Explanation of sensor flicker/banding phenomenon

Catalog

1、 Basic concepts

2、30fps, 1 In frame banding Process demonstration

3、30fps, Between frames banding Process demonstration

4、25fps, Between frames banding Process demonstration

5、 summary

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1、 Basic concepts

• flicker

The energy transmitted in the AC network is not stable , But with a fixed frequency , This frequency is commonly referred to as power frequency , For example, China is 50Hz, The United States is 60Hz. The power frequency is determined by the power system .  This energy change caused by power frequency is called flicker.

Let's say 50HZ For example , Alternating current in 1/50s, namely 20ms Change the cycle of , The variation law is shown in the figure ：

And for energy , There is no difference between positive and negative , So the period of energy is 1/100s, namely 10ms.

therefore , The brightness of ordinary incandescent lamps powered by alternating current has actually been 10ms The cycle changes with the change of AC , It's just that human eyes can't perceive the change of picture brightness .

• banding

sensor Catch flicker The phenomenon of bands formed on the image is called banding The phenomenon , Usually abbreviated to banding（ The root cause is sensor Pixels per line , The difference in energy received results in ）, As shown in the video , The screen will flash , I feel the ripples of water jumping ; Specifically, there may be the following manifestations （ These performances do not necessarily occur at the same time ）：

--- The brightness of different lines in the same frame is different , There are light dark stripes , As shown in the figure ;

--- The brightness of the same line in different frames is different , There is a ripple jump like the water ripple in the video ;

--- The overall brightness of the front and back frames is different , The brightness of the picture changes obviously

2、30fps, 1 In frame banding Process demonstration

• as follows GIF The image demonstrates the power frequency environment ,sensor The change in brightness of each line of exposure within a frame . There were 10ms、8ms、12ms Three exposures .

-- 10ms In the exposure state , area P It's always the same （ Because the exposure time is 10ms, That is, the exposure time of each line is also 10ms, A wave crest cycle is exposure 1 That's ok , So each row gets the same amount of energy ）. 

-- 8ms and 12ms In the exposure state , area P1 and P2 Will have regular changes （ Because the exposure time is 8ms、12ms, The integral area is different , This results in a different amount of energy per row ）.

Exposure yes sensor The energy accumulation of each pixel , Here we can use area to express , The area is calculated from the integral .8ms and 12ms The exposure frame will appear banding The phenomenon ;

3、30fps, Between frames banding Process demonstration

• as follows GIF The image demonstrates the power frequency environment sensor The brightness change of each exposure line between frames .

-- 30fps, The time interval between frames is 33ms.

Between different frames ,8ms In the exposure state , area P、P1 and P2 Will have regular changes （ Because different frames go together , The starting position of exposure is different , So the integral area is different , Different energy obtained ） , Then there is a jumping pattern with water ripples ;

4、25fps, Between frames banding Process demonstration

• as follows GIF The image demonstrates the power frequency environment sensor The brightness change of each exposure line between frames .

--- 25fps, The time interval between frames is 40ms.

---  The intra frame brightness change of each frame is the same （ Between different frames , Same line , The periodic point at which the exposure starts is the same ）.

stay 50Hz Under power frequency ,25fps It avoids the non-stop jumping between frames in the video mode banding The phenomenon , And it presents a stable banding The phenomenon , therefore ,25fps It can't eliminate banding.

5、 summary

• eliminate banding The only way to do it is ： stay 50Hz The exposure time under power frequency is 10ms Integer multiple ; stay 60Hz The exposure time under power frequency is 8.333ms Integer multiple .

• Whether it's 30fps still 25fps Will not affect the exposure behavior within a frame .

Reference resources ：Sensor banding Explanation of phenomena