WHAT IS DIGITAL VIDEO RECORDING (DVR)?

DVR is an equipment which digitally records multiple video signals generated by Closed-Circuit Television (CCTV) devices.  Images are recorded into Hard Disk Drive.  DVR is based on personal computer (PC) architecture.  Consequently, advantages of PC based DVR follows:

WHAT FEATURES SHOULD ONE LOOK FOR IN A CCTV DVR?

All DVRs are definitely not made equal!  There are several factors that are critical to consider when purchasing a DVR, especially when comparing price.  The most important factors to look at are the number of cameras supported, frames per second (fps), hard drive space, network connection/remote viewing capability, motion detection, scheduling, and ability to burn video and most probably audio to a CD.

You need to be especially careful when purchasing embedded DVRs or Stand-alone DVRs (i.e., Digital Multiplexer Recorders, DMRs).  Some do not have even basic features such as remote viewing and CD burning and some have very little hard drive space.

The DVR that will work best for your application will depend on several factors including the number of cameras that you will have and the frames per second that you need.  When determining the number of camera inputs, it is important to consider future needs as well as current needs.

The frames per second (fps) relates to how many pictures it will record in a second.  Real time recording is about 30fps on each camera.  To calculate the fps per camera, take the total fps in the system and divide it by the number of video inputs. 

For example, a 60fps digital video recorder with 4 video inputs would result in about 15fps per camera.  Real time recording is 30fps.  The technology has finally gotten to the point now where real time recording is affordable.  If you are recording cash registers or something similar then you should invest in real time recording.  If not, then a lesser recording speed should suffice.  You can still see a clear picture even though it will have a little hesitation or jerkiness on playback.

The amount of hard drive space is very important because it will limit how many days of recording you can store before the system has to start recording over the oldest video.  The general rule of thumb is that each camera will use about 2 to 3 gigabytes (Gb) of hard drive space a day.  Real Time and Embedded DVRs tend to use about twice as much as that per day.  So, for example a PC-Based 4 camera 60fps system with 120Gb of hard drive space will use about 8 to 12Gb total per day, giving you from 10 to 15 days of recording before it needs to start writing over the old video.  DVRs with recording upon activation of motion detection will use about 0.5 to 1Gb of hard drive space a day.  Statistics shows that with 120fps recording with motion detection set at about 70% sensitivity, 16 cameras on relatively average busy areas could fully utilize an 80Gb hard drive space in 22 days before it starts writing over the old video.

Some features are going to affect the amount of hard drive space used.  One important feature is scheduling.  There may be certain cameras that you only want to record during working or daylight hours.  The scheduling feature is what you need to set this up.  Another important feature is motion detection.  If you can set up your recorder to only record when motion is detected will significantly extend your recording life.

Having a CD burner built into the machine is a very important feature because if a problem is detected you need to be able to save it on a CD for others to be able to see it.  If there is no CD burner on the machine then make sure you have network ability so you can connect to it from another machine so you can burn video that way.

Other features you should look for are the ability to view the cameras remotely, easy and comprehensive search capabilities.  The user interface should be easy to operate.  PC-based systems come standard with these options.

WHAT IS THE DIFFERENCE BETWEEN A PC-BASED DVR AND AN EMBEDDED DVR?

A PC-based Digital Video Recorder is basically a personal computer that has been modified with hardware and software to work as a DVR.  An Embedded Digital Video Recorder is a machine that has been manufactured specifically to work as a DVR.  In embedded DVRs, there is typically one circuit board with simple software burned into the chip.  There are advantages and disadvantages to each type of DVR.

The advantages of an embedded digital video recorder is that they are extremely stable and reliable since they contain fewer parts.  The software is often written in basic machine code or Linux code which tends to be more stable than Windows software.  Also the picture that you get on the monitor usually looks better (especially when viewed full screen) than PC-based DVRs because there is less compression.  The disadvantages are they have less options.  Most do not have remote viewing capability.  They generally have slower recording rates.  Sometimes embedded DVRs do not have CD burner so the only way to get video out of the machine is to copy it via the LAN (if it has the connectivity) to another computer or to hook up a VCR to it.  Since they generally have less compression they use more hard drive space so you can fit fewer days of recorded video on it.  And you do not have as many options to upgrade the hard drive space as the PC-based systems.

The advantages of the PC-based digital video recorders is that you have many more features and options available on the units.  For example, some of the options you get on the PC-based machines that you don't get on the embedded is the ability to set many options like motion detection and you can completely control the PC-based DVR remotely.  The software is easily upgradeable when new enhancements are made.  You interact with the software via mouse or an optional keyboard so its much more easier to use (with the embedded systems you set them up with buttons like in a VCR).  A CD ROM burner is included so storing video off the unit is easy.  Compression is usually better so it uses less hard drive space and you can customize how much hard drive space you want on the unit.

HOW DOES A CCTV DIGITAL VIDEO RECORDER (DVR) WORK?

