When designing a CCTV system there are four factors that control the maximum
image quality that can be obtained. One or more of these factors can degrade the system
like a chain with a weak link the weak link in the system limits the quality of the image.
One or more of these factors can be responsible for a reduction in picture quality
particularly if the system components are not well matched from a quality standpoint.
The quality of an image is normally measured in "Lines of Resolution". The term
"Lines of Resolution" refers to a method of defining the amount of reproducible image
detail in the picture. When a manufacturer specifies the "Lines of Resolution" on their
equipment, they are referring to VERTICAL LINES OF RESOLUTION. This is the total
number of vertical black and white stripes that can be reproduced across the screen from
left to right.
Since Cameras, Cable Transmission Loss, and video Recorders may have their
definition of picture quality defined in terms of Frequency Response or Lines of Definition
(rarely, both at once), it would be useful to be able to correlate the two picture quality
measurement systems.
First of all, it is vital to know what is meant by "Lines Of Definition". The number
of lines composing the actual video picture is not what is meant by, "lines of definition". In
fact they are not lines at all, but rather the maximum number of times alternate black and
white dots could be made to appear at that location without smearing the two together to
cause a grey dot to appear. As the frequency response becomes lower and lower, fewer
successive black and white dots can be distinguished on any one line.
High definition pictures therefore require a greater "frequency response" capacity in
the equipment. The upper end of the frequency response of a cable system is considered to
be when the original signal has been attenuated by 3 dB, or a 50% reduction in power
below that originally transmitted. The frequency response in Mega-Hertz can be computed
from the "Lines of Definition" specification by dividing the number of "Lines" specified
by 107. Similarly, the number of "Lines" in a specification can be computed by
multiplying the "frequency response" in Mega-Hertz by 107. By keeping the number 107
in mind, the confusion between the "Lines of Definition" and "Frequency Response" can
be readily resolved.
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Cameras also have horizontal lines in the picture, but all cameras have the same
number of horizontal lines. The NTSC video color standard in use in the united States and
Canada all have 525 horizontal lines, only 480 of which are visible in the picture, the rest
of the lines are off screen and are used for picture synchronization. These horizontal lines
should never to be confused with vertical lines of definition.
The process for measuring line resolution uses one black vertical line followed by
one white vertical line, this is referred to as a "Line Pair" As the physical size of the line
pairs are reduced you will reach a point where the line pair corresponds to two pixels in
the camera. This is the point of maximum line resolution for the camera.
Most security cameras are CCD (Charge Coupled Device) type chip cameras. At the
heart of a chip camera is a silicon wafer. Light reflected off of an image enters through the
lens of the camera and strikes this wafer. The wafer has grooves etched both vertically and
horizontally. The spacing between the grooves create the vertical and horizontal lines of
resolution. The rectangles formed by these intersecting spaces are called pixels. The total
number of pixels for any camera is determined by multiplying the vertical and horizontal
picture lines together.
A cameras maximum line resolution is totally dependant on the number of vertical
lines etched on the CCD element. The price paid for a camera also increases with the
number of vertical lines of resolution. However, there are many other factors that influence
the maximum line resolution that any camera can deliver.
The image quality of picture transmission on CCTV systems is influenced by four
different factors that combine to create the overall picture quality level. These four factors
1. The camera must be able to discern fine details in the picture. This condition is
met when there are a sufficient number of "pixels" available to define the level of quality
desired. These pixels are etched into the CCD chip of the camera, so they cannot be
changed after the camera is purchased. The only way to increase the picture detail a
specific camera is capable of producing is to buy another camera with the required number
of pixels and replace the old camera. Other factors may limit the picture quality, but there
can be no higher quality than can be displayed in the number of pixels available in the
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2. The second source of picture quality and the most common limitation is the
frequency response of the transmission medium being used to convey the picture from the
camera to the viewing location. When the picture is being sent by way of a Coaxial Cable
the loss in detail is a linear function of the distance that the cable must traverse and the
intrinsic loss incurred by the composition of the cable. It is possible to compensate for this
loss, effectively creating a zero loss cable path by automatically compensating for the loss
of the coaxial cable. An amplifier with high frequency compensation designed to cancel
the cable loss can be installed at the receiving end of the system. One such amplifier is the
APR474 Automatic Picture Rejuvenator, which is able to compensate for cable loss up to
5000 feet of RG59/U cable automatically.
