Learn about the selection criteria and installation of vehicle-mounted cameras in one article

At a certain resolution, the points on the image are collected in an interlaced scanning manner. When a certain point is scanned, the grayscale of the image at that point is converted into a voltage value corresponding to the grayscale through the image sensor chip, and then the voltage value is output through the video signal end. The vehicle-mounted camera continuously scans a line on the image, and the output is a continuous voltage signal. The ups and downs of the voltage signal reflect the grayscale changes of the image in that line. When a line is scanned, the video signal end outputs a level lower than the minimum video signal voltage (such as 0.3V) and maintains it for a period of time. This is equivalent to a voltage "groove" immediately after each line of image signal. This "groove" is called a line synchronization pulse, which is a sign of scanning line change. Then, after skipping a line (because the vehicle-mounted camera is interlaced scanning), start scanning a new line, and so on until the video signal of the field is scanned. Then, a field blanking area will appear. There are several composite blanking pulses in this area, one of which is much wider (that is, the duration is longer than) other blanking pulses, called a field synchronization pulse, which is a sign of scanning field change. The field sync pulse marks the arrival of a new field. However, the field blanking area happens to span the end of the previous field and the beginning of the next field. The video signal of the next field will not arrive until the field blanking area is over. The on-board camera scans 25 images per second, and each image is divided into odd and even fields, with odd fields first and even fields later, so 50 images are scanned per second. In the odd field, only the odd lines in the image are scanned, and in the even field, only the even lines are scanned.

There are two important indicators for vehicle-mounted cameras:

Effective pixels and resolution. Resolution is actually the number of line sync pulses per field, because the more line sync pulses there are, the more lines are scanned for each field. In fact, resolution reflects the vertical resolution of the on-board camera. Effective pixels are often written as the product of two numbers, such as "320x240", where the first number represents the fineness of a single-line video signal, that is, the line resolution; the second number is the resolution, so effective pixels = line resolution × resolution.

Car camera classification:

Cameras are divided into black and white and color. To achieve the purpose of line finding, it is only necessary to extract the grayscale information of the picture, without extracting its color information, so a black and white camera is used in this design. Compared with using a color car camera with the same resolution, this can reduce the burden of single-chip microcomputer sampling. The camera is mainly composed of a lens, an image sensor chip and a peripheral circuit. The image sensor chip is its most important part, but the chip must be equipped with a suitable peripheral circuit to work. The chip and the peripheral circuit are made on a circuit board, which is called a "single board". If the single board is equipped with a lens, a shell, leads and connectors, it constitutes a commonly seen camera, such as a camera for chatting; if only the single board is equipped with a lens, this is a "single-board camera". Single-board cameras are rare in daily life. Companies that produce single boards usually sell them to other companies, and other companies package these single boards according to their own requirements.

How to choose a car camera:

The first step: ask about the photosensitive chip, which is the chip solution used in the camera.

Generally, the photosensitive chips on the market can be divided into two types: CCD (charge coupled) and CMOS (metal oxide). The advantages of the former are good imaging levels, high clarity and color reproduction coefficient, and are often used in high-end digital video cameras and digital cameras. The disadvantages are that the price is relatively expensive, the power consumption is large, and the chip size is large. The disadvantages of the latter are just the same as the former. The price is relatively low, the power consumption is also small, and the chip size is also small. With the rapid development of CMOS technology, the effect of CMOS image performance is getting better and better. Some of them have exceeded the CCD effect during the day, and even some high-end professional cameras currently use CMOS photosensitive chips. CCD sensors are slightly better than CMOS sensors in terms of sensitivity, resolution, and noise control, while CMOS sensors have the characteristics of low cost, low power consumption, and high integration. However, with the advancement of CCD and CMOS sensor technology, the difference between the two is minimal. Therefore, when choosing a camera, don't be too demanding whether the camera uses a CCD image sensor. In addition, some CMOS cameras now also add image control technologies such as automatic gain enhancement of image light source, automatic brightness adjustment, white balance control, automatic adjustment of saturation and contrast, which can completely match the effect of CCD cameras.

The second tip: understand the material of the camera and the number of lenses.

Camera lenses are generally made of two materials: glass lenses and plastic lenses. Plastic lenses are very cheap, but the imaging effect is obviously inferior to glass. Cameras with better quality use 4-layer glass (full glass) lenses (referred to as 4G), and there are 5G lenses, which actually add an iris or a filter to enhance the filtering effect on the basis of 4G. However, different 4G glass lenses vary greatly in quality, price and specific reflection on the imaging effect. How should we identify them? To put it simply, we can use the light flux of the glass lens to identify them. The light flux of the lens is expressed in f-value. The smaller the f-value, the more light passes through the lens at the same time, the more saturated the imaging color, and the better the lens. Therefore, when choosing a lens, you should use a lens with a relatively small light flux f-value. Another point is clarity, which depends on whether there will be local blurring of the entire image. However, many small factories now often reduce the number of lenses or use cheap plastic lenses in order to save costs and pursue high profits. Although these products are much cheaper and look very attractive, the actual imaging effect is really unsatisfactory. Therefore, when buying a camera, you must not blindly pursue cheapness, but try to choose a glass lens. Another point is the imaging distortion of the wide-angle lens. The general wide-angle lens will have a large barrel distortion (distortion) when restoring the image, which makes the image restored by the camera differ greatly from the shape of the object in reality, and the visual effect is greatly reduced; with the update and development of technology, wide-angle lenses with image distortion correction have been launched on the market, which has greatly improved the problem of barrel distortion (distortion), and it has become possible to restore the image to a real wide angle with zero distortion. Therefore, there are two main factors to consider in terms of the lens: one is whether the lens is a glass lens with a large light flux, and the other is whether the distortion of the lens is too large if it is a wide angle, and whether it will affect the actual imaging effect.

Tip 3: Compare night vision effects (low light performance)

There are two solutions to achieve clear imaging under low illumination (at night). One is to add infrared lights for infrared fill light, and the other is to use ultra-low illumination photosensitive chips with lenses with large light throughput and software adjustment to achieve night vision function. The advantage of the former is that it does not require any external light source, and can only rely on its own infrared light to emit light, and the imaging distance is 2-5M; the disadvantage is that the color distortion is serious during the day, and the image is only black and white at night, and it is easy to reflect white and expose light. The advantage of the latter is that only weak external fill light is needed to achieve clear imaging and the color image is rich and realistic; the disadvantage is that weak light must be used for fill light (for example, reversing), and imaging cannot be achieved without fill light or any light. From a practical point of view, the latter solution using ultra-low illumination photosensitive chips will be more applicable and more popular with consumers.

Tip 4: Understand the imaging principle of pixels

The pixels of mainstream products on the market are generally around 300,000, which means that the TV lines are around 420 lines. The so-called 420-line effect is similar to the effect of playing genuine DVD discs on daily home VDs. Earlier, some products with about 100,000 pixels were also released. Due to the relatively low technical content and poor effect, they soon withdrew from the stage of history. At this time, some people may ask, is the higher the pixel, the better? In fact, this is not the case. As we know, there are many photosensitive units on the image sensor, and each photosensitive unit corresponds to a pixel. The more pixels, the larger the image format generated. As for whether the image is clear, it is not enough to just look at the pixels. It is also closely related to other aspects such as lens material and software processing. Therefore, when buying a camera, you must remember that there is no absolute relationship between the height of the pixel and the clarity of the image. The same 300,000-pixel 420-line product can have a huge difference in image restoration effect and layer processing.