From CCTV Information
The principal part of a CCTV system is the camera. There are many types of camera and many ways in which they are used. In this chapter, the different sorts of cameras and the fundamentals of their operation will be examined. It will also explain the terms describing the performance of the cameras. This will enable an understanding of the data sheets available for the myriad of cameras available on the market. There is now no standard method for manufacturers to present data defining camera performance. Therefore, their literature should be studied carefully before making a selection and comparisons made against a common standard.
Types Of Camera
Internal cameras are usually designated for use indoor without the need for environmental protection. Normally the cameras are simply fitted with a lens to view the required area and mounted on a wall or ceiling bracket. If the camera is in an area such as a corridor or other place where the light level doesn’t change, then a simple manual iris lens may be used. The light level may change because there are windows or skylights in the area being viewed. Alternatively, if twenty-four hour operation of the camera is needed then an automatic iris lens or another means of electronic sensitivity control must be used. (See electronic shutter cameras.)
Frequently the styling of an internal camera is important because an architect or similar person will want the camera to blend into the surrounding decor. In those cases, the camera may be mounted inside some kind of housing. There are many housings of different styles available, from simple cases through to domes, wedges and other types. Internal housings are also used for other reasons. It may be important that the camera is not seen at all, in which event a covert housing is used to hide the camera or disguise it as something else.
Housings may also be used to give a measure of protection in certain situations. There are many types of enclosures that can be used to protect the camera from vandalism, dust, or other contaminants.
External cameras are usually described for use in outdoor situations. They are nearly always housed in some form of weatherproof housing, an exception being where the camera case itself is water-resistant. The external camera housing normally contains a heater and thermostat to prevent the glass window at the front from misting at low temperatures.
External cameras always need some form of electronic sensitivity control. This is because, over the course of the day and night, the light level may well change by a factor of over a million times. At the time this book went to press the most effective way of giving such electronic sensitivity control is an automatic iris lens fitted with a neutral density spot filter. Chapters 4 and 14 provide more detailed information on lenses and lighting.
Electronic Shutter Cameras
There are an increasing number of cameras being introduced with ‘electronic shutters’; electronic devices that are controlled by the amount of light falling on the imaging device. In effect, it is the electronic equivalent of the variable speed mechanical shutter fitted to early cine cameras. In these, the amount of light was measured by a photoelectric cell, an increase in light causing the shutter to revolve faster and vice versa. The same problems apply to both devices. At very high light levels, there is a limit to the speed at which the shutter can effectively operate without the picture flaring. At very low light levels, the exposure time is so long that moving images become blurred. Some manufactures have claimed that these cameras eliminate the need for an automatic iris lens. This is doubtful in all conditions. They are ideal for indoor conditions where there is a limited range of light levels. As always, the manufacturer’s specification should be consulted carefully to check the light range covered. Another problem that should be appreciated is that because the iris is invariably set at the maximum aperture the depth of field is greatly reduced. See automatic light control and electronic shutter later in this chapter.
Since CCD cameras (see later in this chapter) have been available, the size of cameras has reduced considerably. These miniature cameras are available in a number of styles in two main groups, either where the camera is a complete unit or where the image sensor is separated from the camera electronics. Complete cameras are available at the present time with dimensions similar to the size, say, of a pack of cigarettes. If even smaller sizes are required, the cameras with separate sensor heads have sensor blocks of only 25mm cubed. One restriction to the minimum size of camera is due to the necessity of fitting a lens and mounting the camera. The ultimate is a camera of current design that is about the size of a thumbnail, including all the electronics.
Line Powered Cameras
Normally a CCTV camera has to have some kind of power source, either wired from a central point or from a local mains spur. Obviously there is a cost involved of providing the necessary cabling or supply points for such cameras. Some camera manufacturers have addressed this issue by making cameras to which the power for the camera is sent down the same coaxial cable used to bring the video signal back from the camera. CCTV systems using line-powered cameras, then, cost less to install in terms of supply cables or mains spurs. There are, however, two disadvantages. First, some cameras need a specialised power supply unit to feed the camera and separate the video for the monitor. Furthermore, with long cable runs it is not possible to amplify the video signal from the camera because the power cannot travel through the video amplifier. This is also a problem if there is ground loop interference on the camera as it is not possible to use a video isolation transformer with line powered cameras.
