Many videos now have HDR (High Dynamic Range Image) effects. However, in the field of car camera, there are still flicker and halo, so there is no HDR and anti-flicker. Let's take a look at the related content with the car electronics editor.
STMicroelectronics will launch a car-mounted camera sensor solution in March and April 2018 with HDR and anti-flicker functions, which can be used in driving recorders, ADAS ring fields, mirrors, etc., and it is expected to be used in electronics in the future. glass.
Figure: Future automotive electronic rearview camera
To this end, Mr. Zhang Chengyi, Technical Marketing Manager of ST Imaging Products, came to Beijing to introduce the technical principle and ST solution to the media.
The difference between car camera and mobile phone camera
Cars pursue HDR and no flicker, and why are mobile phones without these indicators?
*HDR (high dynamic range image). The biggest difference between car camera technology and mobile phone, the car is not too much attention to how many pixels, such as 10 million, 20 million pixels, mobile phone photos are taken one frame (one frame). However, car products are concerned with speed, continuous shooting, so the pursuit of dynamic camera indicators such as 720P and 1080P, as well as HDR. Because when driving, the outdoor environment has very dark or very bright scenes at any time. Just take a picture of the mobile phone as long as it is bright enough, but the car may suddenly enter the shade or tunnel when it is particularly bright. It is very bright and very dark and can be imaged at the same time.
Correspondingly, general photography such as mobile phones does not pay enough attention to the dynamic range. For example, if a person stands in the sun or next to a tree, you can point (focus) the person on the screen, and you can make the person clearly, but the white side is white; if the tree next to the point (focus), the face is black. It is.
However, when you drive, you can't first click on the screen while driving. This may cause the bright place to be too bright, and the dark place to be too dark. This mobile phone technology is used in driving recorders and has poor results. So the first key indicator in the car is the ability of HDR, the width between the darkest and the brightest.
* No flicker. The flashing is mainly due to the LED light source. Although the LED light has advantages such as energy saving and high efficiency from the perspective of illumination (Note: One of the characteristics of the LED lamp is dimming, so it can save energy. However, the dimming usually adopts PWM modulation, but the light source using PWM modulation LED seen on the road is seen on the road. There is no standard frequency, there is no phase coherence between the light sources), and it is often used for neon lights, traffic lights, LED lights, etc. However, due to the PWM modulation, the LED is a flashing light source, but the human eye does not see flicker, but the camera/machine will capture flashing or halo due to its wide bandwidth.
For example, driving recorder, if the resolution is high enough, you can see the flashing of LEDs; if the resolution is low, you will see a large halo - this is the effect of flashing, because the image sensor is exposed, exposed , three exposures..., have been repeatedly stacked in the exposure (Figure 1 line 1).
Figure 1 ST's dual diode scheme compared to the traditional 3 exposure scheme (in this figure: during the exposure period of line 1, the short exposure does not catch the "on" of the second line, so black/strobe will occur; and ST The solution, in the long, medium and short exposure period, did not lose the signal)
ST's unique dual diode solution
To this end, ST's new product must first achieve the effect of suppression, that is, you can see that the picture is stable - the human eye does not flash, it does not flash, the human eye sees it is dark, the camera looks dark.
How to achieve no flicker? ST has a special dual-diode process.
As shown in Figure 1, the second line is a flashing LED light source. The general image sensor is that when light comes in, it will take image, that is, there is an exposure time, and then turn off (blanking) after taking the light. However, in the process of closing, it is assumed that the external light (line 2) is bright and blinking. The outer light source (line 2) is off/on, off/on-off time is longer. The opening time is short, and the human eye is not aware of the "off".
Figure 1, the traditional 2 or 3 exposure time HDR sensor: in direct sunlight, the long exposure is always saturated, while the short and medium exposure is too short to miss the LED pulse (Note: HDR usually consists of multiple LDR (low) Dynamic range images are synthesized, while LDR corresponds to different exposure times, such as long, medium, and short exposures. Long exposures are suitable for dark areas and short exposures are suitable for bright areas.).
Specifically, in the conventional car sensor (line 1), in order to make it bright and very dark, the effect of long, medium and short exposure is adopted - long exposure takes a long time. So now the light is very dark, but if the time is long, you can still get the signal. However, in the short exposure (first line), the LED is turned off, so it will flash.
To this end, ST introduced a dual-level solution (lines 3 and 4 of Figure 1). Originally made a long, medium and short exposure under one condition (Fig. 1 line 1), now one diode is designed for long exposure, and the other is for medium and short exposure.
The hardest part here is the process. How to make the diode in the sensing layer on the surface of the sensor equal to the two below it (note: one is long, one is short), and then combined, the speed is combined enough. Fast? ST is also because it has its own process.
Specifically, the difficulty is that the sensor of a sensor has many layers of stacks, some of which are called sensing layers, one of which is a Diode, in which two are cut, stacked one by one in the semiconductor process. One, long exposure, one short exposure.
So, is it true that competitors have learned this method and will do it? In fact, each family has its own unique efforts. But ST's view, if you want a single diode (Figure 1 line 1), long, medium and short exposure in order to avoid certain problems, HDR performance will certainly sacrifice some. Therefore, it is difficult to combine both flicker and wide dynamics.
ST's combination scheme
ST's sensor solution is VG6768 (image sensor) + STV0971 (image processor). The VG6768 (image sensor) is 145 dB HDR; the industry has generally been around 120 dB, and the higher end is 130 dB. It can be seen that the VG6768 has a large performance advantage.
summary
HDR and no flicker are the goals pursued by car cameras. In the past, ST, which specializes in consumer electronic camera sensors, introduced the VG6768 (image sensor) + STV0971 (image processor) solution through an innovative dual diode process, achieving 145 dB HDR with no flicker.
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