SPECIFICATION FOR THE RIGHT CCTV CAMERA IS NOT ALWAYS AN EASY PROCESS.
First, there are many factors that have to be taken into account: technical specifications, the application and its requirements, as well as any physical constraints the site may impose. With ever increasing product ranges available in the marketplace, and technology constantly evolving to optimize performance, reliability and functionality, it is quite a challenge to make an informed decision to meet the requirements for the job whilst remaining within projected budget. Understanding the many variables within CCTV camera technology today can only be an advantage in helping you make the right choices.
At the heart of the CCTV camera technology is a CCD sensor (Charge Coupled Device) that converts light into an electrical signal. This electrical signal is then processed by the camera electronics and converted to a video signal output that can then be either recorded or displayed on to a monitor.
However, the treatment of the video signal is then dependant on the type of camera. CCD chip cameras can be divided into two principal types: analog or the more recently introduced digital versions.
These can be sub-divided further into the following:
· Medium resolution monochrome
· Medium resolution color
· High resolution monochrome
· High resolution color
· day/night cameras that provide color in the day and monochrome at night.
To complicate matters even further, each of the above is generally available with different levels of performance -like a car model varying from 'base features' to 'top of the range'.
MONOCHROME OR COLOR?
The human eye remembers and recalls things better if they appear in color - it's easier to track down a brown-haired person wearing a red sweater and blue jeans than a dark, gray-clad figure that would be produced in monochrome.
Color cameras carry an additional premium in price compared with monochrome cameras. But they are also less sensitive making night usage an impractical option unless good lighting is available.
Monochrome cameras can offer Infra Red (IR) sensitivity allowing their use with covert IR illumination possible. This can be particularly useful where planning permission makes extra lighting impractical or the security requirement is such that intruders should not be alerted to the existence of CCTV surveillance.
ANALOG OR DIGITAL?
Until recently most cameras have been of the analogue type, producing good quality images at an affordable price. However, the introduction of Digital Signal Processing (DSP) has increased both the flexibility of using security cameras whilst enhancing the quality of the color images produced.
At the heart of DSP lies computer microchips, or 'chip sets' which have replaced the conventional integrated circuits in the camera head. This enables DSP camera manufacturers to offer installer friendly, feature-rich products.
The market for DSP technology falls into two broad categories: 'standard' and 'premium' DSP. Standard DSP cameras generally offer more consistent picture quality than their analogue counterparts, operating over a wider range of lighting conditions. Premium DSP cameras, however, have much richer functionality. This includes programmable intelligent backlight compensation (BLC), Video Motion Detection, remote set-up and control using a serial data link; built-in character generator and on-screen menus. These features make Premium DSP cameras the ideal choice for complex surveillance conditions such as those encountered in town centers.
Some situations may require a standard DSP camera, but with a specific premium feature. A good example of this is the Vista NCL634 color camera. Using digital signal processing, the NCL634 splits the screen into 64 zones. The DSP function calculates the average brightness within each of these zones and then compares it with those in all 64 zones. The camera can then adjust the picture detail for areas that are in silhouette.
This innovative feature is ideal in awkward lighting situations, e.g. a camera looking towards a shop window. In the morning, the sun may be in the top left corner of the window, but then moves across the field of view during the day, causing poor picture quality in most cameras. Intelligent backlight compensation is a function that will ensure crisp detailed pictures automatically throughout the day.
CCD CHIP SIZE?
CCTV cameras generally use COD chips that are designed for the consumer camcorder market. Originally, the chips used were half-inch image diagonal, but the drive for reduced size led to the development of third-inch and more recently quarter-inch chips.
The half-inch chips are capable of producing the highest sensitivity and resolutions owing to the simple fact that they are able to gather more light. Third-inch chips now form an increasing part of the market and as product development continues their performance is approaching that of their larger brothers.
Quarter-inch chip sets are a relatively recent development and are being widely used in consumer camcorders. Currently their use in CCTV is still somewhat limited because of the lack of availability and range of quarter-inch format lenses.
As a general rule, quarter-inch cameras provide the lowest cost and performance while half-inch cameras provide premium performance and are more expensive.