A CCTV Digital Video Recorder (or DVR for short) is essentially a computer that saves security video images to a hard drive.  Most security cameras in use today capture an analog picture.  The DVR converts the analog signal to digital and then compresses it.  Digital compression results in a much better picture than analog compression and its more efficient.

Many cameras can be connected to one DVR.  DVRs generally comes with 4, 8, 16, 32 camera inputs.  The DVR will allow you to view all of these images at once or one at a time, and all of the video is saved to the hard drive.  Additional switches, quads, or multiplexors are not required.

HOW DO YOU SEE PICTURES FROM A REMOTE SITE?

You can view the camera video over the internet using a modem which is slow but can display 1 or 2 frames every 5 seconds.  Better is a Digital Subscriber Line (DSL) or cable modem connection which can generally display a frame per second.  When viewing remotely, the refresh rate is restricted by the communications medium (the network connection speed).  When viewing or playing back locally, the display is dependent of the unit's frame rate (fps).

WHAT ARE THE ADVANTAGES OF MPEG4 OVER MJPEG?

The newer lines of network cameras are using MPEG4 protocol over MJPEG to broadcast it's Audio/Video data.  This is because the compression quality of MJPEG is so poor.  A 16 camera system using MJPEG would need about a Terabyte of hard drive space to store only 5 days of video.  Also, the remote transmission rate of MJPEG is very slow for the same reason.  The same 16 camera system using MPEG4 could store about  20 days of recording on a Terabyte.  And the transmission rate of the MPEG4 video is fantastic

WHAT ARE DISK-ON-MODULE (FLASH DISKS OR FLASH MODULES)?

Disk-On-Module is a flash memory based storage device which emulates a standard magnetic hard disk.  It is a very low power device suitable for use in portable and embedded systems limited by space.  Industrial variants are available with extended temperature range and shock and vibration resistance.  No special software or firmware needs to be installed.

Disk-On-Module products are 100% Integrated Data Electronics (IDE) - Compatible Flash Disks that can be integrated onto any Single Board Computer.  These are designed to replace conventional low capacity disk drives or M-Systems SSD Disk-On-Chip Flash Modules.  They are suitable for industrial applications and harsh environments, since they are more durable than conventional hard disk drives.  No Drivers, No BIOS, No Worries!

On special application PC-based Digital Video Recorders (DVRs), these Disk-On-Modules are used to store Linux code and other DVR application software to be loaded onto the personal computer that has been modified with hardware to work as a DVR.  Results are: 300% faster than Disk-On-Chip; MTBF of 1,000,000 hours (i.e., 114 years); shock and vibration-resistant; and has an access time of less than 0.1 millisecond as compared to the hard disk storage's average access time of 8.5 milliseconds.  FASTER LOADING OF LINUX CODE AND OTHER DVR APPLICATION SOFTWARES UPON POWER ON OR RESTART!

WHAT IS CCTV?

CCTV stands for Closed-Circuit Television.  It is a television system intended for only a limited number of viewers, as opposed to broadcast TV. 

WHAT IS A CCTV CAMERA?

A CCTV camera is a unit containing an imaging device that produces a video signal in the basic bandwidth.

WHAT IS A CCTV INSTALLATION?

It is a CCTV system, or an associated group of systems, together with all necessary hardware, auxiliary lighting, etc., located at the protected site.

WHAT IS A CCTV SYSTEM?

An arrangement comprising of a camera and lens with all ancillary equipment required for the surveillance of a specific protected area.

IS CCVE DIFFERENT FROM CCTV?

CCVE stands for Closed-Circuit Video Equipment.  It is an alternative acronym from CCTV.

WHAT IS A CCD?

A CCD (Charge-Coupled Device) is a semiconductor used in video cameras, digital still cameras and cameras built into cellular phones.  The CCD's role is to transform light captured by a camera into electric signals. Anywhere from a few hundred thousand to a few hundred million light sensors are contained in approximately one square centimeter of the CCD chip. The CCD is a device that records images by transforming light falling on light sensors into an electric charge and converting the charge into a voltage change. The CCD is an electronic eye, so to speak.

Sony has accumulated a wealth of knowledge and expertise in advanced technology with regard to imaging products since the tremendous success they had with the "passport sized handycam, TR55" which was launched in 1988.

The technology supporting Sony's high picture quality is the fundamental design of the CCD.  Design features include the HAD sensor, the on-chip microlens and technologies such as the Super HAD CCD, SIL, DIL and other structures which they have developed in order to achieve improved light sensitivity.

WHAT IS SUPER HAD CCD?

The HAD (Hall Accumulation Diode) sensor is a photo sensor originally developed by Sony.  This sensor resulted in higher light sensitivity, reduced noise, afterimage reductions and other improvements.

