Common misconceptions about the performance of individual parts of the system
Light levels: A salesperson was heard to say recently, "that's a great camera it's got a good lux level and it's cheap." This is not an indictment of the salesperson it is a criticism of the level of training provided. It is also not untypical of the jargon used by people desperate to create an impression of knowledge. The camera in question was in a distributors' catalogue and described as 'sensitivity .6 lux.
Resolution: 580 lines; 750(H) x 580(V); 435,000 pixels. What do they all mean? The most useful method of specifying resolution is by the number of lines in this case 580 horizontal. With the increasing number of CCD cameras it is becoming common to state the number of horizontal and vertical pixels, in this example 750 x 580. If this is the only value given it can easily be mistaken for resolution in lines and appears to be better than it is. The last value is the total number of pixels in the chip, impressive but no practical use. Apart that is from the salesperson who needs to impress,-- "their camera only has 580 lines, mine has 435,000 elements!"
An approximation to convert horizontal pixels to equivalent lines is TV LINES = PIXELS X 0.7. I.e. 750 pixels is approximately 525 lines.
Colour cameras generally have lower resolution than monochrome. Be careful not to be taken by the specification for some colour cameras that specify resolution as 450 lines Y/C. These can only be used in certain situations and the resolution can only be obtained when using a compatible monitor and associated control equipment. (See similar note under monitors).
Auto exposure control: Sometimes called electronic iris. This is a development in CCD cameras and electronically controls the amount of light reaching the CCD sensor. Several manufacturers claim that this eliminates the need for an automatic iris lens. Using manual iris lenses instead can make a significant saving on a system. If this type of camera is to be used from full daylight to dusk then use caution. Call the manufacturers' technical department, describe the application and go by their advice.
Sensor sizes: Early cameras had a circular tube as the sensor therefore the size was decided by the diameter of the tube, which is the diagonal measurement of the picture. This is still the case today so although CCD sensors are flat rectangular chips the nominal size is the diagonal measurement.
The sensor sizes shown in the diagram must be considered in relation to the lens selected. This is because lenses are also designed for a particular size of sensor. See further notes under lenses.
Note: It is a little known fact that the video output from a colour camera should be 1.2 volts peak-to-peak compared to 1.0 volts for a monochrome signal.
Lens functions: The CCTV lens performs two main functions. First it determines the scene that will be shown on the monitor, this is a function of the focal length. Second it controls the amount of light reaching the sensor, this is a function of the iris. The focal length may be fixed-- or variable as on a zoom lens. The iris may be manual-- or automatic and controlled by the camera.
Lens mountings: All CCTV lenses are based on what is known as the 'C' mount. This is a photographic standard that specifies the thread type and dimensions. There is now a generation of lens mounts for CCTV known as the 'CS' mount. The first point to make is that the thread type and its dimensions are identical for both types of lenses. Therefore, either type of lens may be screwed to cameras having either type of mount without causing any damage. The wrong combination will be impossible to focus but there is no apparent mechanical indication to an installer that the wrong combination has been used.
The difference between the two types is the optical distance from the back of the mounting flange to the face of the sensor. This is known as the 'flange back length.' In the case of the 'CS' lens this distance is shorter. This allows the use of smaller glass elements in the make up of the lens and fewer elements to be needed. The result is a lens that is more compact and cheaper to manufacture. The differences are shown in the diagram.
The lenses are not totally interchangeable. A 'CS' lens may only be used on a camera with a 'CS' format mounting. A 'C' mount lens may be used on a 'CS' mount camera by adding a 5mm-adapter ring.
Lens sizes: It can be very confusing to establish the actual field of view that will be obtained from a combination of sensor size and lens specification. Lenses are specified as being designed for a particular sensor size. A lens designed for one sensor size may be used on a smaller size but not the reverse. The reason is that the extremities of the scene will be outside the area of the sensor. Many people in the CCTV industry have grown up with the 2/3" camera as being the most popular and are familiar with the fields of view produced. However the 1/2" and 1/3" cameras are now being extensively used and therefore there are important factors that must be taken account.
