CCTV kamerák

A typical CCTV camera consists of:

• lens that collects the light coming from the scene (emitted or reflected from the objects),
• image sensor, which converts the light into electrical signal,
• signal processor that processes and optimizes the signal,
• power control circuit that adapts supply voltage to the requirements of the camera circuitry.

Image sensor is the heart of each camera. The array, composed of pixels, converts the incident light to an electric signal. Each pixel generates an electric charge proportional to the light intensity. The electric charge is collected and processed into an electrical signal that can be used for imaging purposes.

The original array provides no information on the color of the light, only about the light intensity. To obtain a color image, it is necessary to use special color filters placed on the array. Each pixel is covered with red, green, or blue filter, so it is effectively illuminated only by the light of the corresponding wavelengths. The camera electronics interpolates the resulting color of each pixel based on information from several neighboring pixels.

The main factor for good image quality is technology - the type, precision and features of the image sensor. Digital image processors and automatic control systems, although important, can adjust the signal only within certain limits.

CCTV cameras are currently equipped with image sensors using one of the following technologies:

• CCD (Charged Coupled Device),
• CMOS (Complementary Metal Oxide Semiconductor)

CCD sensors and CMOS sensors differ in the way they provide the data from the arrays. A CCD circuitry uses only one amplifier and A/D converter, whereas in a CMOS chip each pixel includes individual amplifier and A/D converter. This means more electronics near the photo diodes, resulting in lower efficiency i.e. less sensitive image sensor. However, CMOS technology is much cheaper and widely used, so there is a continuous development in this field.

Currently, most image processing systems are based on digital signal processors (DSPs). The image sensor and signal processor can be compared to the eye and brain of the camera. The signal processor collects and modifies the signal generated by the sensor. The algorithms are stored in its memory, and can be activated by the user as selectable settings or functions of the camera, intended for various environmental conditions.

Sony Effio is a modern platform providing images with high quality and resolution. It consists of a series of Super HAD CCD and EXview CCD sensors and Effio processors (Enhanced Features and Fine Image Processor).

There are three versions of Effio DSPs: Effio-E , Effio-S and Effio-P.

Effio-E (Entry level version) can cooperate with CCD image sensors having (horizontally) from 510 to as much as 960 pixels (the horizontal video resolution of some cameras with this DSP can exceed 650 TV lines). The DSP performs advanced operations including image analysis and image modifications, depending on the selected functions (e.g. motion detection, privacy masking, noise reduction, HLC, ATR).

Effio-S DSP has been enriched with:

• 3D noise reduction - comparison of consecutive frames with separation of color and B/W information - more precise noise reduction with less motion blurring artifacts,
• E-Zoom - digital zoom without losing image quality,
• Sense-Up function (digital slow shutter speed multiplying sensitivity in extremely low-light),
• Digital Image Stabilization (DIS),
• more advanced ATR (Adaptive Tone Reproduction) which digitally improves image quality of scenes with very strong differences in light intensity

Effio-P is the most professional signal processor of the series. In relation to Effio-S, it supports the most advanced image sensors that scan the image twice (with different exposure parameters). This feature is called "true" WDR (Wide Dynamic Range) and effectively reduces overexposure.

The table below shows the differences between digital signal processors of Effio series:

Geometric size of the sensor, measured in inches. There are cameras with 1", 2/3", 1/2", 1/3", 1/4" and 1/6" sensors. The most popular size is 1/3". Generally, the larger sensor the higher sensitivity (within the same category).

The size of the image sensor determines the minimum size of the lens. In general, larger sensors can provide higher image quality. This is due to the possibility of implementing more pixels. At the same time, many applications require miniaturization of the camera. Currently, even 1/4" sensors can provide satisfactory results.

Sensitivity

This parameter determines the ability of a camera to generate an image in low light and is usually given in lux (lx).
The sensitivity of a camera is the minimum value of the light intensity that is necessary to produce a specified amplitude of the composite video signal with a defined value of the signal to noise ratio.
This parameter of cameras is often unreliable. Most manufacturers do not specify the conditions under which they have measured it. It is important whether the sensitivity of a camera has been measured with switched-off automatic gain control (AGC). The necessary information should include the place of measurement of the light intensity - at the image sensor or on the object (in the latter case the necessary details are also the F-number of the lens and reflectance of the object's surface).

