 FAQs
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As with everything, CCTV can be as simple or as complex as the occasion demands. The significant difference between CCTV and intruder or access control systems is that different degrees of skill, experience and training are required from designers and installation engineers. Another factor is that fault finding requires particular experience on the part of service engineers. This is not to denigrate the essential skills of alarm engineers, but it is a fact that their training is not directly transferable to the needs of CCTV installation or servicing in anything other than basic systems.
This article looks at the needs of the more complex
industrial and commercial systems. There are, of course, plenty of do-it-yourself CCTV systems on the market that provide
excellent quality and performance.
As with all debates
there are two points of view, in this case the expectations of the customer and the capability of the installing
company. Either can be very high or very low depending on their previous experience. Let two examples illustrate this from
the author's direct experience. Neither problem needed any great knowledge of CCTV theory to resolve. Mainly the application
of common sense and interpretation of manufacturer's specifications.
During a visit to a site demanding a very high level of security, the views on two CCTV monitors were
noticed as being appalling, with almost no definition. This was not commented on at the time because the visit was for a totally
different reason. Later, however, a comment was made and the customer stated that the installation was technically very difficult
and the installing company had said that this was the best that could be achieved under the arduous conditions that prevailed
at this site. Engineers had returned on many occasions and could not improve the picture quality. The expectation of this
customer was, therefore, that with all the technical resources of this large installation company, he was obtaining the optimum
performance. An inspection of the site was made and the conditions were indeed demanding. It was discovered that the cameras
were up to one mile away from the control, and underground, the illumination being by means of infra red lights. The cameras
were connected back to the control by a very large coaxial cable. Furthermore, at the control the video signal was fed to
a video motion detection system, and then on to five separate monitors.
A series of tests was suggested to which the customer agreed. It was intended to replace the coaxial
cable with a 'twisted pair' but site conditions precluded this possibility. A video launch amplifier, designed to send a video
signal down a coaxial cable, was fitted at the camera end and a line corrector at the control end. This resulted in a dramatic
improvement in picture quality and, for the first time, the video motion detection system worked correctly. An additional
light was installed at the camera end to increase the levels and allow the lens to operate at a smaller aperture, which increased
the depth of field. There were still problems with the remote monitors until it was discovered that they were all terminated
at 75 ohms. Simply setting all except the last monitor to 'high z' cured this problem.
This was a classic example of a poorly designed and badly installed system. The
tragedy was that the customer thought that it was the best that current technology could produce. He may well have been put
off CCTV as a practible proposition for future applications.
The
second example was when I was asked to look at a system that had been designed by a salesperson, apparently based on a cursory
knowledge obtained from a distributor's catalogue. The manufacturers of the installed equipment produce most impressive brochures
and sets of data sheets, and the products are well regarded in the industry.
The system comprised four cameras around the outside of an industrial building, connected to a quad
screen splitter and a time lapse video recorder. One of the cameras was also connected to a video motion detector covering
the main entrance.
The customer's expectation
of the system was that, if a vehicle entered the yard outside normal working hours, the system would go into alarm. The identity
of the vehicle and any other activity around the building would be recorded. Here was the first flaw in communication; the
customer's interpretation of 'identity of the vehicle' meant reading the number plate, but he also wanted to see the entrance
and where the vehicle went.
There was not much of the system
that functioned at all. The video motion detector was sometimes set off by passing traffic, sometimes not; sometimes by vehicles
entering the yard, sometimes not. The front camera was fitted with a wide angle lens to view the whole yard and had absolutely
no chance of reading number plates. In addition, it was mounted about two metres up a wall, facing east over open country.
The quad screen splitter was set to record a full
picture on alarm, so the other cameras were not being recorded. Even if the splitter was not set in this mode and recorded
in quad screen, the pictures would be too small to be of use.
The time lapse video recorder was set in 24 hour mode and switched to real time on alarm. The problem was
that, with only a few incidents over- night, reviewing the tape was a tedious procedure (assuming the system had worked as
intended).
In principle, the system seemed
reasonable on paper and the customer would have no basis for doubting what was presented to him. However, the individual items
were not correctly selected. The person who designed the system had obviously come across various pieces of equipment through
exhibitions and advertising, but had no depth of understanding of the specifications or what questions to ask of the manufacturer.
