Selection of front-end equipment for TV monitoring system

Selection of front-end equipment for TV monitoring system

In the TV monitoring system, due to improper equipment selection, the image quality is reduced or a lot of money is wasted. Reasonable selection of TV monitoring equipment and integration of systems with higher performance and price ratio are the goals that security colleagues have always pursued. The article introduces how to choose the front-end equipment of the TV monitoring system from the perspective of practical application, and provides lessons for the integration of the system.
Keyword TV monitoring system equipment selection TV monitoring system front-end equipment mainly includes: camera, lens, PTZ, protective cover, bracket, control decoder, spotlight and so on. At present, there are many manufacturers of these devices, with different brands, models and functions. Therefore, reasonable selection of these devices is very important to improve the image quality of TV monitoring systems. Let's talk about some experiences in combination with the actual project.
1 The choice of camera The camera is the core equipment of the TV monitoring system. According to user requirements, it is very important to choose reasonably. TV monitoring systems for security use generally use CCD cameras because of their small size, light weight, low inertia, high sensitivity, good image uniformity, good impact resistance, and long lifespan compared to vacuum tube cameras. In actual projects, if the illuminance of the monitoring target is not high, and the user has higher requirements for the clarity of the monitored image, the black and white CCD camera should be used; if the user requires color monitoring, the auxiliary lighting device should be considered or the color black and white automatic conversion CCD camera, this camera automatically switches to black and white camera when the monitoring target illumination can not meet the requirements of color camera.
After determining whether to use a black-and-white camera or a color camera, the issue to be considered is the technical specifications of the camera. The main indicators considered are as follows.
1.1 Resolution (sharpness)
Represents the camera's ability to distinguish the details of the image, usually expressed by TVL. It depends on the number of pixels of the CCD chip, the resolution of the lens and the bandwidth of the camera system.
The horizontal resolution of the black and white camera is generally about 450TVL. Considering the construction and other factors, the final resolution of the system can meet the 380TVL specified in China's industry standard GB / T16676-1996.
The horizontal resolution of the color camera is generally selected to be greater than 350TVL, because the human eye is difficult to distinguish the color finer, so this choice can also meet the requirements of the 270TVL color monitoring system in GB / T16676-1996.
1.2 Sensitivity Under the condition that the lens aperture is fixed, the minimum target surface illumination required to obtain the specified signal level is obtained. For example: using F1.2 lens, when the surface illumination of the object is 0.04Lx, the amplitude of the camera output signal is 350mV, that is, 50% of the maximum amplitude, then the camera's sensitivity is said to be 0.04Lx / F1.2 . If the illumination of the surface of the subject is low, a gray image with difficult to distinguish layers will appear on the monitor screen. According to experience, the sensitivity of the selected camera is generally appropriate when the illuminance of the surface of the object is 1/10.
1.3 The signal-to-noise ratio is the ratio of the signal voltage to the noise voltage. The typical value of the signal-to-noise ratio of the CCD camera is between 45 and 55dB. The general TV monitoring system should choose about 50dB, which not only can meet the requirements of the industry standard that the system signal-to-noise ratio is not less than 38dB, but more importantly, when the environmental illumination is insufficient, the higher the signal-to-noise ratio, the more the camera image Clear.
1.4 Working temperature -10 ~ +50 ℃ is the temperature index of most camera manufacturers. Depending on the temperature changes in the area of ​​use plus protection or special protection.
1.5 Power supply voltage The foreign camera AC voltage adaptation range is generally 198 to 264V, and the resistance to power supply voltage change is strong; the domestic camera AC voltage adaptation range is generally 200 to 240V, the resistance to power supply voltage change is weak, and it is generally stable when used in the system Voltage power supply.