The diagram shows the effect of using one lens on two different sizes of sensor. The result of using a larger lens format on a smaller camera format is to create the effect of a longer focal length, that is, a narrower angle of view.
To summarise then:
- A lens designed for one format may be used on a smaller format camera but will produce a narrower angle of view.
- A lens designed for one format may not be used on a larger format camera.
- Assuming a focal length has been assessed based on a particular format of camera and lens. It is then decided to use a smaller format camera. The same field of view will only be obtained if a shorter focal length lens is used.
- Always check the angle of view for the particular lens and camera combination it is intended to use.
Size: As with camera sensors the size of monitors is the diagonal measurement of the screen. The distance at which it is to be viewed generally decides the size of monitor. Typical figures in metres are:
Another consideration is for viewing multiscreen displays. A 15" monitor is normally the minimum for viewing a quad display. For multiplex displays that can show up to sixteen pictures then a 17" is the minimum with a 21" preferred.
Resolution: With monochrome monitors resolution is not generally the limiting factor. As always specifications need interpretation for instance most monitor resolution figures are given as number lines at the centre. A figure of 600 lines at the centre may only be 400 lines at the edges. The difference is likely to be greater the larger the monitor for two reasons. First the problem of maintaining accuracy of the scanning magnets over a larger area. Second, it is more difficult to produce larger monitors with as fine a coating as smaller monitors. This is why the picture on a 9" monitor always looks sharper than when seen on a 17" monitor.
The resolution of colour monitors is less than can be obtained with monochrome. This is because three spots are needed to make each point-- red, green and blue. Typical resolutions for colour monitors are from 280 lines to 350 lines. There is the same fall off towards the edges of the screen as with monochrome monitors. Some colour monitors are specified as 450 TVL Y/C. Take care because these monitors only produce this resolution when using cameras and control equipment that produce separate chrominance and luminance signals.
A simple video switcher is designed to direct the signal from one of a series of cameras to a monitor. Most switchers have a control to enable the monitor to sequence automatically through each camera in turn. The time between each sequence is generally adjustable. A switcher should be selected that incorporates what is known as 'vertical interval' switching. This delays the actual moment of switching until the blanking period of the sync pulse in the composite video signal. The result of this technique is to prevent picture 'bounce' between successive pictures. One picture simply replaces the previous one without any rolling caused by waiting for the next sync pulse. Some switchers can provide output to two monitors. One monitor can be locked on to a specific camera while the other sequences.
Matrix switchers are now becoming common place in the market due to the development of microprocessor technology. This type of switcher can process the signals from a large number of cameras to many monitors. There can also be many control positions, each of which can call up any of the cameras. In a railway system for instance it is possible to have two hundred stations each with twenty cameras. Each station would have individual control of its own cameras to sequence or select. All the stations would be connected back to a central location that could control all four thousand cameras. The central control could then be divided into say ten regions each with a control and bank of monitors for its own group of cameras.
Recording of CCTV cameras has had a fairly mixed press over the last few years. Obviously the failures to identify culprits and lousy pictures seen on programs like Crimewatch have not helped. There are though hundreds of systems incorporating video recording that have paid for themselves time and time over. The most successful systems are obviously in conditions of good consistent lighting using good quality colour cameras and a well-maintained video recorder. This then is the key to successful video recording, the right conditions, the correct equipment and proper maintenance of the system.
Until recently, one problem has been the limitation of a video recorder to provide only 240 lines resolution. This is a function of the maximum bandwidth that can be used on the standard width VHS tape. It does not really matter what quality of camera and monitor is used. The limiting resolution is that of the recorder. That is why the superb pictures seen on the screen during commissioning and handover of a system are not reproduced when an incident occurs. Reusing the same tape repeatedly frequently aggravates this. Also, by lack of maintenance on the recorder. The problems of poor video recordings could be dramatically reduced if customers insisted that the manufacturers' recommendations for maintenance and limits of tape use were strictly followed.