Board Mounted Cameras
Board mounted cameras are normally small CCD cameras mounted on the printed circuit board of another system. They are used to give a picture as part of the function of the system. The best example of board mounted cameras is those used in video entry phone systems. In these systems, a complete CCD camera with a lens is mounted on the PCB of the door entry unit. The board-mounted camera gives pictures to residents, on small dedicated monitor units, of the person operating the bell push.
Types of Image Sensors
The first CCTV cameras to be used were based around special vacuum tubes with a light sensitive coating on one end. Light striking this coating caused electric current to flow down the tube, proportional to the amount of light falling at each point on the coating. The circuits of the camera then converted the current to the video signal.
This was a good initial design and gave cameras that had good sensitivity and resolution. However the cameras were bulky and the tubes had a limited life span, requiring regular, expensive tube changes. CCD cameras, when introduced, were smaller, lighter and required practically no maintenance. This has led to their widespread replacement of tubed cameras in CCTV systems, where CCD cameras are now used in practically all new installations. For this reason, no further discussion of tubed cameras will be made in this book.
CCD is an abbreviation of Charge Coupled Device. This is the name given to a group of optical detector integrated circuits made from semiconductors (see diagram 3.1). A lens focuses light onto the surface of the CCD image sensor. The areas of light and dark are sensed by individual photo-diodes, which build up an electrical charge proportional to the light. That is to say that the brighter the light on an individual photo-diode the bigger the charge developed. These photo-diodes are arranged in a matrix of rows and columns and are given the name picture cells or Pixels. The charge is removed from each pixel by rows of CCD cells. These CCD rows are like ladders for charge, enabling step-by-step the charge on each pixel, and consequently the light level on it, to be read off by processing electronics.
When the first CCD cameras were developed, it was important that they could replace existing tube cameras without having to change lens sizes. Therefore, the first CCD cameras were created in 2/3” format. As CCD sensor technology has improved, the format of CCD cameras has decreased to 1/2 inch, 1/3 inch, and most recently to 1/4 inch and 1/8th inch to make cameras smaller and cheaper. The associated lenses are also much more compact, but not necessarily cheaper due to the much higher accuracy required to grind a smaller lens. The dimensions of the imaging devices are shown in Chapter 4.
An amplifier is needed to boost the signal from the CCD sensor electronics up to the level where it can be used on a monitor. A synchronising generator is also used in the CCD camera to generate the signals that read the light level charge off the CCD and the synchronisation pulses used by the video monitor to re-create the image. The mixer section combines the video and synchronisation signals to produce the composite video signal used by the monitor.
There are many advantages of CCD cameras that have led to their wide spread replacement of tubed cameras. First, CCD cameras use less power and need no high voltages like the tube. As mentioned in the section on miniature cameras, CCD cameras can be very much smaller than tubed cameras. The picture linearity is better with CCD cameras as tubed cameras used a magnetic field to scan the image sensor. It is extremely difficult to make a magnetic field that is completely even over a given area. This meant that the pictures from tubed cameras were sometimes distorted by the magnetic field, bulging out at the edges (barrelling) in bulging in (pin-cushioning). CCD cameras do not use magnetic fields and consequently do not have this geometric distortion.
CCD cameras are also a good deal more rugged than tube cameras. Viewing the sun or another bright point could easily damage the surface of the tube and the tubes regularly needed replacement as a routine maintenance task. CCD cameras do not have this problem and are not damaged by high light intensities, nor do images become burned into the surface over long periods. This, and the ability of CCD cameras to survive vibration and mechanical shock, gives very much reduced maintenance cost for CCD cameras.
Colour CCD Cameras
Colour CCD cameras are basically the same as monochrome cameras. However, there are additional components that have important effects on the performance of the camera
Light passes through the lens and through a colour correction filter on to the CCD. The CCD is sensitive to infrared light, which is present in normal daylight. This infrared light produces false signals from the CCD that affects the purity of the colours reproduced by the camera. The colour correction filter removes the infrared light before it hits the CCD and ensures the colour purity of the camera. However, it also means that infrared illuminators cannot be used with normal colour cameras as the colour correction filter removes all the lighting created.
The actual CCD image sensor comprises of an array of pixels like a monochrome camera. However, each pixel is subdivided in to three smaller light sensitive areas that are constructed to be sensitive to red, green and blue light respectively. Consequently the pixels are larger in size than for monochrome CCDs and the number of pixels which can be fitted on to a colour CCD of a given size is less than a monochrome CCD of equal dimension. This is why, generally, monochrome cameras still have resolution which is higher than colour cameras. The colour correction filter and colour sensitivity of the pixels also tend to make colour cameras less sensitive to light that monochrome cameras. Typically, colour cameras have sensitivities between 1 lux and 2.5 lux whereas monochrome cameras have sensitivities between 0.01 lux and 0.1 lux.