The on-chip microlens is a technology for collecting more light. When a CCD* processes an image, the larger the number of pixels used by CCD the clearer the image will be. (Pixels on the CCD correspond to dots of light in the final image). On the other hand, the smaller the area per pixel, the less light that can be collected. This results in a decrease in light sensitivity. In order to pursue both an increase in the number of CCD pixels and reduction in overall CCD size, a small lens is put on each pixel allowing each pixel to collect more light.

The Super HAD CCD improves sensitivity exponentially by modifying the above mentioned on-chip microlens. The Super HAD CCD structure was develop to collect more light. As well as minimizing the ineffective field (the area between pixels that does not collect light) on a microlens, the shape of the on-chip microlens was also optimized. Thanks to this technology, even though the area per pixel has been reduced, the sensitivity per unit area has improved.

WHAT IS A PINHOLE LENS?

Pinhole lens is a fixed focal length lens, for viewing through a very small aperture, used in discrete surveillance situations.  The lens normally has no focusing control but offers a choice of iris functions.

WHAT IS A COAXIAL CABLE?

The coaxial cable is the most common type of cable used for copper transmission of video signals.  It has a coaxial cross-section, where the center core is the signal conductor, while the outer shield protects it from external electromagnetic interference.

RG-59:  A type of coaxial cable that is most common in use in small to medium-size CCTV systems. It is designed with an impedance of 75-ohm. It has an outer diameter of around 6 mm and it is a good compromise between maximum distances achievable (up to 300 m for monochrome signal and 250 m for color) and good transmission.

RG-11:  A video coaxial cable with 75-ohm impedance and much thicker diameter than the popular RG-59 (of approximately 12 mm). With RG-11, much longer distances can be achieved (at least twice the RG-59) but it is more expensive and harder to work with.

RG-6:  A video coaxial cable with 75-ohm impedance which allows the connection of CCTV cameras beyond 150 meter cable-length (maximum of 300 meters).  It has an attenuation (loss) of 13.1dB/meter as compared to RG-59's 29.5dB/meter.

WHAT ARE BROADBAND COAXIAL SYSTEMS?

Broadband coaxial systems are communication systems that use broadband networking techniques on coaxial cable.  The 300 megahertz (MHz) bandwidth of a coaxial cable is divided into multiple channels through frequency division multiplexing.  The channels can transmit signals at different data rates, allowing diverse applications to share the cable by means of dedicated channels.  Channel bandwidth may range from a few kilohertz to several megahertz.  A single cable may carry both digital data and analog data (voice, television) simultaneously.  Access to the cable is provided through radio-frequency transceivers (modems) assigned to a particular channel.  Frequency-agile modems may be used to communicate on different bands at different times.

There are two classifications of broadband coaxial systems.  In a one-way system signals travel in only one direction in the cable.  This kind of system is common in cable TV (CATV) systems.  In a two-way system signals can travel in both directions on the cable.  All traffic that originates from network nodes travels on the inbound channels to the headend.  The headend is the origin of all traffic on the outbound channels, routing all messages on inbound channels to the proper outbound channel to reach their destination.  Network nodes transmit messages on inbound channels and receive messages on outbound channels.  Two-way systems fall into midsplit or subsplit categories.  Midsplit systems divide the cable bandwidth equally between inbound and outbound channels.  Subsplit systems put inbound traffic in the 5 - 30 MHz bands and outbound traffic in the 54 - 100 kHz bands.  This format is the easiest way to retrofit onto a one-way CATV (Community Antenna Television) system, and leaves the VHF TV channels on their normal "off-the-air" frequency assignments.

WHAT IS A BANDWIDTH?

Bandwidth is the complete range of frequencies over which a circuit or electronic system can function with minimal signal loss, usually measured to the point of less than 3 dB.  In PAL systems the bandwidth limits the maximum visible frequency to 5.5 MHz, in NTSC to 4.2 MHz.  The ITU 601 luminance channel sampling frequency of 13.5 MHz was chosen to permit faithful digital representation of the PAL and NTSC luminance bandwidths without aliasing.

WHAT IS A BASEBAND?

Baseband:  The frequency band occupied by the aggregate of the signals used to modulate a carrier before they combine with the carrier in the modulation process. In CCTV the majority of signals are in the baseband.

WHAT IS A TV FRAME AND HOW WOULD YOU COMPARE IT WITH A FIELD?

A frame refers to a composition of lines that make one TV frame.  In CCIR/ PAL TV system one frame is composed of 625 lines, while in EIA/NTSC TV system of 525 lines.  There are 25 frames/second in the CCIR/PAL and 30 in the EIA/NTSC TV system.

A field refers to one-half of the TV frame that is composed of either all odd or even lines.  In CCIR systems each field is composed of 625/2 = 312.5 lines, in EIA systems 525/2 = 262.5 lines. There are 50 fields/second in CCIR/PAL, and 60 in the EIA/NTSC TV system.

CCIR: Committee Consultatif International des Radiocommunique or, in English, Consultative Committee for International Radio, which is the European standardization body that has set the standards for television in Europe. It was initially monochrome; therefore, today the term CCIR is usually used to refer to monochrome cameras that are used in PAL countries.