The resolution of a camera specifies the ability to distinguish fine detail on the screen. It is given in TV lines (TVL) for analog cameras and in pixels for IP cameras.In terms of resolution analog cameras can be divided into:

• low-resolution cameras: below 480 TVL,
• standard resolution cameras: 480 ... 600 TVL,
• cameras with increased resolution: over 600 TVL.

This parameter informs how many times the power of the signal is higher than the power of the noise generated as a background by the camera electronics.
The measurement of the S/N value has to be preformed without any functions modifying the output signal (AGC, BLC, HLC, etc).

The noise generated by a camera cannot be totally eliminated, only minimized. Its level depends on the quality of the sensor and electronics, particularly on the amount of heat generated by the components. A lower operating temperature means less noise. For example, some cameras used in astronomy operate below -50^oC.

S/N ratio higher by 3 dB means a reduction of noise by half. So, comparing cameras with S/N ratios of 48 dB and 51 dB, one can notice a marked difference in image quality, especially visible in low light conditions.

Wide Dynamic Range (WDR)

The dynamic range refers to the ability of the sensor to simultaneously capture details in the brightest and darkest areas of the scene. It is defined as the ratio of voltage corresponding to the brightest point to the lowest voltage corresponding to the darkest point. "Standard" CCD sensors can reach the ratio of 1000:1, which corresponds to 60 dB.

Image sensors with a wide dynamic range can have much higher ratios, up to 120 dB, which is 1000 times more than in the case of conventional sensors. It significantly improves the visibility of details in difficult lighting conditions, especially in the brightest and darkest areas of the image.
Such sensors are most useful when the light comes from the back of a scene (e.g. against a window), is very variable (e.g. by a door) or very different across a scene.

WDR is implemented in two ways:

One of them is used in Pixim sensors - each pixel is in fact a separate A/D converter capable of adjustment to the actual lighting conditions in this point.

The second one consists in double scanning of the sensor with diametrically different exposure parameters. The resultant image is a combination of the two frames. This is the principle of WDR system in cameras based on Effio-P DSP.

For each camera the manufacturer determines the maximum range of temperature changes within which it can properly operate with the rated parameters. In the case of compact cameras the operating temperature range is usually -20^oC to 45^oC. Various cameras may employ special means for maintaining permissible operating conditions, such as sealed enclosures, heaters, fans, cooling systems.

Each camera requires its rated supplying voltage, with given tolerance, and consumes the rated current.
NOTICE: At the startup of a CCTV system it is necessary to measure voltage a each camera, with full load of the system (switched-on heaters and IR illuminators), which is very important due to the fact that the auxiliary equipment can consume 2/3 of the total power supply.
The symptoms of low voltage include loss of color, a broad horizontal bar moving vertically on the screen, sync problems, or even no operation of the camera at all. Unstable power supply may significantly reduce the lifetime of cameras. Excessive load of the power source may also trigger overload protection (switch-off of the system after some time).
More on power supply for CCTV systems

Analog cameras have BNC video outputs with 75 ohm impedance to be matched with the same impedance of the transmission medium (coaxial cables). The maximum transmission distance depends on the parameters of the cable (100-800 meters). In the case of copper cable TRISET-113 E1015_500 the maximum distance is about 600 meters.
More on video transmission in CCTV systems

EAI or EI is, simply saying, automatic adjustment of sensitivity of the camera to illumination conditions. EAI is useful in environments with little illumination change (e.g. rooms). The advantage of electronic iris is opportunity of using simple lenses with fixed or manually set iris.

Automatic Iris (AUTO IRIS - AI) is the function of a camera allowing to control special type of lenses; it enables to match fixed proportion of light falling on image sensors, no matter the illumination conditions are. Electronic shutter is set to 1/50s, however lens with AI is being closed and opened adequately to illumination intensity. The camera and lens with AI are able to work properly even in environments with large illumination changes, where simple cameras are useless.