Also, it was apparent that the system had been designed down to a price under pressure from the customer. It is a sad indictment
of parts of the industry that sales people try to find the cheapest solution rather than concentrate on finding the right
solution. Even sadder is the probability that many do not have the training to know the right solution. The following changes
were made to produce an acceptable working system.
The
video motion detector initially installed was a type only designed for indoor use, or limited outdoor use in fairly constant
lighting. A phone call to the manufacturer confirmed this. The unit was replaced with a budget version of a more sophisticated
video motion detector with a range of more sensitive set-up parameters. This functioned as expected.
The lens on the front camera was changed to one with a longer focal
length so that it only viewed the area that would catch vehicle number plates. Also, it was moved higher up the wall and looked
down at the area to avoid the problem of looking directly at the rising sun.
The camera viewing along the front of the building was swung around to view the general area of the
entrance.
The quad screen splitter was replaced with a
low cost multiplexer so that all cameras were continuously recorded when alarmed. This enabled any camera to be viewed individually
in full screen, or all four at once for general analysis.
The
time lapse video recorder was replaced with an event recorder that only recorded in real time when an alarm was active. This
provided rapid reviewing of the previous night's, or even previous week's, events without the need to plough through hours
of inactivity.
The result was a system that
achieved the expectation of the customer, although at considerable unnecessary cost to the installer. The cost of the final
equipment installed was not a great increase on the original and would certainly not have lost the company the contract.
Whilst there are companies that offer a competent and professional
service, there are many more that do not have the depth of technical knowledge to design larger systems that produce acceptable
results. In the case of CCTV in particular, there are many factors that must be taken into account in designing an effective
system. In some instances there may be restrictions caused by the Laws of Physics of which the customer should be appraised.
Some of these factors are listed below.
· There is a limited depth of field in focus in low light conditions when the lens iris is fully open; it is also a function of the focal length and the object distance. The depth of field decreases when the 'f' number is smaller (the aperture is larger); the focal length is longer; the object distance is shorter.
· There is a shift in the area of view in focus between daylight and illumination by infra red light. It may be necessary to accept a compromise between day and night focus with a fixed focal length lens, or to decide which is the most important.
· The image should remain in focus throughout the range of a zoom lens. If the focus has to be adjusted every time the zoom is altered it indicates that the back (mechanical) focus adjustment of the camera is incorrect.
· The limiting resolution of a system may be a video tape recorder. Irrespective of the quality of the cameras, this is generally from 240-400 TV lines for monochrome systems and 240-330 lines for colour.
· A type of video recorder known as S-VHS (super VHS) is available and often quoted as producing 500 lines resolution (for colour only). This is not always practical for use with industrial video systems because the input to the recorder must be made up of separate luminance and chrominance (Y/C) signals. Apart from the need to run two separate coaxial cables to every camera there are few switchers or multiplexers on the market to accommodate this type of input. Having said this there are new products appearing on the market that overcome this problem. As always, check carefully with the manufacturer before using them based on the advertising.
· Manufacturers' specifications for cameras can be very confusing to the uninformed. Many installers simply look at the factor called' sensitivity' expressed in Lux. It is necessary, however, to look further; is this at fl.2, fl.4? Is it at 79%, 100% reflectivity? Is it with AGC on or off? Is it with a 'gamma' of 1 or.45? Is it' usable' video or full video? Unless all these factors are taken into account, a true comparison of specifications cannot be made. There can be a three times difference between cameras of similar actual specification. Are you really getting what you are paying for? If in doubt seek professional advice.
In conclusion, therefore, these are just a few of the factors that must be taken into account when designing a CCTV system; it is not like a 'point and shoot' camcorder. It is important that the customer is made aware of the limitations as well as the advantages of the system being proposed. In particular, the view as seen on the monitor should be defined accurately or demonstrated. There is no lens in the world that can emulate the human eye, yet we all live every day with the most incredible and adaptable lens and tend to imagine that a modern camera can reproduce similar views. We see in three dimensions with continuously adjusting focus but a camera sees only a flat plane with restricted depth of field. The question of what the customer thought would be seen compared with the actual view obtained probably causes more debate and ill feeling than any other aspect of CCTV
The final selection of lens is frequently a compromise from the ideal and should be discussed with this in mind. In this way the expectations of the customer will always be achieved and a satisfied customer pays the bill.
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