The digital CCTV enigma - Part 2

March 2003 CCTV, Surveillance & Remote Monitoring

In the concluding part of this article, Phil Mailes continues to discuss the merits of digital CCTV in layman’s terms.


Resolution is the measure of image quality and is measured in pixels. Remember that the higher the image quality the more data. Images with a lot of data will take longer to transmit, will utilise more network bandwidth and will require more space on the hard disk to store. For example a 320 x 240 pixel image (approx VHS quality) is 1/4 the size of a 640 x 480 pixel image (approx S-VHS).

To give you some idea consider the two pictures below, both were taken using a Minolta Dimage F100 digital camera. One was captured using the following settings: 2272 x 1704 pixels (high resolution) with no compression, whilst the other was captured using 640 x 480 (medium resolution) with high compression. The first file was 11,352 Mb whilst the latter was 90 Kb, can you spot the difference?

Figures 1 + 2: As can be seen there is very little difference between the two images. However, if you were to blow up the two images the following would be the result:
Figures 1 + 2: As can be seen there is very little difference between the two images. However, if you were to blow up the two images the following would be the result:

Here the results of using the lower resolution can be clearly seen; however, when using CCTV cameras, if they are positioned correctly, the need to zoom in is negated and the overall cost of ownership of the system can be drastically reduced.

Figure 3: Zoom in using Figure 1
Figure 3: Zoom in using Figure 1

Figure 4: Zoom in using Figure 2
Figure 4: Zoom in using Figure 2

Other considerations

Hard disks

Digital CCTV recorders are basically a PC with one or more hard disks included for the storage of the video images. As detailed above, dependent upon the compression algorithm used, the amount of compression selected, the refresh rate utilised and the resolution adopted the storage requirements will differ widely. This will mean that more hard disks will be required as the specification is increased.

Hard disks are now quoted in terms of the number of transactions written to them before failure, rather than the length of time before failure. Recording video is a very transaction intensive process, as information is continuously being written to the disk/s. The law of averages dictates that the more disks utilised for storage the more likely it is that one will fail. As digital CCTV systems mature in the market place we are starting to see that the hard disk is the most common point of failure. Given this scenario it is important to select a DVR with the largest capacity disk drives available as it is better to have fewer large disks than more small capacity disks. Additionally, DVRs with easily removable disks that can be removed and replaced from the unit without having to strip down the unit should be considered.

Some manufacturers will treat three hard disks in a digital recorder as a single volume, thus if one disk should fail all of the stored information is lost on all three drives. Other manufacturers treat each disk as a separate volume, therefore if one drive fails only the information on that drive is lost, whilst the information on the others remains intact.

To overcome the disk failure issues many manufacturers utilise RAID storage devices, these will allow for disk failure without loss of video information.


From the information detailed above it can be deduced that DVRs can only store a limited amount of information. The number of hard disks that can be mounted within the units limits this. Even though some DVRs can accommodate up to 1 Terabyte of hard disk space this will soon fill up. Most DVRs are set up to have two areas of storage, the first being for event driven 'locked' video and the other being for continuously recorded video. The system is continuously recording, or caching, the video and when an event occurs it will 'grab' the video associated with that event from the cache, according to pre- and post- alarm video configuration, and place it in a locked file. The locked files cannot be overwritten by the system unless the operator manually purges them. The cache is overwritten on a first in first out basis. The ratio of locked video space to free space on a disk is dynamically allocated and so after a period of time the disk will fill up with locked video.

When this situation occurs it is necessary to purge the files, in which case all of the information is lost or clips can be archived. Should the user wish to access this information at a later time it will be necessary to archive the video. Similarly, where there is a requirement to store large amounts of continuous video, it too can be archived, either automatically or manually. Information can be archived to two types of media, either tapes (video juke boxes) or arrays of disks.

Furthermore, smart manufacturers are starting to deploy central archiving capabilities meaning that video images from remote DVRs are transmitted across the network and stored centrally on either tape or disk. The likes of IBM have been doing this for pure data for many years, not only for simplified storage and retrieval, but also for disaster recovery. They are now offering their considerable experience and technology solutions to the industry to the point that they can store petabytes of video information and allow access to many simultaneous video streams thus enhancing retrieval and analysis of video clips by multiple people.

Bandwidth utilisation

Bandwidth utilisation is the amount of the 'pipe' taken up by the video information being transmitted over it. The frames per second, the algorithm and the compression ration will all effect the utilisation. When planning a system, the buyer should get some estimates on the utilisation, based upon the scenarios they have selected. Some systems are designed to be good network citizens and take bandwidth utilisation into consideration from the outset, whereas some systems are designed to be analog VCR replacements. There is room in the market for both products, but they are two separate solutions for two separate needs. A networked digital recorder can be used as a VCR replacement but would tend to be expensive. A digital VCR replacement, would struggle to effectively sit on a wide area network.


Another issue that is rarely discussed is the utilisation of a central database. Many manufacturers produce units with a database included, this is fine for a site with a single unit, however if a site has two or more units then it will need two or more databases ie, one per unit. The utilisation of a central database means that all recorders can be configured from a single computer, it is possible to run a report or get status, event or configuration information from every unit in the system from a single computer. It means that the management of events can be accomplished from a single location including the archiving of multiple recorders back to a single archive server. Thus a single database reduces management complexity, increases usability and therefore reduces the overall cost of ownership.


Within the industry, there still seems to be two distinct camps: that of CCTV and that of access control, neither of these two camps really gets it. The fact is that it is really just data acquisition.

CCTV manufacturers are merely moving along producing a plethora of digital VCR replacements. Access control manufacturers are still producing their products with an RS232 port to send ASCII commands to the CCTV system, to drive the system upon an event.

Very few have really embraced the two technologies and produced a single holistic solution. Even as a result of the recent spending spree on company purchases within the industry, we have seen holding companies purchasing both access control and CCTV manufacturers, but we are yet to see any true integration of the two technologies.

For example, the benefits to the user of a single platform that would allow programming, control and monitoring of both the access control and CCTV system from a single screen would be considerable. Consider the cost savings of such a system utilising a single user interface ie, one lot of training, a single database to administer, a single application to upgrade, duplication of servers and workstations eliminated, etc.

Historical analysis utilising two discreet systems, albeit interfaced, can be cumbersome. To track an event, information has to be reviewed on an access control system and a report generated, giving the date and time of the event in question. An operator will then have to review that date range on the digital CCTV system to access video clips. As the two separate systems operate on two separate PCs, the clocks on both are likely to drift out of synchronisation, for example 13:30 on one system, may be 13:50 on the other, further hampering investigation and adding to user frustration.

True integration will provide the ability to attach video clips to alarms and events on a single workstation, thus an operator should be able to click on an alarm and launch the video clip of the event that caused that alarm. Using a single database on a single server means that the times of the alarm occurrence and video clip will always be in sync.

Several manufacturers have launched Digital Video Servers that are capable of running other applications, for example, it is now possible to have one PC (video server) running digital video, access control, ID management, guard tour, etc, therefore again lowering the cost of ownership.


It can be seen from the above that the decision to embrace digital CCTV is a wise choice, however, the selection of the technology is about what works for you in your environment and within your budget. After all, video is a visual media and it is a simple task to view the resultant results of the various factors that affect digital CCTV and determine for yourself the best solution rather than have inappropriate and thus potentially expensive technology forced upon you.

For more information contact Phil Mailes, Lenel Systems International, 0944 0 148 381 5230,

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