The separate brightness signals for red, green and blue are amplified separately and the used by signal processing circuits to produce the luminance (Y) signal (by combination as described in chapter 2) and the chrominance (C) signal (by phase and amplitude modulation of the 4.434MHz colour sub-carrier as described in chapter 2). The Y and C signals are then combine with the composite sync pulses to produce a composite colour video signal. Many colour cameras also feature a separate connector where the Y and C signals are output separately for connection to Super VHS video recorders and monitors, for improved resolution.
Two coaxial cables must be installed between the camera and the S-VHS video recorder. The Y-C output of the recorder must be connected to the Y-C input of the monitor. This is normally achieved using a pre-made S-VHS cable with mini-DIN connectors on each end. However, the benefit of investing in this cabling plus an S-VHS recorder and high-resolution colour monitor (400 TVL at centre) will be noticeably better live and playback pictures in terms of resolution. Resolution of typically 400 TVL will be possible when viewing live action pictures (compared with about 350 TVL using the composite video output of the camera). Resolution of typically 400TVL will be possible when viewing pictures recorded on theS-VHS video recorder (compared with about 240 TVL compared with a standard VHS recorder). The down side is the cost. An S-VHS system like this may cost twice as much as a standard VHS system using composite video.
Advantages Of CCD Cameras
- No geometric distortion.
- No coils, magnets, or glass tube.
- Not prone to ghosting or image burn.
- More compact and resistant to vibration.
- Not affected to electromagnetic interference.
Initially CCD cameras could not provide the same degree of resolution compared to tubed cameras. The dynamic range was less and produced fewer shades of grey. However, improvements in CCD sensor design have meant that the current generation of CCD cameras produces excellent images of high resolution and accurate colour reproduction.
Digital Signal Processing (DSP) CCD Cameras
In conventional CCD cameras the functions of amplification, signal processing and mixing are carried out by analogue circuits, which work on changing the voltages of the signals by various means. Adjustments to picture quality are made by small adjustable resistors which are set up to give the best overall performance across a range of camera operating conditions (light levels etc.) This approach is very cost effective and gives good quality pictures in most lighting conditions However, these adjustments are, at best, a compromise and the effects of tolerances in the values of the electronic components and changes over the lifetime of the camera can cause the quality of pictures obtained from the camera to vary greatly.
In DSP cameras digital circuits, as shown in figure 3.5, carry out the signal processing and mixing.
The signals from the CCD are connected to an analogue to digital converter (ADC). This converts the brightness level from each point into a number. In this way, the entire picture captured by the CCD at any moment is represented by a group of numbers. These numbers are processed at high speed by the digital signal processor, which does mathematics on the numbers in order to produce the video signal at the output of the camera. The digital signal processor gives the other name used for digital cameras, DSP.
The composite video signal or Y-C video signal is produced by a digital to analogue converter (DAC) which takes the finished information from the digital signal processor and produces the composite video described in chapter 2. Most DSP cameras still produce these analogue composite video and Y-C signals as this is currently the most popular format required by the other equipment in the video system; monitors, switchers, multiplexers, VCR’s etc. DSP cameras do have the possibility to produce the video signal in a digital form and it is likely that this will become popular when a worldwide standard is agreed for sending video pictures digitally in CCTV systems.
A microprocessor controller sets the settings of the camera, controlled by the DSP circuits. This is a small computer built in to the camera, which controls the mathematics used by the DSP circuits to build the video signal. The controls of the camera are usually a series of push buttons on the camera, which are scanned by the controller. With these buttons the user can select and adjust the picture quality and performance of the camera using a series of menus overlaid on to the video picture by the controller. Obviously, the extra circuitry required by a DSP camera make them more expensive than a conventional analogue camera. However, there are a number of benefits for this extra expenditure in terms of features that are not available from conventional analogue cameras. These include:
- Stability - the adjustments to the camera are made by changing number values on an on-screen menu and not by small screwdriver adjustments. Consequently the settings of the camera are easily repeatable and tend not to change over time.
- Menu programming - provides an easy and rapid way to adjust the camera for the best picture during installation.
