashing screen" as a promotion means. Today, a large number of monitors are still using non-flash screens for publicity.

The notebook field is sandwiched between the desktop and the mobile phone, and the panel has never been sprayed, and the backlight is also a pit. There are a large number of low-frequency PWM dimming products in the notebook market, some of which are only PWM under low brightness, and some are always PWM. A wave of notebook manufacturers' new products, even high-end products, are still dimming with low-frequency PWM (strongly condemned by the machine guy).

Fortunately, the LCD screen of mobile phone is hardly troubled by low-frequency PWM dimming. Even if the brightness of some mobile phone LCD screens fluctuates, it is not at the same level as the low-frequency flicker of organic light-emitting diodes (the next part of popular science will talk about their differences in detail). However, just like the large-scale use of PWM dimming on desktop displays, the low-frequency PWM dimming used by Samsung and LG, the manufacturers of organic light-emitting diodes, reminds human beings of the horror of being dominated by PWM. ...

If we say that from 16:9 screen to 18:9 full screen is progress, from full screen to Liu Haiping is retrogression. From the moment when the OLED screen uses low-frequency PWM dimming, it has regressed into the pit of historical cycle.

In the second half, we will do a popular science "How to evaluate the harm degree of low-frequency PWM, the personal detection method of PWM and the survival guide of eye protection in the era of one screen", so stay tuned.

References:

IEEEstd 1789-20 15, Report of International Illumination Society CIE TN006:20 16, IEC TR 6 1547- 1, GB/T 9473-20/kloc-0.

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