GETTING THE HIGHEST PERFORMANCE FOR YOUR CCTV SYSTEM DOLLAR
GETTING THE HIGHEST PERFORMANCE FOR YOUR CCTV SYSTEM DOLLAR
By: Frank McClatchie
FM SYSTEMS, INC.
Getting more performance on a CCTV system usually involves at least two different parameters and usually two are at odds to each other, the more of one, the less of the other. The two parameters in this case are PICTURE QUALITY and DISTANCE between the Cameras and the Monitors. Usually the greater the distance the poorer the quality and the greater the quality required the shorter the distance that can be covered. We will see that that there is a way to get the highest quality picture and the greatest distance for low dollars. But first some needed technologies.
PICTURE QUALITY
Picture quality can be defined in terms of LINES. In fact, many Cameras, Monitors, and Recorders are now specified in these terms. But just what does this term mean? After all, Cameras in the U.S.A. all produce 525 lines per picture, no more, no less. Of course Camera manufacturers do not actually change the number of lines comprizing a picture ! What the camera specifications are describing is the number of picture elements (or vertical Lines) that could be distinguished on any given horizontal line. Obviously, the more vertical lines that can be displayed, the higher the quality of the picture that could be produced by that camera.
That leads to the first reality of CCTV design. It is vital that all parts of a CCTV system have similar Lines capabilities, since the quality of the overall system will be limited by the component that has the lowest Lines capability in the system ! Of course some components may have very high Lines capability, but this does not matter as long as the cost is not raised higher than necessary. Since some parts of a CCTV system are specified in terms of Lines and other equipment in terms of frequency response, the following table equates Lines to Frequency Response, so may help to resolve these concerns.
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LINES TRANSMITTED
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BAND WIDTH REQUIRED
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330
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3.1 MHz
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380
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3.6 MHz
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400
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3.7 MHz
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460
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4.3 MHz
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470
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4.4 MHz
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480
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4.5 MHz
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570
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5.3 MHz
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TRANSMISSION MEDIA
Having chosen the optimum combination of Cameras, Monitors, Recorders, and othe associated equipment, the next concern is how to connect these various pieces of equipment together. For really long distances Satellite transmission, Microwave, and Fiber Optic systems come to mind, but these lie outside the scope of this article. For our purposes, we will be concerned with Coaxial Cable and Unshielded Twisted Pair (UTP) cables that may be as long as 5000 feet or so. Since all coaxial cables used for CCTV systems have a characteristic impedance of 75 Ohms, then all Cameras, Monitors, Recorders, etc. also have terminal impedances of 75 Ohms. Another aspect of coaxial cable is that it is an unbalanced transmission medium with a central wire surrounded by insulation and, usually a conductive jacket of woven shielding wires.
By contrast, UTP cables are a balanced transmission system. consisting of two insulated wires (lightly) twisted together and bundled with other such pairs into a cable. The characteristic impedance of this transmission system is about 105 Ohms, so must be coupled to CCTV terminal equipment with 75 to 105 Ohm unbalance-to-balanced conversion devices, usually called Baluns. Passive Baluns are usually a form of impedance converting wide band transformer, while electronic equipment meant to interface with UTP cables usually do not require seperate Baluns, but are equipped to connect directly with UTP cable. Both the coaxial and UTP cable loose energy as they get longer. In each case, the higher the frequency (or Lines), the greater the loss at any given distance. This means not only that the longer the cable the weaker the signal, but even more so, the fuzzier the picture becomes. Just how much loss of detail is involved in cable of various lengths?
One reasonable measure of picture definition loss is when more than ½ of the energy is lost at the frequency associated with those “Lines”. Assuming that the Camera is capable of generating the requisite “Lines” and the Recorder can reproduce those “Lines” and the Monitor can display those “Lines” then the limiting factor for quality transmission becomes the loss incurred in the Coaxial Cable or UTP transmission facilities. The tables below show the “Lines” loss at various distances for unequalized Coaxial Cable and Twisted Pair transmission facilities.
LINES COAX LENGTH UTP LENGTH 330 686 feet 289 feet 400 615 feet 259 feet 470 585 feet 246 feet 570 570 feet 198 feet
BOTH PICTURE QUALITY AND GREATER DISTANCE
If the Coaxial Cable loss is corrected by a GB60 or GB464 Receiving Amplifier / Equalizers, the cable length can be extended up to 2500 feet for any “Lines” quality level up to 570 “Lines” and thus completely eliminate all losses due to the cable facility. The picture quality at the end of 2500 feet of coaxial cable will be exactly as could be observed directly at the Camera Output once the cable has been equalized by the GB60 or GB464 Receiving Amplifier / Equalizer.
If UTP cable pair loss is corrected by a GB60-UTP or GB464-UTP Receiving Amplifier / Equalizer, the Twisted Pairs can be extended up to 3000 feet at full quality level up to 570 Lines and thus completely eliminate all losses due to the twisted pair facilities. The picture quality as seen at the end of 3000 feet of equalized cable will be exactly as could be observed directly at the Camera output once the cable pairs have been equalized by the GB60-UTP or GB464-UTP Receiving Amplifier / Equalizer.
Even longer UTP cable pairs may be traversed by pre-equalizing the UTP cable facility at the Camera location using the TPS-2000 Twisted Pair active Video Sender. This Video Sender can pre-equalize zero feet, 1000 feet, or 2000 feet (selectable) of UTP cable and when used with the GB60-UTP or GB464-UTP Receiving Amplifier / Equalizer can fully equalize UTP cables up to 3000 feet long.
Further extension of fully equalized Twisted Pair facilities are possible by placing a TPT-4000 Twisted Pair Repeater / Tap at the cable mid-point. In this way UTP pairs can be extended from 5000 feet (using the TPS2000 Sending Equalizer and GB464-UTP Receiving Equalizer), to 5000 feet using the TPT-4000 Twisted Pair Repeater / Tap. The TPT-4000 Repeater also provides a 75 Ohm tap to observe the video signal at that location for active secondary monitoring or as a simple test point.
The spacing between any two locations (i.e. Camera to Tap or Tap to Terminal) may be any distance up to 5000 feet by adjusting the input and output equalizers accordingly. By adjusting the amplifiers and equalizers while measuring the Sync Pulse and Color Burst to 40 IRE Units at each location where the video response must be “flat”, a perfect system alignment can be assured.
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