Mid-priced third-inch cameras make up the bulk of cameras used in the market today.
UNDERSTANDING HARD DISK RECORDERS
The late 1980's saw the introduction of video multiplexers offering users the facility to record pictures on to single videotape, eliminating the need for VCRs dedicated to recording a single camera output.
The use of time-lapse VCRs as a storage medium for those images is well known, as are their inevitable drawbacks - introduction of noise, wear and tear and the simple requirement that the tape needs to be rewound to access information. In a practical situation the reviewing of tapes to secure the important face shot" or "scene of crime" can involve long and tedious work.
The late 1990's have seen the emergence of Hard Disk Recorders (HDRs) that are essentially multiplexers with a computer hard disk memory to store images. HDRs are excellent at reproducing high quality images with little noise or picture degradation and are extremely useful in calling up an alarmed picture.
A problem that HDRs faced, however, was that computer memory was still relatively expensive compared with a storage medium such as videotape.
The struggle for many HDR manufacturers was to produce a machine that provided the features and performance required with sufficient memory to make it a practical machine at a realistic price. Now, with computer memory being available in hundreds of Gb at a relatively low cost, Hard Disc Recorders have finally come of age. The advantages over VCR's are many. HDRs are able to record in VHS mode (the same quality as a standard VCR), SVHS mode (the standard used by the highest quality VCRs and giving over 60% better resolution than standard VHS) and SVHS+ (not available with VCRs).
Further to the above a VCR and it's tapes begin to wear and deteriorate from the moment they begins recording while Hard Disk recording should remain at the same high standard throughout it's working life. This means that even in VCR mode the quality will, in most cases, be superior to what would be achieved on a standard Video Recorder.
In addition to the above HDRs offer a number of additional features not available with Multiplexers.
1. The ability to view and control the system from computers around the world.
2. Interconnectivity to Computer networks.
3. Built in Motion Detection for setting alarm events and immediate retrieval.
4. The ability to go direct to a time or incident without the need to search through hours of videotape.
WHAT IS A MULTIPLEXER?
Put simply, a multiplexer allows several camera signals to be recorded onto one videotape. To do this it synchronises the camera signals (lines them up in time) and marks each one with a code, allowing every camera to be replayed independently from tape, regardless of how many cameras are recorded on that tape. In addition, each image is stamped with a time and date caption.
Many multiplexers also provide the ability to view several cameras simultaneously on one or more monitors. These groups of pictures, when displayed on one monitor, are usually called multi-scene pictures. This is particularly useful when there are a large number of cameras across a site.
As with so many things today, there is a vocabulary which one must learn to be able to assess the functionality of any particular unit.
MULTIPLEXER JARGON?
SIMPLEX: This term is used to describe a multiplexer that will record pictures to tape or display multiple pictures on a single monitor. A simplex multiplexer will not perform both functions simultaneously. When a simplex multiplexer is used to replay tapes, it will stop recording.
DUPLEX: This term is used to describe a multiplexer that will record pictures to tape and display multiple pictures on a single monitor. A duplex multiplexer will continue to record even when a tape is being replayed.
MULTI-SCENE: When several images are displayed on a single monitor screen simultaneously, this is called a multi-scene view.
FRAMESTORE: This is an area of memory within a multiplexer that is used to store pictures when they are digitised. Generally speaking, a simplex machine can be considered as having one framestore, a duplex machine has two.
DIGITISTATION: The process of turning an analogue signal (most camera signals) into a digital signal (a collection of binary 1's and 0's used to represent an analogue value.)
ASYNCHRONOUS CAMERA INPUTS: Most multiplexers use asynchronous camera inputs. This means that cameras within a CCTV system do not have to be referenced to a single timing source (gen-lock). This usually makes the system both cheaper and easier to install.
TIME DIVISION MULTIPLEX: All video multiplexers perform time-division multiplexing. This refers to the process of recording all connected cameras by taking a snapshot of each camera in turn. In other words, a slot of time is allocated to each camera.