Fiber Optic or Microwave transmission equipment can also transmit a picture up to
their maximum distances, but in general, they are much more expensive than direct CCTV
cable using the automatic cable equalizing system, APR474 Automatic Picture
Rejuvenator cable equalizer. Fiber Optic transmission systems always use an AGC system
that "clamps" a picture at the peak of white, instead of at black level, which is standard for
video. This condition prevents a "fade to black", which in turn reduces the dynamic range
of the video picture giving you fewer levels of grey to black. Also any increase in the input
luminance level by the camera "IRIS" will cause the lower negative portion of the video
waveform and the SYNC pulse to be compressed below the normal level, which can result
in picture rolling and other visible picture abnormalities at the DVR. The DVR is also
sensitive the under and over levels of video which will result in no picture being displayed
at the output of the DVR during certain picture conditions. A normal cable installation
using the APR474 will solve the cable distance and picture quality problem without the
need for expensive Fiber Optic equipment.
3. Almost all CCTV systems use a video recording device either analog "VCR" or
digital "DVR" to record the images from the cameras. With an analog VCR the picture
detail is limited by the band-width of the input amplifiers and the ability of the tape heads
to lay down and pick up the information from the magnetic tape. With the digital "DVR"
the picture information can be reduced by the input amplifiers, but the main cause of the
picture quality issue is usually the amount of compression used on the incoming video.
Video has a real-time image changing 60 times per second. The DVR will reduce the
number of frames it receives in order to reduce the amount of data storage needed. The
manufacture will also limit the video band-width in order to reduce the storage space as
well. This reduction of band-width directly affects visible picture quality and is never
specified by the manufacture of the DVRs. To test this on your DVR just look at the live
picture on your monitor then play the recording back on the same monitor. The picture
should be the same if it is not then you are seeing the effects of DVR compression and
band-width reduction.
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4. The forth factor is the Monitors ability to display high resolution pictures. This is
set by the maximum number of pixels on the LCD or Plasma display and for the older
CRT type monitors it is set by the frequency response of the input circuitry and in some
cases the number of holes in the phosphor screen.
Obviously, it does no good to have one or more parts of the system capable of
substantially greater detail than the rest of the system. Once the Camera, DVR, and
Monitor are chosen, the greatest control of the picture quality that can be exercised is by
connecting the APR474 Automatic Picture Rejuvenator to the receiving end of the cable.
Applying an APR474 to the receiving end of the cable automatically cancels all the losses
incurred by the cable, so that there is no loss of any kind remaining in the transmission
path. The picture will have the same quality that could be seen on a test bench using the
selected Camera, Recorder, and Monitor with zero feet of cable connected between them,
no matter how long the actual Coaxial cables really are.
The frequency response diagram below graphically shows the quality advantage
obtained when an APR474 is connected to the receiving end of a CCTV system of any
length up to 5,000 feet. The graph plots the feet of cable traversed against the detail
remaining in the picture at each distance covered. Notice that even 100 feet of cable will
result in the loss of as much as 10% of the detail present at the camera output, which is
even more than the loss of a coaxial cable equipped with an APR474 alter traversing 5,000
feet of the coaxial cable. Also the graph depicts the cable loss to be 50% when the picture
has traveled over about 600 feet of cable without an APR474 connected. The APR474 will
deliver a picture at 100% of the quality level that the Camera, Recorder, and Monitor are
capable of producing.
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The cable loss alone, for high definition Cameras, Recorders, and Monitors is
higher than shown on the graph for any number of feet traversed., however the corrected
overall frequency response, including the correction factor applied by the APR474 is not in
any way affected by the extra loss induced by the cable at the higher frequencies being
transmitted by the higher definition cameras. This means that the APR474 will correct the
frequency response of cables transmitting medium definition signals or high definition
signals without adjustment of any kind being required from the system operator. In every
case whether high definition or medium definition pictures are being transmitted, the
picture delivered to the receiving location will be the best that the Camera, Recorder, and
Monitor can display.
The APR474 Automatic Picture Rejuvenator was developed and is manufactured in
the United States by FM Systems, Inc., and can be obtained by calling 1-800-235-6960 to
place your order.
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