CCIR 601:  An international standard (renamed ITU 601) for component digital television that was derived from the SMPTE RP1 25 and EBU 3246E standards. ITU 601 defines the sampling systems, matrix values and filter characteristics for Y, Cr, Cb and RGB component digital television. It establishes a 4:2:2 sampling scheme at 13.5 MHz for the luminance channel and 6.75 MHz for the chrominance channels with eight-bit digitizing for each channel. These sample frequencies were chosen because they work for both 525-line 60 Hz and 625-line 50 Hz component video systems. The term 4:2:2 refers to the ration of the number of luminance channel samples to the number of chrominance channel samples; for every four luminance samples, the chrominance channels are each sampled twice. The D1 digital videotape format conforms to ITU 601.

CCIR656: The international standard (renamed ITU 601) defining the electrical and mechanical interfaces for digital television equipment operating according to the ITU 601 standard. ITU 656 defines both the parallel and serial connector pinouts, as well as the blanking, sync and multiplexing schemes used in both parallel and serial interfaces.

WHAT IS A COMPOSITE VIDEO?

In CCTV, Composite Video is the basic electrical signal which starts at the camera and goes to the control room via a transmission system.  it has a maximum amplitude if 1 volt, peak to peak, and it is made up of the following parts:

Video Signal:  When light falls on a CCD chip, it generates a charge in the pixels, which is directly proportional to the light falling on them.  More light means a greater charge.  This charge is then read out from the CCD chip and is converted into a video signal. The methodology of reading this charge from the chip depends upon the type of CCD chip.  The greater the amount of light on the pixel, the larger the amplitude of the video signal. In a composite video, the maximum amplitude of the video signal is 0.7 volts. In other words, the white or the bright part of the picture will have a signal strength of 0.7 volts, while the black or dark parts will have a signal of 0 volts.

Vertical Sync Pulses:  A video picture is made up of video frames. In NTSC there are 30 frames per sec, while PAL has 25 frames per sec.  To avoid picture flickering in CCTV, this video frame is divided into 2 fields, that is, the odd and even fields.  These two fields are separated out at the camera point and then combined once again at the monitor end.  This is also called interlacing of fields.  At the end of each frame or field, a vertical sync pulse is added.  This sync pulse tells the electronic devices in the camera and other CCTV component that the field has come to an end and gets them ready to receive the next frame or field. The duration of the pulse depends upon the time the electronic devices take to receive the next field. The amplitude of this pulse is a 0.3 volts. This when added to the video signal, gives a total amplitude of 1 volt, peak to peak.

Horizontal Sync Pulse:  A video frame is made of lines.  In NTSC there are 525 lines per frame, while PAL has 625 lines per frames.  Each point in the line reflects the intensity of the video signal.  At the end of each line, a horizontal sync pulse is added. This sync pulse tells the electronic devices in the CCTV system that a line has come to an end and to get ready for the start of the next line. This also has a amplitude of 0.3 volts. 

The above is a quick overview of the components of a composite video. Below are some statistics and additional information about a video signal. 

WHAT ARE THE DIFFERENT FREQUENCIES UNDER THE PAL AND NTSC SYSTEM?

The following table details the different frequencies under the PAL and NTSC system:

                                                                NTSC                               PAL

       Frame Frequency                               30 per sec.                        25 per sec.

       Duration of each frame                        1/30 sec.                           1/25 sec.

       No. of fields per frame                         2                                       2

       Field frequency                                  60 per sec.                         50 per sec.

       Duration of each field                          1/60 sec.                           1/50 sec.

       No. of lines per frame                          525                                   625

       No. of  lines per field                           262.5                                312.5

       No. of lines per sec.                           525 X 30 = 15750               625 X 25 = 15625

       Duration of each line                          1/15750 sec or 63.5 us       1/15625 sec or 64 us

WHAT ARE HORIZONTAL AND VERTICAL BLANKING?

Retrace or fly-back is the time required to move from the end of one line to the start of the next line or from the end of one field to the start of the next field.  No picture information is scanned during the retrace and therefore must be blanked out.  In television blanking means " going to black level".  The retrace must be very rapid, since it is wasted time in terms of picture information.  The time needed for horizontal blanking is approximately 16% of each horizontal line.  The time for the vertical blanking is approximately 8% of the vertical field.

                                                               NTSC                                PAL

       Field duration                                    1/60 sec.                           1/50 sec.

       Vertical blanking                               1/60 * .08 = 1333 us           1/50* .08 = 1600 us

       Line loss due to vertical blanking        1333/63.5 = 21 lines           1600/64 = 25 lines

       Line duration                                     63.5 us                              64 us

       Horizontal blanking                            63.5 * .16=10.2 us              64 * .16=10.25 us

       Visible trace time                              53.3 us                              53.75 us

WHAT ARE HORIZONTAL AND VERTICAL SYNCHRONIZATION?