UPDATE RATES: This is probably the most often talked about subject when referring to multiplexers - and the least understood! The update rate is the period of time between the first and subsequent recordings of any particular camera. Since the cameras within a multiplexer based system are recorded in a time-division multiplex, and each camera is allotted a slot of time, the update rate will be dependent upon the number of cameras recorded.
For example, if 4 cameras are recorded - for this example we'll call them 1,2,3 & 4, the sequence of images on tape will be 1,2,3,4,1,2,3,4,1,2,3,4 etc. When camera 1 is replayed from tape, images 2,3 and 4 will not be displayed, but the time allotted to them on the tape will elapse before the image of camera 1 is updated. When using time lapse VCR's that extend the length of time a single tape will record, the update rate will be a function of the VCR recording mode (i.e. 12, 24, 48-hour modes etc.) and the number of cameras recorded. THIS IS ALWAYS INDEPENDENT OF THE MULTIPLEXER USED
DIGITAL STORAGE – MORE FACTS AND HYPE!
Digital storage is likely to be flavor of the month for a long time yet, as is the thriving hype and misinformation industry. I originally produced an article on digital recording in the July 1996 issue, when it really was very much in its infancy. Incidentally the article was sub-titled ‘The hype and the facts’, so in one respect nothing has changed since then except that manufacturers now have bigger numbers to confuse us with.
I expected that digital recording in CCTV would develop at a rate comparable with the PC industry but this has not happened. For instance in 1996, a CD-ROM drive was many hundreds of pounds and a CD writer was several thousand pounds. Now you can buy a CD-ROM for as little as £35.00 and a CD-RW for not much over £100.00. Similarly PC central processors have increased tenfold in power and speed at significantly lower prices. One area of great interest to digital storage of video images is the capacity of hard disc drives (HDD). This has increased from about 1/2Gb in 1996 to common 18Gb today with 36Gb available, still not the 100Gb I hoped for in the original article.
There is no question as to the benefits of digital recording for event recording, ATMs, etc. This article is looking to the future of continuous recording as we currently enjoy with the VCR.
A lot of progress has been behind the scene with developments and availability of various compression techniques to, create more efficient storage of data with smaller file sizes. It may be worth revising some of the techniques involved in digital recording. The following is a brief extract from The Principles and Practice of CCTV 2nd edition.
PRINCIPLES OF DIGITAL VIDEO RECORDING
In digital recording each field is divided in to an array of individual points or pixels. At each one of these points, analogue to digital converters convert voltages representing the color and brightness at that point to a binary digital number. This array of binary digital numbers can then be stored digitally in a file with a name cross referenced against time and date. A single frame of monochrome video needs about 450Kb (Kilobytes) of space for storage and single frame of color needs about 650Kb. This is the uncompressed size that would be needed for storage on hard disc or other storage medium.
Consequently to store the same number of images as a videotape, a total storage capacity of about 280Gb (Gigabytes) would be needed for one camera. This is considerably larger than hard discs and other media generally available and would also be tremendously expensive. Consequently some means is required of reducing the amount of space required without adversely affecting picture quality. The technique of reducing the amount of space required is generally referred to as compression.
The video frame contains a large amount of redundant information that can be eliminated without a great loss in perceived picture quality. Consequently, common types of compression used are known as "lossy compression" because the redundant information is discarded. Most compression methods are effective up to a certain point, or "Knee", beyond which the image quality quickly degrades.
To assist in reducing the amount of size required for storage the video signal can be represented in a form known as YUV. The YUV format consists of the Y (luminance) and UV (color difference) signals (for further descriptions of luminance and video signal components see section 2). The advantage of using YUV format is that fewer bytes are needed to digitize the video. Normally, recording all of the color components; red, green, blue (RGB recording) would need three bytes, one byte for each color. By using YUV format the luminance can be digitized as one byte and the color difference signal as one byte. Consequently only two bytes are needed rather than three, a saving of one third of the storage space required. This technique can be used together with compression to minimize the amount of space required for storage.
