The equipment we handle in CCTV is at the cutting edge of electronics technology, and as such often requires specialist skills and equipment to diagnose and repair faults when they occur. And yet there is a surprising amount that the front line CCTV service engineer can do in the area of fault rectification in equipment, and in this article we shall look at some of the more common fault conditions and the associated rectification methods.
The three common operating voltages for CCTV cameras are 12 Vdc, 24 Vac and 230 Vac. There are a number of factors which govern the choice of operating voltage; 230 Vac is favourable for external installations where the higher potential offers efficient heater and P/T motor operation. However, if the installing engineer is not qualified to perform the necessary inspection and testing of the electrical installations, then the additional cost of an electrical subcontractor may make it difficult to offer competitive quotes.
This is one of the reasons why 24 Vac – which falls into the extra low voltage (ELV) category and is thus exempt from many of the tests required by low voltage (LV) installations – has become very popular in small to medium size installations. Not only is it simpler to install than 230 V systems, but the problems of volt drop are not as acute as with 12 Vdc, and the ac nature of the supply permits the use of the line lock synchronisation facility offered by many cameras.
Fig 2 shows a typical 24 Vac power supply capable of supplying up to 16 cameras.
The common problems experienced with power supplies are those of ground loops, induced noise and voltage drop. The subject of ground loops was covered in detail in the April 2002 edition of Security Installer, so we shall move straight to the subject of induced noise.
All CCTV installations are prone to the effects of RFI and EMI (radio frequency interference / electromagnetic interference) and we go to great lengths to eradicate the problems by using co-axial, twisted pair, or fibre-optic cables to carry the video signal. However interference can enter via the supply voltage cables and, therefore, the installer should take steps to ensure that the same precautions are afforded to the installation of ELV supply cables as would be taken when installing intruder alarm system cables. In other words, avoid long parallel runs of ELV cable in close proximity to LV cables – especially those feeding industrial machinery, fluorescent lighting circuits, etc. Also avoid installing cables in close proximity to any equipment that is likely to generate interference such as electric motors, industrial light fittings, welding equipment, etc (See Fig 1).
Where noise or interference is evident on one or more pictures in a system employing ELV supply cables (either ac or dc), it is often worthwhile eliminating the possibility of supply borne interference by temporarily feeding one offending camera from a local supply source – perhaps even a 12 V battery.
Inspecting mains earth
Noise ingression via a 230 V low voltage supply is more difficult to prove (and eradicate!). We looked at the issue of checking for mains borne noise using an oscilloscope in the March 2002 edition of Security Installer.
Where mains borne noise has been proven, the cause may be a poor earth path and therefore a full inspection of the mains earth is suggested as there may be other safety issues lurking in the background. Where the earth is proven to be satisfactory the only other solution is to consider including mains conditioning equipment in the supply, or looking for a different mains source. The option you choose will often be determined by cost.
Voltage drop is a well known phenomenon in security system installations and we shall only give it a passing mention here. Where an ELV power supply is located some distance from a camera, check that the supply voltage at the camera is well within the operating limits of the camera once all cameras on the line are functioning. Intermittent fault symptoms can result in ELV systems where a camera supply is fluctuating around the lowest operating point; and fluctuations will occur as iris, zoom and P/T motors cut in and out.
Power supply units themselves are fairly straightforward and are generally reliable. 24 Vac units tend to employ a torroidal transformer which offers maximum efficiency for minimum physical size. With ac and dc units, one or more outputs may be available and these are normally protected against overload by 20mm quick blow fuses, although more expensive versions may employ thermal fuses with auto reset following removal of an overload condition Where glass fuses are employed, a blown output fuse would normally indicate a short circuit or overload condition on the line, however bear in mind that if the fuse has not ruptured violently then it may simply be a case of failure of the fuse wire due to deterioration with age.
Camera tests
Unlike some items in a CCTV system, there is little that can be done with cameras from a servicing point of view without having both specialised training and a considerable amount of (expensive) service equipment. However there are a few tests that should be performed before returning a camera for service or, in the case of a less expensive model, throwing it away.
Where the fault symptom is simply ‘no picture’, it is worth checking that power is actually getting to the camera. I know that this is stating the obvious, but it is amazing how many engineers forget to do this. Also check that the iris is not fully closed. This can occur because of faulty wiring to the lens, or a defective iris mechanism. There are two methods of checking for a closed iris; you can either look into the lens and see if the iris veins are open, or you may remove the lens from the camera. Removing the lens should cause the camera to produce a lighter output from the monitor screen, although there will be no actual image.
Some cameras contain 20mm fuses and, in the case of a completely dead camera, it may be worth checking the condition of these. Beyond this there is nothing else that can be checked internally in a CCTV camera in the field.
Camera settings
CCTV cameras offer a lot of features, however it is important during installation to ensure that these are set up correctly if optimum performance is to be achieved.