The blanking pulse puts the video signal at the black level, the synchronization pulse starts the actual retrace in scanning.  Each horizontal sync pulse is inserted in the video signal within the time of the horizontal blanking pulse and each vertical sync pulse is inserted in the video signal within the time of the vertical blanking time.  The following is the frequency of each synchronization pulse:

                                                               NTSC                                PAL

       Vertical                                             60 Hz                                50 Hz

       Horizontal                                         15750 Hz                           15625 Hz

WHAT IS A COLOR SIGNAL?

A color video signal is the same as monochrome except that the color information in the scene is also included, which is transmitted separately.  The following two signals are transmitted separately:

Luminance signal:  known as the Y signal, it contains the variations in the picture information as in a monochrome signal and is used to reproduce the picture in black and white.

Chrominance signal:  known as the C signal, it contains the color information. It is transmitted as the modulation on a sub carrier. The sub carrier frequency is 3.58 MHz for NTSC and 4.43 MHz for PAL.  In a color receiver, the chrominance signal is recovered and combined with the luminance signal to give a color picture.  In a monochrome receiver, the chrominance signal is not used and the picture is reproduced in black and white.

WHAT IS THE CONSTRUCTION OF THE COMPOSITE VIDEO SIGNAL?

The composite video has the following parts:

- Camera signal output corresponding to the variation of light in the scene

- The sync pulses to synchronize the scanning

- The blanking pulses to make the retrace invisible

- For color signals, the chrominance signal and color sync burst are added.

WHAT ARE CAMERAS?

Cameras are the starting point of the video signal and are therefore a critical component of a CCTV system.  The word camera comes from the Latin " camara obscura" and means "dark chamber".  Artists in the Middle Ages used a dark box to trace images.  Since then the camera has come a long way.   Today there are three types of cameras most commonly used.

1.  film cameras

2.  photographic cameras

3.  video cameras

The construction and type of Charge-Coupled Device (CCD) chip used in a camera is important.  Some of the better quality cameras have superior chip design incorporating many innovative features like On-Chip Lens (OCL), Back Light Compensation (BLC), excess charge drainage technology, etc.

WHAT IS THE IMPORTANCE OF CAMERA SPECIFICATIONS IN CHOOSING THE RIGHT TYPE FOR YOUR USE?

Any camera data sheet has a number of specifications shown like resolution, sensitivity, signal to noise ratio, camera voltage, chip type, and operating temperature.  Some data sheets are detailed, while others are quite sketchy and cover the bare minimum.  To classify a camera, most people will first look at the resolution and sensitivity in the data sheet.  These two specifications are the most important.  There are usually confusions surrounding these terms and we would like to explain them in simple terms.

Resolution - is the quality of definition and clarity of a picture and is defined in lines.  More lines equals higher resolution, equals better picture quality.  Resolution depends upon the number of pixels (picture elements) in the CCD chip.  If a camera manufacturer can put in more number of pixels in the same size CCD chip, that camera will have more resolution.  In other words the resolution is directly proportional to the number of pixels in the CCD chip.

In some data sheets, two type of resolutions, vertical and horizontal, are indicated as follows: 

Vertical resolution = no. of horizontal lines and it is limited by the number of horizontal scanning lines.  In PAL it is 625 lines and in NTSC it is 525 lines.  Using the Kell or aspect ratio factor the maximum vertical resolution is 0.7 of the number of horizontal scanning lines.  Using this the maximum vertical resolution is 625 x 0.7 = 470 lines for PAL and 525 x 0.7 = 393 lines for NTSC. 

Vertical resolution is not critical as most camera manufacturers achieve this figure.

Horizontal resolution = no. of vertical lines.  Theoretically, horizontal resolution can be increased infinitely, but the following two factors limit this:

-  It may not be technological possible to increase the number of pixels in a chip.

-  As the number of pixels increase in the chip, the pixel size reduces which affects the sensitivity.

There is a trade-off between resolution and sensitivity. If only one resolution is shown in the data sheet, it usually is the horizontal resolution.

HOW DO YOU MEASURE RESOLUTION?

There are different methods to measure resolution:

1.  Resolution Chart - The camera is focused on a resolution chart and the vertical lines and horizontal lines are measured on the monitor.  The resolution measurement is the point were the lines start merging and they can not be separated.

The resolution of the monitor must be higher than the camera.  This is not a problem with B/W monitors, but is a problem with most color monitors as they usually have a lower resolution as compared with a camera.

2.  Bandwidth method - This is a scientific method to measure the resolution.  The bandwidth of the video signal from the camera is measured on an oscilloscope.  Multiply this bandwidth by 80 to give the resolution of the camera.