Cameras with AI function are equipped with special output controlling AI lens. Depending on the output signal, the lens will close or open the iris, stabilizing intensity of light coming to the sensor. It also helps to keep good contrast in whole observation area.

Control of lens with AI can be performed in two ways:

• Video - IRIS: control by the signal proportional to illumination. Camera provides signal that controls lens motor amplifier, which opens or closes the iris.
Lens with Video-AI has two potentiometers:
• Level: used to set brightness level that should be kept,
• ALC: enabling to set reaction time to illumination changes.
• Reaction mode can be changed from the option marked as PK or P (Peak) to value A or AV (Average); in first case the reaction is adequate to the brightest pixel in the screen, in second case - to average level of the whole screen. Brightness level is being changed from value marked as H (High) to L (Low).
• DC IRIS: lens controlled by direct current signal. Camera has signal output directly controlling the motor of DC-IRIS lenses, which opens and closes the Iris.
In the case of DC-AI lens, both potentiometers are inside the camera. As DC lenses are cheaper, it is worth to check in advance whether the camera has the controller enabling to use them.

AES (Automatic Electronic Shutter) - opening time is set depending on intensity of light falling on image sensor; the change occurs in the range from 1.50s to 1/100000s. Sometimes, in more technologically advanced cameras, we can find manual shutter adjustment. It should be remembered that longer exposition, although increases camera's sensitivity, decreases refresh rate of the image - time lapse effect appears (with long exposition times).

Other chosen types of automatic control are:

Reduction of the noise resulting from a low light level, useful in the case of static scenes.

3D DNR is an innovative development of the popular DNR function. 3D filter cuts the noise caused by low light, which improves S/N ratio and effective resolution of the image. This provides a fairly clear picture in very low light conditions.
3D DNR compares subsequent image frames. If, as a result of this analysis, it finds a difference between the images resulting from noise, it removes the very small grains, replacing them with average values.
Additionally, the function can significantly reduce the amount of the data that is recorded on the hard disk of the DVR. This reduction can reach up to 70%.

Automatic Gain Control is implemented in many CCTV cameras but only more advanced models enable the user to adjust its level. This adjustment allows to set optimum sensitivity in difficult light conditions. The user should realize that higher gain increases noise as well. Optimum AGC level is usually determined by trial and error approach.

This is a function that influences representation by the camera all the colors, in reference to "white" color. In automatic mode, the reference point for white is the brightest point in the image. Because the camera often monitors an area where the brightest point isn't really "white", the colors in the image can be far from those the human eye considers "real". The solution to the problem is to preset colors corresponding to natural or artificial lighting, as well as to save camera settings for a "white" pattern.

This function is the ability of a camera to compensate the brightness of the subjects with a large amount of background light that would make it practically impossible to see any details of the subjects. Backlight compensation consists in adjusting the gain of the camera to improve exposure of the subjects that are in front of a bright light source. It allows to identify the foreground. Unfortunately, the background also becomes even brighter.

This function is very useful in locations exposed to vibrations (for example from passing cars or wind), especially in the case of using lenses with long focal length. The image stabilization is the result of suitable processing of consecutive frames. The algorithm detects the "global" movement that is caused by the motion of the camera and significantly reduces the blur in the image.

The sensitivity of a CCTV camera can be significantly enhanced by slow shutter speed. Cameras with Sense-Up (or Digital Slow Shutter - DSS) function can be used in very low light, which is especially useful when operating at night, for example, at 0.25 lx (light intensity of the full moon in a cloudless sky).
The proper performance of that function depends on the kind of environment and the slow-down factor - e.g. x4 will allow for four times more light than normally gets into the camera. However, with fast moving objects close to the camera, it is not recommended to set this parameter above x8 - it would effect in a blurred image. The highest values (there are cameras with the maximum factor of x512) can be used only for static scenes.
So the Sense-Up function cannot eliminate the need for cameras operating with infrared illuminators. In total darkness or in the case of fast-moving objects they are indispensable.