TYPES OF COMPRESSION
The technology for compressing video pictures originated in the storage of still photographs on computers. The most commonly used standard, JPEG, takes its name from the Joint Photographic Expert Group by whom it was developed. Using JPEG compression, the knee occurs at about 8:1 compression. The most commonly used standard is Motion JPEG for which the knee occurs at about 15:1 compression. Consequently, M-JPEG reduces a 450Kb file to only 30Kb. While this is still too large to fit the same number of images as a video tape on to a hard disk it is small enough to permit, say, 2 frames per second to be recorded for 24 hours on to a 6Gb hard disk, which is a size generally available, costing a few hundred pounds.
The Motion Picture Expert Group specifically for the digitization of moving images devised another more recent compression standard. This standard is given the name MPEG. This standard makes use of the redundancy between adjacent frames.
MPEG-1 contains three types of encoded frames. Intracoded frames (I-frames) contain all of the video information required to make a complete picture. Predicted frames (P-frames) are generated by previous I-frames or P-frames and are used to generate future P-frames. Bi-directional Predicted frames (B-frames) are generated using both previous and future frames. A complete sequence of frames is made up of a series of these different frame types with more than one I-frame for every 10 P- or B-frames. This process is known as inter-frame correlation and allows compression ratios of 100:1 to be achieved.
MPEG-2 is the format used in the latest Digital Video Disk (DVD) technology, which can store about 90 minutes of VHS quality video and audio on to only 650Mb of storage space, such as a CD-ROM. However there are a number of disadvantages to MPEG compression. Firstly, in order for MPEG to achieve high compression it needs the video signal not to change abruptly from frame to frame. Since many video recording applications require multiplexing because more than one camera must be recorded, the rapid change from frame to frame, as cameras are switched defeats the inter-frame correlation technique used in MPEG.
RECORDING DEVICES
There is now a far greater range of recording devices available at easily affordable prices than before. This is a brief review of the main characteristics for each.
ANALOGUE DEVICES
Video Cassette Recorder (VCR)
This is not intended to praise or condemn the humble VCR, simply to include it in the list of available devices. To record 16 cameras over 24 hours will provide a picture update time of 5.12 seconds. The tape can then be removed and stored for as long as the Code of Practice requires. This is frequently 31 days but sometimes 90 days. With S-VHS resolution can be up to 500 lines, depending on multiplexers, cameras, lenses, transmission, etc. Rewind time for an E180 tape is in the order of three minutes; this would be the time to locate a scene at the opposite end of the tape. The stored signal is an analogue video signal and can be replayed on any other make of VCR. The information stored on tape is permanent and can normally only be deliberately wiped off. VCRs require regular, relatively expensive maintenance and frequent replacement of tapes.
OTHER ANALOGUE DEVICES
There are other types of higher quality analogue video recorders such as U-Matic, but these are rarely used in CCTV systems, they are mainly the province of broadcast television.
DIGITAL DEVICES
As discussed earlier, there are many different formats of compression and analogue to digital conversion and different compressed file sizes. There are also many varying claims for the resolution produced for these combinations. A future article will attempt to compare these to a common base. For simplicity of comparisons, this article will be based on a final file size of 20Kb, which is a compression ratio of about 30:1 for a color picture. There will be 16 cameras with a picture update time of 5.12 seconds to compare with conventional recording.
HARD DISC DRIVE
HDDs are found in every computer and have evolved to be extremely reliable devices requiring virtually no maintenance. There are no touching components in a HDD although there are mechanical parts to rotate the disc and move the read/write head. Seek time is virtually instantaneous to retrieve a scene from any part of the disc by many search parameters. It is possible, although unlikely, to accidentally delete all the data from a hard disc. Current disc capacity is up to 36Gb, with 18Gb being readily available; this is likely to increase dramatically over the next few years.
The example would require 5.4Gb per 24 hours; a 36Gb disc would provide 6.67 days of continuous recording, (26M images). The options therefore would to accept an archive period of just less than 7 days or transfer the full disc to another removable medium for longer archiving. The medium could be another HDD or a DAT.