The common settings found in cameras are gamma, gain and AGC (Automatic Gain Control), BLC (Back Light Compensation), iris type, and synchronisation method. The first three of these in general affect the overall picture quality whilst the latter two, if incorrectly set, can have adverse effects on the system performance.
Gamma correction circuits are employed in all video cameras. It is a deliberate distortion of the luminance output waveshape introduced to compensate for the non-linear characteristics of the phosphor in the CRT. If that seems a bit of a mouthful, take a look at the explanation in Fig 3.
Switching the gamma on and off on a CCTV camera appears to simply alter the contrast level, however you will note from Fig 3 that the effect is non-linear and, with the gamma turned off, the CRT will produce greater contrast changes at the lower and higher brightness levels, leaving the mid-grey levels somewhat bland. As a general rule the gamma correction should be turned on, offering a linear contrast range. However you may experience cases where the CRT phosphor characteristic does not match that of the gamma correction in the camera and the contrast range appears too stark at the high and low levels. In such cases a more linear appearance may be obtained with the gamma correction turned off. The point to remember is, don’t simply use the gamma correction setting as an alternative gain control! Which leads us to our next point – AGC.
Gaining control
The gain control is in a literal sense a contrast control because it affects the peak to peak level of the luma output signal by altering the gain of the processing amplifier inside the camera. Automatic gain control is a little more complex because it constantly adjusts the gain in an attempt to maintain a constant output level irrespective of input (light) level variations.
The danger of applying too much gain in a situation where the light levels are quite high is that the video output signal can become too high (much greater than the standard 1 Vpp). This can result in clipping of the sync pulses and subsequent vertical bounce and/or horizontal pulling. Conversely, too little gain can result in poor contrast and an overall grey picture.
Where the lighting conditions are reasonably stable it is generally a good idea to switch off the AGC and set the gain manually, leaving the iris to deal with minor lighting variations. However in adverse lighting conditions AGC is essential if a reasonably stable output is to be maintained.
The most noticeable effect of operating at high gain is under very low lighting levels where a background grain is clearly visible over the picture. Under these conditions the low output from the CCD image chip results in a very poor S/N ratio (signal to noise) and thus the high gain of the following amplifiers causes a boosting of the noise (grain) as well as picture information. There is little that can be done to overcome this problem from our point of view, although digital processing circuits capable of cleaning up the similar effect caused by poor aerial signal levels in domestic TV receivers have been available for many years, so perhaps manufacturers could consider employing the same technology in CCTV cameras (hint!).
Over exposure
Back light compensation is a very useful feature, if used correctly. The installer must bear in mind that with this switched on the iris will open up to the extent where the bright areas caused by windows or doors will be over-exposed and will contain no useful picture information. This is fine as long as you are only interested in the subject entering into the area, however if monitoring of the whole area is required, other solutions must be sought such as the use of a second camera facing in a different direction to provide overall area monitoring.
This is not the forum to go into detail about the differences between EI, DD, Video Drive and AI iris types, however it must be pointed out that a camera will not perform correctly (if at all) if these settings are incorrect. Perhaps the most common problem is where the electronic iris is left switched on when an auto iris lens has been fitted. This produces an effect known as hunting where, following a rapid and large change in light level, both circuits react. If, for example, the light level were to suddenly increase, the EI would normally react first and thus, when the mechanical iris closes a moment later, the light input becomes too low. This causes the EI to open once again, followed quickly be the mechanical iris, so the light level is once again too high. This cycle repeats a number of times until the iris circuits stabilise.
The condition described above results in a pulsating brightness level on large light transients, however EI and AI conflicts can produce other interesting (and sometime confusing) effects such as vertical banding across the picture.
Synchronisation
A camera will offer up to three methods of sync pulse generation; internal, line lock (LL) and gen lock. Gen lock, where a master sync signal is fed to each camera via a separate co-axial cable is not favoured for CCTV applications because of the extra cable costs and, in any case, there are other viable options. The line lock facility (where the vertical sync pulses are referred to the 50Hz mains supply) is obviously not available on 12 Vdc cameras.
Synchronisation between cameras becomes less of a problem as more manufacturers produce switching and multiplexing equipment with digital video processing which includes methods of re-timing the video input signals such that they all appear at the output in sync.
Nevertheless, there is still a lot of the older equipment out there which requires the cameras themselves to be synchronised to one another, and when a camera is replaced it is important that the service engineer sets up the sync accordingly, otherwise the customer will more than likely be complaining of a picture jump that was not evident on the old camera.
Where the LL facility is being used, it is more than likely that the phase control will require adjustment when a new camera is first installed. This may be done through trial and error, but this method may lead to a number of call-backs to the site before everything is performing satisfactorily. A better method is to align the vertical sync period of each camera to a ‘master’ camera (generally camera number 1) using either an oscilloscope or a phase meter. The method is illustrated in Figs 4 and 5.
