TV monitoring system front-end equipment selection

In TV monitoring systems, it is common to see image quality degradation or a large amount of money wasted due to improper equipment selection. Reasonable selection of TV monitoring equipment and integration of systems with high performance-price ratio are the goals that colleagues in the security industry have always pursued. This article introduces how to reasonably select the front-end equipment of the TV monitoring system from the perspective of practical application, and provides experience that can be used for reference for system integration.
Keywords TV monitoring system equipment selection
The front-end equipment of the TV monitoring system mainly includes: cameras, lenses, pan/tilts, protective covers, brackets, control decoders, spotlights, etc. At present, there are many manufacturers of these equipment, with different brands, models and functions. Therefore, it is very important to reasonably select these equipment to improve the image quality of the TV monitoring system. Let's talk about some experience in combination with the actual project.
1 Selection of cameras
The camera is the core equipment of the TV monitoring system. It is very important to reasonably select it according to user requirements. At present, TV monitoring systems used for security generally use CCD cameras, because compared with vacuum tube cameras, it has the advantages of small size, light weight; low inertia; high sensitivity; good image uniformity; good impact resistance; long life, etc. In actual projects, if the illumination of the monitored target is not high, but the user has high requirements for the clarity of the monitored image, it is advisable to use a black and white CCD camera; if the user requires color monitoring, it should be considered to add auxiliary lighting devices, or use a color-black and white automatic conversion CCD camera. This camera automatically switches to black and white when the illumination of the monitored target cannot meet the color camera requirements.
After determining whether to use a black and white camera or a color camera, the next issue to consider is the technical indicators of the camera. The main indicators to be considered are as follows.
1.1 Resolution (clarity)
refers to the ability of the camera to distinguish image details, usually expressed in TV lines (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 clarity of a black and white camera is generally selected to be around 450TVL. Considering factors such as construction, the final clarity of the system can meet the 380TVL specified in China's industry standard GB/T16676-1996.
The horizontal definition of color cameras should generally be greater than 350TVL, because the human eye has difficulty distinguishing colors more finely. This selection can also meet the requirement of 270TVL for color monitoring systems in GB/T16676-1996.
1.2 Sensitivity
When the lens aperture size is constant, the minimum target surface illumination required to obtain the specified signal level. For example: using an F1.2 lens, when the illumination of the surface of the object is 0.04Lx, the amplitude of the camera output signal is 350mV, that is, 50% of the maximum amplitude, then the sensitivity of this camera is called 0.04Lx/F1.2. If the illumination of the surface of the object is lower, the monitor screen will be a gray image with difficult to distinguish levels. According to experience, it is more appropriate to select a camera with a sensitivity of 1/10 of the illumination of the surface of the object.
1.3 Signal-to-noise ratio
That is, the ratio of signal voltage to noise voltage. The typical value of the signal-to-noise ratio of a CCD camera is between 45 and 55dB. In general TV monitoring systems, you should choose about 50dB, which not only meets the industry standard requirement that the system signal-to-noise ratio should not be less than 38dB, but more importantly, when the ambient illumination is insufficient, the higher the signal-to-noise ratio, the clearer the camera image.
1.4 Working temperature
-10 ~ +50℃ is the temperature indicator of most camera manufacturers. Add protection or special protection depending on the temperature changes in the use area.
1.5 Power supply voltage
The AC voltage adaptation range of foreign cameras is generally 198 ~ 264V, and the ability to resist power supply voltage changes is strong; the AC voltage adaptation range of domestic cameras is generally 200 ~ 240V, and the ability to resist power supply voltage changes is weak. When used in the system, a voltage stabilizer is generally required.

2 Lens selection
There are many types of lenses, and there are several ways to classify them. We will only talk about how to choose a camera lens from the control method.
2.1 Manual aperture fixed focus lens
This lens is used to monitor fixed targets and in situations where the illumination changes little. This lens is relatively cheap. The required focal length can be estimated using the following formula.
f=A•L/H
Where: f—lens focal length
A—camera imaging surface height
L—object to lens distance
H—object height
The larger the focal length, the smaller the field of view angle and the narrower the monitoring range; the smaller the focal length, the larger the field of view angle and 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 to rotate forward or reverse, thereby adjusting the aperture size. When the field of view illumination changes by more than 100 times, choose this lens. However, it should be noted that if the illumination of the field of view is uneven, especially when the contrast between the monitored target and the background light is large, the camera effect of this lens is not ideal.