HDDs can be removable slot-in devices, therefore it would be practicable to remove a full disc and replace it with a blank pre-formatted disc to continue recording, just as is done with VCR tapes. One example of this is digital recording in trains where it is not practical to review incidents on the train. The hard disc is replaced with a blank disc and the original taken back to a central control for reviewing.
A major advantage of hard disc recording is that any part of the disc can be reviewed without interrupting the continuous recording.
DIGITAL AUDIO TAPE (DAT)
DAT drives are miniature audiocassettes incorporating magnetic tape similar to a VCR and can have capacities up to 50GB. (A 50GB tape costs in the order of £40.00). One common use of DAT drives would be to download from a HDD when it is full for archiving. As many tapes as necessary could be used to provide the total storage time required. Rewind time would be about 3 minutes for a full tape. Although search parameters may be similar to a HDD, the seek time could be comparable to a VCR. If involved searches are required, the DAT could be downloaded to a HDD for faster output. It should be noted that transfer rates of data could be quite slow, from 1 to 12 Mb/sec. At the best rate, transferring 50Mb could take over one hour or up to four hours at the slower rates. Similar comments apply to a DAT as to a VCR cassette; there is a thin magnetic tape being drawn across read/write heads. Again similar to VCRs, because the cassette is a fixed size, greater capacity is achieved by using thinner tape.
DIGITAL VERSATILE DISC (DVD)
This used to be known as Digital Video Disc, but is now used for all types of data storage. It utilizes the same principle as a CD in that indentations are burned on to the disc by a laser. The same laser reads these indentations. These drives are now readily available as read/write devices at modest prices and can be used exactly the same as a HDD. Capacities of discs are quoted as being 2.6 or 5.2Gb, the latter uses both sides. A 5.2Gb single sided disc will be available shortly. DVD drives can read a range of devises such as, CD-ROM, CD-RW, PD format, DVD-RAM and DVD-ROM. Beware though of quoted capacities because data protocols use quite a lot of space. A DVD formatted as FAT 16, which is the international standard for CD-ROMs, reduces the capacity to 2GB. Another format is used for long continuous files of video is known as UDF, in this case capacity is limited to 2.32GB. At the previously noted files sizes and number of cameras this would equate to nearly 9 hours of continuous recording, (11M images). This medium could be useful for downloading excerpts from a HDD drive for evidence or distribution. If formatted to FAT 16 it could be replayed on any PC. Most DVD drives include MPEG1 compression software so that recordings could be made directly from a composite or S VHS input and replayed on a PC with MPEG1 decoding. (Most have this). DVD discs cost about £13.00 for 2.6Gb or £19.00 for 5.2GB.
CD WRITABLE AND RE-WRITABLE DISCS (CD-R, CD-RW)
CD writer drives are now available for under $200.00, with CD-R discs less than $1.00 each. The capacity though is limited to 640MB with several caveats. This is nearly 3 hours (.3M images) of recording on the previous basis. CD-RW discs are written to ISO9660 standards so any CD device may read them. Now for the caveats, and these apply to your CD writer that you use for every day applications. The header information requires 27Mb, so this leaves only 627Mb for data. If you write several separate sessions, then 5 sessions needs 79Mb for headers leaving 561Mb for data, and 10 sessions leaves only 490Mb for data. Writing speed is up to 900Kb/sec, so 600Kb of data would be read in about 11 seconds. As with the DVD, this would be an inexpensive medium for transferring data.
STANDARDS
As with most systems, there are no common standards for video data storage in the CCTV industry. The various systems on the market incorporate most of the types of compression mentioned earlier. Add to this many methods of encryption and watermarking and there are the makings of a massive problem of the use of digitally recorded video. Life was simple when we had VHS, there were even problems when S VHS was introduced and that was only two standards, although they are internationally agreed.
CONCLUSION
There is no doubt that digital recording is now a potent force in the CCTV armory and will prove to be the most effective and efficient method of video recording and archiving. It is still a case of ‘caveat emptor’, be suspicious of the specification that states 8 video inputs and offers continuous 24 hour recording with a 8Gb hard disc. You will probably find that this is only for one camera with a 15Kb file size and 5 frames per second.
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