Example. If the bandwidth is 5Mhz, the camera resolution will be 5 * 80 = 400 lines

Typical Resolutions of Cameras

                                              Monochrome cameras                Color Cameras

Low Resolution                        380 - 420 lines                           330 lines

High Resolution                       570 lines                                   470 lines

WHAT IS SENSITIVITY AND MINIMUM SCENE ILLUMINATION?

Sensitivity measured in foot-candles or lux indicates the minimum light level required to obtain an acceptable video picture. 

There is a great deal of confusion in the CCTV industry over this specification.  There are two definitions "sensitivity at faceplate" and "minimum scene illumination".

1.  Sensitivity at faceplate indicates the minimum light required at the CCD chip to get an acceptable video picture.  This looks good on paper, but in reality does not give any indication of the light required at the scene.

2.  Minimum scene illumination indicates the minimum light required at the scene to get an acceptable video picture.  Though the correct way to show this specification, it depends upon a number of variables.  Usually the variables used in the data sheet are never the same as in the field and therefore do not give a correct indication of the actual light required.  For example a camera indicating the minimum scene illumination is 0.1 lux.  Moonlight provides this light level, but when this camera is installed in moonlight, the picture quality is either poor or there is no picture.  Why does this happen?   It is because the field variables are not the same as those used in the data sheet.

How does it work?  Usually light falls on the subject.  A certain percentage is absorbed and the balance is reflected and this moves toward the lens in the camera.  Depending upon the Iris opening of the camera, a certain portion of the light falls on the CCD chip.  This light then generates a charge, which is converted into a voltage.  The following variables should be shown in the data sheet while indicating the minimum scene illumination.

1.  Reflectance

2.  F-Stop

3.  Usable Video

4.  AGC

5.  Shutter speed

Light from a light source falls on the subject.  Depending upon the surface reflectivity, a certain portion of this light is reflected back which moves towards the camera.  Below are a few examples of surface reflectivity:

Snow = 90% 

Grass = 40%

Brick = 25%

Black = 5%

Most camera manufacturers use an 89% or 75% (white surface) reflectance surface to define the minimum scene illumination.  If the actual scene you are watching has the same reflectance as in the data sheet, then there is no problem, but in most cases this is not true.  If you are watching a black car, only 5% of the light is reflected and therefore at least 15 times more light is required at the scene to give the same amount of reflected light.  To compensate for the mismatch, use the modification factor shown below.

Modification factor F1 = Rd/Ra

Rd = reflectance used in the data sheet

Ra = reflectance of the actual scene

The reflected light starts moving towards the camera.  The first device it meets is the lens, which has a certain Iris opening.  While specifying the minimum scene illumination, the data sheet usually specifies an F-Stop of F1.4 or F1.2. F-Stop gives an indication of the iris opening of the lens.  The larger the F-Stop value, the smaller the Iris opening and vice-versa.  If the lens being used at the scene does not have the same Iris opening, then the light required at the scene requires to be compensated for the mismatch in the Iris opening. 

Modification factor F2=- Fa² / Fd²

Fa = F-Stop of actual lens

Fd = F-Stop of lens used in data sheet.

After passing through the lens, the light reaches the CCD chip and generates a charge, which is proportional to the light falling on a pixel.  This charge is read out and converted into a video signal.  Usable video is the minimum video signal specified in the camera data sheet to generate an acceptable picture on the monitor.  It is usually measured as a percentage of the full video.

Example: 30% usable video = 30% of 0.7 volts (full video or maximum video amplitude)  = 0.2 volts.

The question here is:  Is this acceptable?  Unfortunately there is no standard definition for usable video in the industry and most manufacturers do not indicate their definition in the data sheet while measuring the minimum scene illumination.  It is recommended that you be aware of the useable video percentage used by the manufacturer while specifying the minimum scene illumination in the data sheet.  The minimum scene illumination should be modified if the useable video used in the data sheet is not acceptable. 

Modification Factor F3 = Au/Du, where Au = actual video required at the site as % of full video, and Du = usable video % used by the manufacturer

AGC stands for Automatic Gain Control.  As the light level reduces, the AGC switches on and the video signal gets a boost.  Unfortunately, the noise present also gets a boost.  However when the light levels are high, the AGC switches off automatically, because the boost could overload the pixels causing vertical streaking etc.

The data sheet should indicate if the AGC is On or Off while measuring minimum scene illumination.  If the data sheet indicates AGC is "on" yet, if in reality the AGC is "off" then the minimum scene illumination in the data sheet should be modified

Modification Factor F4 = Ad/Aa, where Ad = AGC position in the data sheet, and Aa = Actual AGC position.  If AGC off = 1, then AGC on = db figure from the data sheet.

These days, most cameras have an electronic shutter speed, which allows one to adjust the timing of the charge, read of the CCD chip.  The standard read out is 50 times (PAL) and 60 times (NTSC) per second.  If the shutter speed is increased to say 1000 times per second, that means the light required at the scene should be 20 times more (for PAL).  Increasing the shutter speed allows the picture to be crisper, but requires more light.  Use the following modification factor:

Modification Factor F5 = Sa/Sd, where Sd = Default shutter speed (PAL - 1/50 second; NTSC - 1/60 second), and Sa = Actual shutter speed being used

The minimum scene illumination of the camera must be adjusted because of the mismatch between the actual conditions in the field and the variables used in the data sheet.