S-VHS: (Super VHS) There is a new generation of video recorders using the S-VHS system. This provides greater resolution of up to 500 lines but only in colour. In a composite video signal there are two elements that make up the colour information. They are known as the chrominance (C) and the luminance (Y). The chrominance is specific to colour signals and determines the colour content of the picture. The luminance is used in both monochrome and colour signals and determines the brightness. In S-VHS colour signals these two components are separated at the camera and transmitted as separate signals. Therefore point one- the camera has to provide separate Y/C components. This requires two coaxial cables to be run from each camera. The video recorder must be able to accept the separate Y/C signals as also does the monitor. The use of this improved quality is limited because at present there are no switchers or multiplexers that can pass the Y/C components. Note that there is no improvement with a monochrome signal.
There has been much written about digital recording recently and is not within the scope of this article to reiterate all the advantages and loopholes in specifying this type of recording. However, there are still tens of thousands of analogue recorders still in use and thousands still being installed. Analogue recorders are still the main recording medium for many small installations.
Weatherproof housings must be about the most mundane aspect of a CCTV installation. Or so it seems because many engineers simply consider the housing as a protection against the elements. However there are many aspects to consider and many suppliers of housings. It is about the cheapest element of an external system yet price seems to be the main factor in selecting which to use. Important considerations should be:
- Ease of access for pre-assembly in workshop.
- Ease of access during installation.
- Ease of access for future service needs.
- Is the camera mounting plate insulated from the case?
- Can the mechanical focusing screws on the camera be accessed? Some are at the back, some at the side and some on top.
- Can the lens be focused and the peak/average settings adjusted on site?
- Can one man remove the cover and work on the inside?
- If there is a telemetry board fitted can it be accessed without removing the camera?
The most common type of housing is that where the camera is mounted on a flat bed. A rectangular box shaped cover drops down over the complete assembly and is held in place by four spring clips. This is great for assembly in the workshop because everything is nicely accessible. The problem comes when a service engineer at the top of a ladder needs to work on it. Many engineers know that it needs four hands to hold the clips clear and two to remove the cover. Especially if it has been on for some time and the cover is welded to the rubber seal. Once the cover is off everything is exposed to the elements and it is no quick job to replace it.
Another type that was in vogue a few years ago and still around is the extruded aluminium design. The housing is a complete box and the camera plate slides out of the back in guides. The cable glands are usually in the rear sealing plate therefore sufficient slack must be allowed for the length of the plate to be slid all the way out. At this point the engineer is faced with the whole assembly and cable in his hands. It now needs two hands to hold it and two more to work on it. With the camera and lens on a loose platform and the cables hanging down it is really fun to focus the camera and set up the lens adjustments.
There is a variation on the first type mentioned but instead of using clips there is a pivot at the front. There are clips at the rear and when released the cover swings right up and forward. This exposes the complete interior and a stay rod holds up the cover. It's just like opening a rear-pivoting bonnet of a car. In another design there is a simple gas strut to hold the cover open. This needs one hand to open the cover and leaves two hands to work on the inside.
Another design is a box like housing with two latches on each side. When the latches on one side are released, the cover pivots open on the opposite side latches. The cover may be opened in either direction. If all four latches are released, the cover can be completely removed.
There are other designs around that may be just as engineer friendly. It is worth spending a little time on this often overlooked item to make future servicing easier and cheaper.
Finally there is the ubiquitous dome. There has been a proliferation of dome variants introduced recently probably more than any other development. In addition they are becoming ever smaller and faster. But they are not necessarily the panacea for all PTZs. The main fallibility of dome housings is in the material and manufacturer of the actual dome itself. Poorly moulded domes can lead to disastrous loss of focus, particularly at long focal lengths. It is always advisable to arrange a test and see the results for the longest focal lengths and distances. If you intend to purchase a dome and fit your own camera/lens make sure that there is sufficient clearance between the lens and the inside of the dome to allow focussing at long focal lengths.
Most domes now allow for quick release fixing of the pan tilt mechanism and plug/socket for the telemetry and video. This can be especially important when fitted at the top of a pole in a high street.
This chapter is supplied by Mike Constant and was originally published in CCTV Today. Mike is the author of 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.