Functionality and Specifications
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
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
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
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
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
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.
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
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.
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.
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
Excellent, no noise apparent
Good, a small amount of noise but picture
Reasonable, fine grain or snow in the
picture, fine detail lost.
Poor picture with a great deal of noise.
Video Camera Selection Guide - White Paper