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  Video Camera - Selection Guide
項目符號 Functionality and Specifications
Motion Detection System
Excellent Image Quality
High Image Capturing Speed
AGC (Automatic Gain Control)
BLC (Back Light Compensation)
Electronic Shutter
White Balance
Signal to Noise Ratio (S/N ratio)


Functionality and Specifications

Motion Detection System
This function is used in conjunction with VisionNet Network DVR. The motion detection function is designed to automatically detect any activities within a selected detection areas, and to start recording of that camera when activity is detected inside the detection area.

Excellent Image Quality
Resolution is the ability of a camera to discriminate fine detail in a scene. The resolution of a camera is usually expressed in terms of horizontal TV lines (TVL). Cameras specifications quote resolution based on the number of horizontal elements that can be captured by the camera and, confusingly, this relates directly to the number of vertical lines that can be discerned. Imagine a camera with a resolution of 570TVL being able to display a signal horizontal lines as 570 individual segments. The higher the number of segments, the more fine detail can be resolved in the image. Obviously the number of horizontal lines displayed vertically (i.e. the vertical resolution) in a given system is fixed according to the standard in use (625 lines in PAL/CCIR; 525lines in NTSC; etc)

Cameras are often described as medium and high resolution. This typically equates to medium resolution of 330 TVL and a high resolution of 480TVL for color cameras; and a medium resolution of 380TVL and a high resolution of 570TVL for B/W cameras. If it is important to be able to resolve fine details in an application, especially in sensitive area, choose a camera with an appropriate resolution.

In the world of Network DVR, TVL image is translated to digital resolution. Namely, (Pixels x Pixels). Typical VisionNet Video Camera supports image capturing resolution up to 512x582 in PAL mode and 420TVL clarity. The generated image is digitalized by Network DVR, The achieved image quality surpasses traditional analog CCTV Camera in the product line of VisionNet Technology.

However, as the larger the image resolution, the larger data will be generated for storage. When it comes to image transmission, it also requires more bandwidth. In practical use, users are able to select different recording resolution in deal with different situation.

High Image Capturing Speed
Smoothness of Digital Video is represented by Frame rate. All the video is composed of a series of Frame (Picture) linking and playing together to form a video. Human eye can see up to 25 Frames per second. VisionNet Video Camera generate maximum 25 Frame in one second. As a result, generated video produces 25Frame/sec, we call it "Real-Time" records. When the record is replaying, viewer can see every movement momentarily, just like real-life.

However, just like the principle of Image Quality, the larger the Frame rate, the larger data will be generated for storage considerably. When it comes to image transmission, it also requires more bandwidth. In VCR-based system, using one tape to record 24-hour mode, the Frame rate is less than 1fps, when it comes to VisionNet typical Network DVR system, capacity of "Storage Buffer" is adequate for one month recording in 12.5fps.

AGC (Automatic Gain Control)
Video Cameos contain a video amplifier which applies gain to the video signal as required (up to a maximum of 28dB). The circuit is designed to compensate for fluctuations in scene illumination which would cause the video output level to be too low. If the video level is adequate, the circuit will not apply any gain to the signal. As the video level drops (e.g. the scene illumination level falls), more and more gain is applied by the AGC circuit to the video signal. The camera only applies as much as gain as is necessary to bring the video signal up to a reasonable level (typically 1V peak to peak).

It should be understood that the AGC circuit cannot work miracles and some light must be present within the scene. Note that as a consequence of amplifying a poor signal, the noise present in the signal is also amplified. Therefore a poorly lit scene with a lot of gain applied to it will appear noisy or grainy. This is usually accepted in deference to the alternative of having no picture at all. Obviously, the ideal solution is to provide adequate illumination for the scene wherever possible.

It is recommended that the AGC feature is left permanently switched on since it will have no effect as long as the scene illumination is adequate. When setting lens levels, switch the AGC off. This way you can be sure that the picture you are seeing is not due to the effects of the AGC circuit. After the lens level has been set up, switch the AGC back on.