- Digital zoom - The DSP circuits have a complete numerical model of each picture and can manipulate these numbers. By performing certain calculations, the DSP circuits can selectively enlarge a section of the picture, producing a zoomed-in image. This is a useful feature but it should be borne in mind that the number of pixels in the CCD is constant and so the greater the amount of digital zoom used, the poorer the apparent resolution of the picture will be.
- Multi-zone backlight compensation - Unlike analogue cameras, which compensate for bright light behind an object by sampling the video voltage across the whole picture, DSP cameras and have a number of separate zones which can be positioned to cover bright light sources. Consequently, this provides better overall picture quality in these situations.
- Automatic quality adjustment - DSP cameras can hold a model of how a good quality video signal should appear. The DSP circuits can then compare this with the picture being produced at any moment, and then actively adjust the camera to provide the optimum picture quality. This can give very good picture quality over a very wide range of lighting conditions.
- Remote set-up and control - like any computer, the microprocessor controller can communicate with other computers over a digital link. Consequently, DSP cameras can be used in systems where they are set up and controlled by a matrix switcher or a PC, even over great distances. This also simplifies camera replacement in the field as when a camera becomes faulty the replacement fitted can have identical settings downloaded very quickly to give identical performance to the original camera.
Twin Colour/Monochrome Cameras
Twin colour/monochrome cameras, are designed to meet a particular requirement in CCTV systems. Sometimes, it is required to have outdoor cameras which produce colour images in the day but which can provide good quality pictures in low light levels at night, perhaps even using infrared illuminators. In the past, the only way to meet this requirement was to use two separate cameras; one monochrome, one colour that were switched over automatically by some type of photocell or control system.
Improvements in CCD technology and the introduction of DSP cameras have led to the availability of colour cameras which produce monochrome pictures at night and which have good sensitivity to infrared illumination. The cameras work as normal colour cameras during the day. The night-time mode is controlled either by the camera itself (by sampling the AGC voltage, see AGC below) or remotely by a control input. In the nighttime mode, the colour sub-carrier is switched off and the camera produces just the monochrome composite video signal.
Dual format cameras do have to overcome the problem of the infrared cut filter. Colour cameras normally have an ifrared cut filter that removes infrared light and ensures accurate colour reproduction by the camera. However, dual format cameras cannot use the colour correction filter at night because this would filter out the light produced by infrared illuminators. Camera manufacturers have solved this problem in two ways. One way is to have small motor that moves a colour correction filter in front of the CCD in colour mode but retracts it in monochrome mode. This has the advantage of ensuring the best colour quality but has the disadvantage that a complex electro-mechanical assembly is built in to the camera and this will lower it’s reliability compared with a camera that has no moving parts. The other solution is to dispense with the colour correction filter entirely. The effect of infrared light is then adjusted by the digital signal processing of the camera. This gives a camera, which is very reliable, but the colour reproduction of the camera will always be a compromise as the amount of infra red light seen by the camera constantly changes and the compensation in the digital signal processing is fixed.
There are already several camcorders on the market that produce a digital output instead of an analogue video signal. These record onto a miniature DAT (Digital AudioTape) in digital form or download straight to codecs. The playback can be either via a digital to analogue converter in to a conventional monitor, or direct by RGB input to a computer monitor. The direct input into a computer monitor will provide a significant improvement in resolution and colour rendering. The recording capability for CCTV is still limited by the current problems of compression and storage capacity, but this is advancing rapidly and soon will not be the main problem. Imagine computer graphic type resolution and quality in a CCTV installation, the day will come.
The majority of advances in CCTV cameras have been as a result of developments in camera technology and miniaturisation in the vast domestic market. There is no reason to doubt that the digital camera technology will soon be available to our industry, although not at the time of publication of this issue. However, it makes sense to propose some of the advantages of this technology when it becomes readily available.
Transmission of video along telephone lines or fibre optic cable requires an analogue to digital converter (ADC) to be incorporated in the transmitter and the reverse digital to analogue converter (DAC) at the receiving end. Using a direct digital output from the camera will render the ADC unnecessary, thus saving cost. When equipment is available that can accept a digital signal then the DAC will not be required providing further savings. It will no longer to use coaxial cable with all its problems of connectors and limited range. Instead, simple twisted pair cables can be used with greatly improved distances and quality.
Multiplexers need to convert the analogue signal to a digital signal to hold in the frame store; again, this will be unnecessary.
Every time a conversion from one form of signal to another is rendered unnecessary, there will be an improvement in resolution and picture quality.
This article is an extract from chapter 3 of The 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.