Ma = (F1*F2*F3*F4*F5) * Md

Ma = adjusted minimum scene illumination

Md = minimum scene illumination as per the camera data sheet

Comparison:

Compare the actual light at the scene (L) with the adjusted minimum scene illumination (Ma).  If the light available is more than the adjusted minimum scene illumination, then the current camera can be used.  If the actual light at the scene is lower than the adjusted minimum scene illumination of the camera, then the camera setting may require adjustment or an alternative solution is necessary.  The following steps will help resolve the issue:

Step 1

Check if camera variables can be changed

- If AGC is switched off, then switch AGC on

- Accept a lower usable video %

- Reduce shutter speed, if possible

- Use a lens with a lower F-stop

- If no success go step 2

Step 2

- Find a more sensitive camera

- Down grade from color to B/W camera

- Add Infrared light if B/W camera is being used

- Add more lighting at the scene

Example

It maybe worth while to study an example so that all the above concepts can be understood correctly.  Let us assume that the camera is focussed on green grass (20% reflectivity).  The actual light level at the scene is 50 lux.  The color camera data sheet indicates the minimum scene illumination is 2.5 lux.  The table below compares the variables as indicated in the data sheet and also the actual situation in the field.

Parameter                               Data Sheet                Actual                        Factor

Reflectivity                               89%                         20%                           4.45

F Stop                                    1.2                            1.4                             1.36

Usable Video                           30%                         100%                          3.3

AGC                                       On                           On                              1

Shutter Speed                         1/50 second                    1/50 second                     1

Minimum Scene Illumination     2.5 lux                       ?        

Actual Light level                                                      50 lux        

Modified Minimum Scene Illumination = ( 4.45* 1.36 * 3.3 * 1 * 1) * 2.5 = 45 lux

This camera would work, as the light level at the scene, 50 lux, is higher than the modified minimum scene illumination of the camera at 45 lux.

WHAT IS THE FUNCTION OF A CAMERA LENS?

The main function of a lens is to focus the scene on to the CCD chip of a camera.  This important function is often under-rated, causing problems after the CCTV system is installed.   A data sheet for lenses usually contains many specifications like focal length, F-Stop, depth-of-field etc. 

WHAT IS THE CONSTRUCTION OF A LENS?

To understand the construction of the lens, it is important to understand the theory of light.  The speed of light when travelling through air is roughly 300,000 kilometers per second or 186,000 miles per second.   When light passes from air into a denser medium at an angle, like glass or water, its speed slows down by the index of refraction of the medium.  The following table gives a comparison for the various mediums. 

Medium                                  Index of Refraction                Speed of Light

Air/Vaccum                            1.0                                      186,000 m/second or 300,000 km/second

Water                                    1.33                                     140,000 m/second or 225,000 km/second

Glass                                    1.5                                       124,000 m/second or 200,000 km/second

Diamond                                2.42                                     77,000 m/second or 124,000 km/second

As the wave of propagation is continuous, this slowing down bends the light beam, when it enters the new medium.  It is similar to a bicycle changing direction when it enters sand from road.  This basic principle is used in the construction of a lens.  Convex and concave lenses are the basic lens types, which make the light beam converge and diverge respectively.  These basic lens types are mixed and matched to give a wide variety of lenses.

WHAT IS CHROMATIC ABERRATION OF LIGHT?

When light is refracted through glass a lens error called chromatic aberration occurs. Simply it is this:  Visible light is made of different colors and each color has a different frequency.  These colors will bend differently compared to each other when they pass through a single convex lens, resulting in a scattered focal point, meaning the picture will not be focused properly.  To overcome this error, several different lenses are grouped together.  This can make the lens construction complex and therefore more expensive.  There are lenses available which do not resolve the chromatic error accurately and are not compatible for use with color cameras, as they will not give a sharp focus for all the colors in the picture.   The same reasoning and logic is applicable for the infrared frequency range also.  For this reason, in many cases, when an infrared illuminator is used with a monochrome camera the picture is not properly focused.

Many people are under the impression that a lens is made up of a single lens.  This is not true.  Besides glass pieces required for correcting chromatic aberration, additional glass is also required for:

1.  To focus the lens on objects at different distances - When the lens focus moves from one object to another at a different distance, or when it follows a moving object, the        lens elements reposition, i.e. the focal point changes and the picture thus always remain clear. This is not a problem with the human eye, which varies the thickness of the lens.  A long way to go to catch up with this advanced technology!

2.  To achieve different focal lengths in a zoom lens - The glass pieces move in relation to each other to achieve different magnification of the object, resulting in different focal lengths in a zoom lens.