BLC (Back Light Compensation)
Backlight compensation is a most important feature for image optimization. Situations frequently arise where the brightness light in the scene is coming from behind the subject of interest. Imagine a camera monitoring a doorway. In this example, the light outside the door is much brighter than the ambient light in the room where the camera is located. The camera's exposure system sets itself according to the average light level in the scene. However as someone opens the door the exposure system reacts to the increased light level and as a result, anybody entering the room appear "dark". The backlight compensation feature can help to overcome this problem.

Normally, the exposure circuit within the camera takes an average reading from the illumination present in the entire scene and uses this to adjust the electronic iris (or the lens iris in the case if a motorized lens). Ideally, the camera would calculate the exposure based on the light level in the part of the scene that is of interest to the viewer. The backlight compensation feature uses a "window" to set the exposure. Everything outside the window is ignored by the exposure system.

Electronic Shutter
In some lighting conditions, particularly fluorescent, the image can be seen to flicker. This is usually caused by the interaction of the shutter with the A.C. frequency of the lighting. The Fickerless setting changes the shutter speed of the camera to a value that will not cause flicker (1/120s for PAL system cameras; 1/100s for NTSC). The disadvantage to fixing the shutter speed in this way is that the sensitivity of the camera will be reduced. This is because the electronic iris feature has effectively been turned off and it will no longer control the optimum exposure setting for the available light conditions.
Shutter Speeds
The electronic shutter available on Video cameras is analogous to the shutter in a conventional 35mm camera. The shutter speed is usually selected using a bank of dip switches located on the side of the camera. A faster shutter speed can arrest the motion of a fast moving object rendering it sharp. Fast shutter speeds allow less light to fall on the CCD and can darken the image. If fast shutter speeds are required, ensure that there is adequate lighting. Selecting a shutter speed manually will override features such as the electronic iris and flickerless settings.

White Balance
The white balance feature compensates for the temperature color 'casts' that different light sources can cause. Color casts can make white appear with a slight hue under different light sources (e.g. tungsten and fluorescent). To see the effects caused by different lighting conditions, point a camera set to auto color mode out of a window. Allow the camera ten seconds or so to balance to the outside lighting, then point the camera indoors at a room scene lit with artificial lighting. Any white areas in the scene will show a definite color tint, After a dew seconds you will see the camera compensate and the white areas will be rendered correctly.

The camera cannot do this unless the color mode is set to auto color balance. It will only correctly reproduce white for the specific lighting type it is set for. Video Camera set the three fixed color modes on their cameras to compensate for indoor (tungsten), outdoor (daylight) and fluorescent lighting types.

Because no signal lighting has a fixed color temperature, accurate rendering of white cannot be guaranteed. Color compensation should only be used if the scene being viewed contains a number of different lighting types and this causes the auto white balance circuit to 'hurt' as it tries to balance itself. For cameras fitted with this feature. VisionNet recommends that it is always set it Auto.

Signal to Noise Ratio (S/N Ratio)
As seems obvious this is the ratio of the level of the video signal to the amount of noise present. Noise in a video is seen as snow or graininess, resulting in a poorly defined image on the monitor or video recording. The unit for expressing s/n ratio is decibels (dB), but do not be too worried because it can be expressed as a ratio. The following table shows the equivalent ratio for values given in dB.

















It can be seen that a s/n ratio of 40Db is equivalent to a ratio of 100:1, that is the signal is 100 times the noise level. Conversely the noise is one hundredth of the signal. Note that at a s/n ratio of 20Db, the noise is 10% of the signal and would produce an unacceptable picture. The following table provides a guide as what quality to expect from various s/n ratios.

S/N ratio dB

S/N ratio:1

Picture quality

60 dB


Excellent, no noise apparent

50 dB


Good, a small amount of noise but picture quality good.



Reasonable, fine grain or snow in the picture, fine detail lost.

30 dB


Poor picture with a great deal of noise.

20 dB


Unusable picture.



For more information:
Video Camera Selection Guide - White Paper

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