2.3 Automatic aperture electric zoom lens
Compared with the automatic aperture fixed focus lens, this lens has two more micro motors. One motor is engaged with the zoom ring of the lens. When it is controlled to rotate, the focal length of the lens can be changed; the other motor is engaged with the focus ring of the lens. When it is controlled to rotate, the lens focuses. Due to the addition of two micro motors, the price of this lens is relatively expensive.
2.4 Electric three-variable lens
Compared with the automatic aperture zoom lens, the aperture adjustment is changed from automatic control to remote control. It also contains three micro motors and is connected to the controller through a set of 6-core control cables. At present, this lens is widely used.
When selecting a lens, in addition to selecting lenses with different control methods according to different occasions and requirements, the following issues should also be considered.
(1) The lens size should be consistent with the image surface size of the camera. For example, a 1/3″ camera should use a 1/3″ lens. When it is difficult to match, a large-size lens can be used with a small-size camera; the opposite is not true.
(2) The lens interface must be consistent with the camera interface. If they are inconsistent, a connecting ring must be added. For example: When a C-type lens is installed on a 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 pan/tilt, protective cover, bracket, and control decoder
3.1 Pan/
tilt The pan/tilt can be simply understood as a base for installing the camera, but this base can rotate in all directions (horizontally and vertically) or (horizontally). Therefore, the use of the pan/tilt expands the camera's field of view. In the TV monitoring system, the pan/tilt must be used in places that need to be patrolled (such as halls, etc.). When selecting a pan/tilt, pay attention to the following points.
(1) The load capacity of the selected pan/tilt must be greater than 1.2 times the actual load. That is to say, the sum of the weight of all equipment on the pan/tilt should be less than the load capacity of the pan/tilt. If the pan/tilt is fully loaded or overloaded, it can work barely, but it is inert when starting, especially when it is more difficult to rotate vertically, which greatly affects the patrol effect.
(2) The pan/tilt head should have good self-locking performance when it stops, and the return angle should be less than 1° when braking.
(3) The noise level of the indoor pan/tilt head should be less than 50dB when under maximum load.
3.2 Protective cover
The device used to protect the camera is called a protective cover. There are indoor and outdoor types. The main function of the indoor protective cover is to prevent dust; while the outdoor protective cover, in addition to preventing dust, is more importantly to protect the camera from working in a harsh natural environment (such as rain, snow, low temperature, high temperature, etc.). This requires not only a strict sealing structure, but also wipers, sprinklers, etc., and the ability to heat up and cool down. This determines that 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 consistent with the nominal size of the camera, that is, a 1/3″ camera uses a 1/3″ protective cover. If it is difficult to match, a large-sized protective cover can be used with a small-sized camera; the opposite is not true.
3.3 Bracket
The component used to fix the camera, which can be wall-mounted or suspended. 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 is easy to cause the bracket to deform, and the pan/tilt will shake when it rotates, affecting the quality of the monitoring image.
3.4 Control decoder
In a TV monitoring system with a pan/tilt, an electric lens and an outdoor protective cover, a control decoder must be equipped. In this way, the control of various actions and functions of the front-end equipment can be completed by operating the corresponding keys on the keyboard in the control room. The control decoder must be of the same brand as the system host. This is because the communication protocols and encoding methods of control decoders produced by different manufacturers and system hosts are generally different. Unless a control decoder specifically states in the manual that the device is compatible with a certain brand of host, it must not be selected.
4 Selection of spotlights
At present, most TV monitoring systems are equipped with spotlights that rotate with the camera for auxiliary lighting. The biggest advantage is flexibility, convenience, and inexpensive. Black and white TV monitoring systems should be equipped with high-pressure mercury lamps; color TV monitoring systems should be equipped with iodine tungsten lamps. Infrared spotlights should be used when hidden surveillance is required at night. One type of infrared spotlight is to add a filter in front of an ordinary lighting lamp, and the other is composed of an infrared light-emitting diode array. The former consumes more energy and often produces "red storm" (due to incomplete light filtering, a small amount of red light is seen by the human eye); the latter rarely produces "red storm", but the irradiation distance is closer. In addition, it should be reminded that infrared spotlights do not work on color TV monitoring systems because infrared light is filtered out by the color filter in the color camera.