WHAT ARE THE FACTORS AFFECTING LENS QUALITY?

During construction, the following factors will determine the quality of the lens:

1.  Number of glass pieces used - More glass pieces combined together in a lens may help in reducing chromatic error, improving focusing etc, but will increase light absorption, resulting in lesser light availability to the camera. There is a trade off between accuracy and absorption.

2.  Absorption factor of the glass - Poor quality glass will absorb more light, again resulting in lower light availability to the camera. Obviously glass with lower absorption factor will cost more.

3.  Coating and polishing - The quality of coating and polishing of the glass can improve lens quality.

4.  Mechanism - Precision and reliability of the mechanism that moves the glass pieces within the lens is important.  Poor quality mechanisms can lead to inaccurate settings which may not be consistent.

WHAT ARE THE DIFFERENT ELEMENTS OF A ZOOM LENS?

A zoom lens is a lens that can be changed in focal length continuously without losing focus.  Magnification of a scene can be changed with a single lens, but every time the position shifts, the lens must be refocused.  If two lenses are combined, it is possible to change the magnification without disturbing the focus.  A zoom lens is made of the following groups:

1.  Focusing lens group - brings an object into focus. It moves irrespective of the zoom ratio or current focal length.

2.  Variator lens group - changes the size or magnification of the image.

3.  Compensator lens group - when moved in relation to the variator group, the compensator lens group corrects the shift in focus.

4.  Relay lens - Since the zoom unit does not converge light, the relay lens group is placed behind it to focus the object on to the CCD chip.

Zoom lens design requires extensive optical path tracing and continuos self correcting performance evaluation effort.  It also involves the use of powerful computers and specialist softwares.

HOW DO YOU CHOOSE A LENS?

Choosing the correct lens for an application is one of the most important decisions while designing a CCTV system.  Experience helps but it is important to work with the end user to understand what field of view is required to be seen on the monitor.  The field of view is the width and height of the scene as viewed by the lens.  It depends upon the focal length and distance of the object.

Any field of view has some critical area which is the target area.  For example when the camera is viewing the gate, the space the car is coming through is the critical viewing area or if one is watching the door, the space occupied by a person walking through the door is a critical a viewing area.  In the same way every scene has a critical viewing area.  This critical viewing area is usually ignored while selecting a lens for an application.  After the installation is complete it is not uncommon to hear comments that the end user wanted to positively identify the person, but is not able to do so with the lens installed. 

Step 1  -  Identify the scene area, which needs to be covered by the lens and estimate the width or vertical height of the scene.

Step 2  -  Estimate the distance from the camera to the scene.

Step 3  -  Calculate the focal length of the lens. The following methods can be used:

1.  Standard formula

The focal length can be calculated using the either the scene width or height formulas

f = c * d / w   or

f = v * d / h, where

c = width of the CCD chip

v = height of CCD chip

d = distance from camera

w = width of field of view

h = height of field of view

f = Focal length of lens

2.  Lens wheel calculator -  Many lens manufacturers provide this lens calculator.  It is quite simple to use and the focal length of the lens can easily be calculated depending upon the object distance and scene dimensions.  The limitation is that it does not tell how large the critical viewing area will be on the monitor.

Step 4  -  As mentioned previously, in any scene there are areas or moving objects which are critical.  It is important to understand what is required, a detection or positive identification.

Detection view - The critical viewing area should cover 5% of the monitor

In any scene there are areas or moving objects which are critical.  It is important to understand what is required, a detection or positive identification.

Detection view - The critical viewing area should cover 5% of the monitor

Action view - The critical viewing area should cover about 10% of the monitor

Identification view - The critical viewing area should cover about 25% of the monitor.

Step 5  -  Calculate the viewing area of the scene and also of the critical viewing area by multiplying the horizontal and vertical dimensions. Divide the critical viewing area with the total viewing area to get the size of the critical viewing area in the monitor.

1. If the proportion of the critical viewing area is as expected, use the calculated focal length.

2. If not, then change the

- focal length till the correct proportion is found or

- change the distance of the camera until the correct proportion is found

3.If you still do not find what you want, you may have to choose a lens which is the nearest to your requirement.

Example:

A 1/3 inch camera is viewing an entrance gate to a factory. The car coming through the gate is the critical view.

1/3 chip; width ( c ) = 4.8mm     height (v) = 3.6mm

distance to gate (d)  = 100 ft

width of gate (w)  = 12 ft

car dimension (front ) =   5 ft X 5 ft

Focal length f = c * d / w  = 4.8 * 100/ 12 = 40mm

scene height h = v * d / f = 3.6 * 100 /40 = 9 ft

Scene area = 12 ft X 9 ft = 108 sq. ft

Critical area = 5 ft X 5 ft = 25 sq. ft

% size of car in monitor = 25 * 100 / 108 = 23.1%

The car will cover about 23% of the monitor.  This will allow the positive identification of the car coming through the gate.