Abstract: The impact of CCD camera quality on network cameras: The quality of CCD cameras used by network cameras (webcam servers, video servers, network video converters, etc.) has a great impact on bandwidth. Using high-quality CCD cameras can greatly reduce The bandwidth of the network camera.
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1. Proposal of the problem
The development of network camera technology, especially the popularity of broadband networks, has made network-based network monitoring applications popular. The advantages of flexible monitoring and control configuration, tight integration with information networks, and long-distance remote monitoring are unmatched by traditional monitoring. How to correctly master and properly apply the network camera is a new topic facing users.
The main problem in the practical application of network cameras, the first is the problem of image compression technology: the second is the problem of network bandwidth data transmission. The essence of the so-called bandwidth problem is how to transmit higher quality images on a limited bandwidth, and of course the improvement of computer operating system software management capabilities (decompression); the development of image data compression technology and the application of high-speed CPU It is the basic technology to solve the problem of image compression of network cameras. The development of many technologies (especially MPEG4) has undoubtedly enabled our network cameras to achieve greater image compression ratio than before. But is it enough to have this? the answer is negative.
2. Improving the quality of the original reference image is the key to image compression
In a certain compression technology and a certain application environment, what factors affect the bandwidth of the network camera? This is whether the CCD camera used in the front end of the system can send a stable and clean image, that is, each frame is used as a new original reference image, or as a constant compressible mutated image. This factor is often overlooked. After the unstable and unclean image is input to the network camera, the network camera recognizes that it is retransmitted as the newly added original reference image. The original invariable compressible mutated image is misjudged to newly add the original reference image, which is characterized by color shift, color roll, picture distortion or scrolling, interference fringes, noise, and the like. Practice has proved that these unstable and unclean images transmitted by network cameras should originally be transmitted as images without mutation, and the transmission bandwidth occupied by these image data must be very large. Why is this result? This is to be understood from the principle of image data compression.
Image data compression is usually achieved mainly through two links: the first link is the compression of the image size, the smaller the image size, the shorter the image data, so as to achieve the purpose of compressing the data volume. The second step is the compression of the image forming process. In the process of continuous image transmission, we only transmit the changed part of the image, and the relatively static part is not transmitted, so that the image data is shortened and the compression is achieved. It also reduces the use of excessive bandwidth.
For example, we monitor the situation in a room. When no one or animal walks in the room, the image we see is still. At this time, it is not necessary for the network camera to repeatedly transmit so many identical pictures, so the static state occupies a small bandwidth. When a person or an animal walks around, the network camera only needs to transmit the changed portion of the image, which greatly shortens the entire image data. Although the image data has been shortened, it still faithfully records all the conditions in the room. Whether the network camera recognizes whether the image is still or active is mainly to detect whether the gray level (or color gradation) of the image changes, and no change is static, and vice versa. However, the factors that affect the change of gray level are not only the moving people and objects, but also the change of light and the influence of the unstable and dirty image noise signals mentioned above. In the eyes of the network camera, even if it is still The room, for these unstable and unclean images it is also considered to be a mutated picture and adds to the image data, which is what we don't need.
3. Use high quality CCD camera to ensure reduced image variation
Using a high-quality CCD camera to increase the signal-to-noise ratio and achieve high-speed tracking white balance is the key to ensuring that the variation of the picture is reduced, thereby reducing the bandwidth of the network camera.
The picture without noise is a clean picture. The performance indicator is the high signal-to-noise ratio. On the contrary, the picture of the CCD camera with poor signal-to-noise ratio is full of noise, and the position of the noise in the picture is not repetitive. The pictures are different. In the network camera, each picture is a full-screen image. After compression, the data is the largest and the bandwidth is the most. Experiments show that such picture data is 30% or more larger than the clean picture data, which is one of the main reasons for the increased bandwidth of many of our users' network cameras.
Another parameter is the speed of tracking white balance of the CCD camera. This parameter actually reflects the operation speed index of the DSP chip used in the CCD camera. The DSP of the ordinary CCD camera has an operation speed of 100,000 times/second. The high quality CCD camera The DSP in the operation speed can reach 10 million times / sec, the whole difference is 100 times. What are the benefits of DSP faster? It is easier to think about the color drift problem, because the speed of the operation directly affects the speed of its tracking of white balance. We can use the fluorescent light environment to illustrate this problem.
The general CCD camera white balance speed is usually between 120 milliseconds and 160 milliseconds, which means that roughly 6-8 fields (50 fields per second for PAL) can be used to complete a white balance change. Certainly can't keep up with the changes in the fluorescent light source. The change of the fluorescent light for 1 second is 100 times, that is, it changes once every 20 milliseconds. Obviously, the general camera can't keep up with this changing speed, so in the environment of fluorescent lighting, the captured image will produce color shift and The phenomenon of color drift, the original still picture will become the "active" image with periodic color changes, so the deceived nature is the network camera. However, if the DSP of the CCD camera is fast, if only one millisecond is used to balance the white balance, a very stable image can be obtained. The enlightenment of this example is that with the continuous improvement of environmental protection and energy-saving requirements, the application of pulsed illumination will become more and more extensive. Under such circumstances, the requirements of the speed of the DSP of the CCD camera used by the network camera are It will become necessary, that is, the requirements for the computing speed of the CCD camera DSP will become an indicator of network monitoring (the same is true for DVRs).
As can be seen from the above analysis, a high-quality CCD camera (providing a signal-to-noise ratio of 52db-60db and a DSP operation speed of 10 million times/second) can greatly improve the application quality of the network camera.
Of course, the indicator that affects the image quality of the CCD camera is also the sensitivity index of the CCD. The CCD camera with high sensitivity index can improve the signal-to-noise ratio when the illumination is low.
It is worth noting that the sharpness index of the CCD camera has little effect on the picture quality, and has a great influence on the bandwidth utilization of the network camera. The higher the resolution, the larger the storage space, and the blind pursuit of the high definition of the network camera not only increases the cost of use, but also increases the transmission, processing and storage of images. This is an unscientific choice.
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