2 Lens selection There are many types of lenses, and there are several types of classification. We will only talk about how to choose the camera lens from the control method.
2.1 Manual aperture fixed focus lens This lens is used to monitor fixed targets and occasions where the illuminance changes little. This lens is cheaper. The required focal length can be estimated by the following formula.
f = A • L / H
Where: f—lens focal length A—camera imaging plane height L—subject to lens distance H—subject high altitude focal length, smaller field of view, narrower monitoring range; smaller focal length, more field of view angle Large; the wider the monitoring range.
2.2 Automatic aperture fixed focus lens When the light flux entering the lens changes, the charge generated on the CCD imaging surface of the camera also changes accordingly, causing the video signal level to change, generating a control signal to drive the micro motor in the lens forward or reverse Turn to adjust the size of the aperture. Choose this lens when the field of view illumination changes more than 100 times. However, it should be noted that if the illuminance of the field of view is not uniform, especially when the contrast between the monitoring target and the background light is large, the effect of this kind of lens is not ideal.
2.3 Automatic iris power zoom lens Compared with the automatic iris fixed focus lens, this lens adds two micro motors, one of which is engaged with the zoom ring of the lens. When controlled rotation, it can change the focal length of the lens; the other motor and the lens The focus ring is engaged, and the lens is focused when controlled rotation. Due to the addition of two micro-motors, this lens is more expensive.
2.4 Compared with the automatic iris zoom lens, the electric three variable lens only adjusts the iris from the automatic control mode to the remote control. It also contains three micro-motors, which are connected to the controller through a set of 6-core control wires. At present, this kind of lens is widely used.
When choosing a lens, in addition to selecting lenses with different control methods according to different occasions and different requirements, the following issues should also be considered.
(1) The size of the lens should be consistent with the size of the imaging surface of the camera. For example: 1/3 "camera should choose 1/3" lens. When it is difficult to be consistent, a large-size lens can be used with a small-size camera; otherwise, it cannot.
(2) The lens interface and the camera interface must be the same. If they are inconsistent, a connecting ring should be added. For example, when the C-type lens is installed on the CS-type camera, a connecting ring must be added to the C-type lens. If the connecting ring is not added, the protective glass of the CCD imaging surface may be damaged, causing damage to the CCD camera.
3 Selection of gimbal, protective cover, bracket and control decoder 3.1 The gimbal can be simply understood as the base for installing the camera, but the base can rotate in all directions (horizontal and vertical directions) or (horizontal direction). Therefore, the use of the gimbal expands the camera's field of view. In the TV monitoring system, the pan-tilt shall be used in the places (such as the hall, etc.) that need to be monitored by patrol. Pay attention to the following points when choosing a gimbal.
(1) The load capacity of the selected gimbal is greater than 1.2 times the actual load. That is to say, the sum of the weight of all the equipment on the gimbal should be less than the load capacity of the gimbal. If the gimbal is allowed to run at full load or overload, although it can avoid strong work, it is more inert at startup, especially when it is turned vertically. Greatly affected the inspection effect.
(2) The gimbal should have a good self-locking performance when the rotation stops, and the return angle should be less than 1?
(3) At maximum load, the noise of the indoor gimbal should be less than 50dB.
3.2 Protective cover The device used to protect the camera is called a protective cover. There are indoor and outdoor. The main role of the indoor protective cover is to prevent dust; while the outdoor protective cover is mainly used to protect the camera under harsh natural environments (such as rain, snow, low temperature, high temperature, etc.) in addition to dust. jobs. This not only requires a strict sealing structure, but also has wipers, spray devices, etc., and has the function of heating and cooling. As a result, the price of outdoor protective covers is much higher than that of indoor protective covers. When choosing a protective cover, it should also be noted that the nominal size of the protective cover is the same as the nominal size of the camera, that is, 1/3 "camera uses 1/3" protective cover. If it is difficult to be consistent, a large-size protective cover can be used with a small-size camera; otherwise, it cannot.
3.3 The bracket is used to fix the components of the camera, and there are types such as wall mounting and ceiling mounting. The selection of the bracket is relatively simple, as long as its load capacity is greater than the total weight of the equipment installed on it, otherwise it will easily cause deformation of the bracket and produce jitter when the gimbal rotates, affecting the quality of the monitoring image.
3.4 Control decoder In a TV monitoring system with a gimbal, motorized lens and outdoor protective cover, a control decoder must be provided. In this way, the corresponding keys of the keyboard can be controlled in the control room to complete the control of the actions and functions of the front-end equipment. The control decoder must be the same brand as the system host. This is because the communication protocols and encoding methods of control decoders and system hosts produced by different manufacturers are generally different, unless a control decoder specifically states that the device and a brand The host is compatible, otherwise it must not be used.
4 Spotlight selection At present, most TV monitoring systems are equipped with a spotlight that rotates with the camera to assist lighting. The biggest advantage is that it is flexible, convenient, and inexpensive. Black and white TV monitoring systems should be equipped with high-pressure mercury lamps; color TV monitoring systems should be equipped with tungsten iodine lamps. Infrared spotlights are used when covert surveillance is required at night. One of the infrared spotlights is to add a filter in front of the general lighting lamp, and the other is composed of an infrared light-emitting diode array. The former consumes more energy and often produces "red storms" (due to poor filtering, a small amount of red light is seen by the human eye); the latter rarely produces "red bursts", but the irradiation distance is relatively short. In addition, it should be reminded that infrared spotlights have no effect on the color TV monitoring system, because the infrared light is filtered by the color filter in the color camera.

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