Sensitivity in infrared range. Typical B/W camera is sensitive to radiation in the range visible to human eye (400 - 770 nm) and also radiation from IR (infrared) range (770 - 850 nm). However, the sensitivity in IR scope is lower, therefore the use of cameras for IR observation is limited.

Conventional color cameras are not capable of operation with IR illumination, due to the fact that they are normally equipped with IR-cut filters necessary for good color reproduction in daylight.

There are also color cameras that automatically switch to B/W mode after the dusk. Switch to monochromatic mode doesn't mean that they will properly work in IR (they still have built-in IR filter).

When we want to use color cameras for night observations, then we have to use Day/ Night type of cameras. These cameras don't have IR filter "removing" this range of light, as in standard color cameras. For example, the camera n-cam 115 M11234 can also work at night (in total darkness - no visible light) with the use of built-in IR illuminator. This construction of camera (lack of IR filter) has though some disadvantages. In normal light (visible light range) such camera has sometimes problems with white balance - it doesn't convey true colors, e.g. black color is commonly "seen" as navy blue etc. The effect is more visible with outdoor work where natural sunlight (as we know - including IR radiation) is dominant. This behavior of a camera may be treated by some installers as improper work of the device. Indoors, where other kinds of light are used (bulbs, fluorescent lamps) this effect is not so visible.

An ideal solution is provided by cameras in which IR filter is automatically removed (system based on servomechanism etc.) - there are no problems with color. However, as we can expect, this kind of cameras may be several times more expensive than these described above. They also require using high quality lenses with impoved focus for IR light.
There is yet another solution - two cameras in single housing - B/W and color. Adequate controller turns on the right camera, depending on the lighting.

Some manufacturers offer dual cameras, with color and B/W modules and appropriate controller switching on one of the camera modules depending on the ambient light level.

Wireless or wired camera. Wireless cameras have built-in transmitters working in 900, 1200, 1500, 2400 MHz ranges, or sometimes at different frequencies. Generally the use of wireless cameras is limited only to non-professional applications. Basic disadvantage of this solution is sensitivity to environmental conditions, mainly electromagnetic noises and interferences. It should also be remembered that for the reason of limited bandwidth used by wireless cameras, there can work only limited number of such cameras in one system. In many countries this kind of operation may require adequate license or authorization.

We describe this matter in details in the chapter: Video transmission..

Grounding problems. Mistakes in leading the zero voltage rail may cause significant quality loss, e.g. appearance of strips coming from the hum of the power network. The zero rails should be linked together in devices like video changers. They shouldn't be connected only via the cameras. It is also very important to separate zero power rail and the zero signal leads, i.e. to avoid using single zero voltage wire.

To eliminate different types of problems connected with power supplying it is well to employ the SV-1000 M1711 separator. It is meant to be additional security device in CCTV installations - for cameras and other devices, protecting them from atmospheric discharges, however it also eliminates problems of potential difference between devices and secures them from currents running through zero signal rail.

Another subject is grounding of metal housings, which is obligatory if camera is powered with AC 230V. For the reason of differences in potential, it is not acceptable to connect zero signal lead with camera housing, therefore it is suggested to use housings allowing galvanic separation of zero signal conductor (including camera's own chassis) from external housing.

Lens mounts

Lens mounts. The most popular types used are C and CS. Basic difference between them is different distance from lens to sensor surface. Most of cameras and lenses are equipped with CS mount which is more universal - using additional adapter ring it is possible to mount C-type devices as well. Reverse adaptation is not possible.

Having problems with setting focus in the lens, the user should adjust the position of the ring to set the distance between the image sensor and the lens (the picture on the left).

Hermetic housings - norms. The IPxy signs define resistance of camera housings to outer environmental conditions. The x - is the number defining mechanical integrity, and y - resistance to humidity. Housings with good enough dust and humidity resistance have labels IP65 or IP66. Totally water-resistant housings have labels IP67 or IP68. Most of compact types of camera housings are not waterproof.