CCTV matrix - real or virtual

CCTV Handbook 2006 CCTV, Surveillance & Remote Monitoring

The traditional analog matrix is under threat from IP video systems which use a so-called virtual matrix. But are they a true replacement and can they offer the many benefits promised by IP-based solutions? Shawn Ciccarelli of CCTV and security equipment distributor Norbain SA, compares the two technologies and separates fact from fiction.

Analog matrix

The core of most traditional analog CCTV systems is the video matrix. This is typically an electronics rack that is situated close to the control room. The matrix is a switch that routes video inputs from cameras to video outputs that are fed to monitors and DVRs/VCRs for recording as required. This is achieved by the operator(s) using desk-mounted keyboard controls.

Virtual matrix

A virtual matrix runs on a data network LAN, WAN or MAN (local, wide or metropolitan area network) that carries information encoded as TCP/IP (Transfer Control Protocol/Internet Protocol). Such networks are now ubiquitous and installed as a matter of course into new buildings as part of the communications infrastructure. The term 'IP network' is now a common phrase to describe this type of network.

In a digital CCTV system the analog video from the camera has to be converted to digital data for transmission over the IP network. This usually takes place at the camera in the form of an analog CCTV camera connected to a separate encoder unit ('transmitter') nearby, or by using a self-contained IP camera. Either way the result is the same, it is the encoded digital video that travels along the network cabling.

Whereas an analog hardware matrix switches video and PTZ (pan/tilt/zoom) controls, a virtual matrix will also handle the processing of alarm and access control data. It will also accommodate the communications required for VOIP (Voice over IP) and bi-directional, full-duplex audio.

The switching of camera feeds to monitors and recorders is achieved by the video management software. The software, which is normally PC-based, manages the control, administration and viewing of the live camera feeds and recorded video over the IP network. The software allows the operator, with the appropriate permissions, to view/analyse live and recorded video from any camera. Operator PC stations running the software can be located anywhere on the network, which allows for the design of truly distributed systems. The network routes the data directly to the appropriate receiver, there is no central switch as with an analog matrix.

The typical NVR solution simply requires a PC platform and hard disk storage. However, for more demanding fault tolerant applications NVRs can be packaged in standalone units with removable hard disk drives.

Performance

The virtual matrix approach can only be justified if the performance of the final CCTV system is at least as good as an equivalent analog system. So let us take a look at some of the more important considerations when comparing technologies.

Image quality

With modern compression techniques the argument that the picture quality of digitally transmitted images is inferior to analog is really redundant. Modern MPEG-4 compression technologies are capable of encoding the complete image at full resolution - 4CIF, and transmitting them around the network at full frame rate (25 fps for PAL, 30 fps for NTSC).

It can be argued that digital video is often superior to analog transmitted images since IP-based transmission is lossless. Image quality obtained at the receiving end, be it monitoring console or networked video recorder (NVR) is identical to that transmitted. No degradation occurs during transmission, recording or playback.

Reliability

An analog matrix is in effect a single point of failure and a catastrophic failure of the incoming video communication lines, switching, monitoring or recording equipment can bring the whole system down.

In a virtual matrix if part of the network, monitoring or recording equipment goes down the rest of the system can continue to operate as normal. In fact it goes further than this -redundant power supplies and network ports are now built into equipment as standard, and these provide 'hot standby' operation.

If one network path fails, another can be automatically swapped in. NVRs can also be distributed around the network providing further levels of redundancy.

Scalability

This is perhaps one of the most important differentiators between the two technologies. To expand a system that uses an analog matrix requires additional coax/fibre cabling, that has to be wired to a single location, and hardware or even a new matrix if the original unit is full. This is clearly expensive in both cabling and extra hardware, particularly if the new additions are remote from the matrix. With a virtual matrix, however, you just add the desired cameras, monitors, recorders or control positions to any point on the network and the job is done. It is totally scalable to an almost infinite degree.

The real answer to expanding existing analog matrix systems is to add an IP video system for the additional cameras and equipment. This fully-integrated hybrid system maintains the original investment in CCTV equipment while providing an expansion path for the future. When the old analog matrix and associated DVRs reach the end of their working lives they can simply be replaced with the virtual matrix and NVRs.

Video recording

An analog system would normally use digital video recorders (DVRs) connected directly to the matrix, whereas in a virtual matrix system networked video recorders (NVRs) are used. It is important to differentiate between DVRs and NVRs, as both are often termed 'digital'. A DVR digitally compresses analog video feeds and stores them on a hard-drive, the term 'digital' referring to the compression and storage technology, not the transmitted video images. The DVR therefore has to be located near the analog matrix. In contrast an NVR stores digital images directly from the IP network.

The advantage of an architecture based on NVRs is that they can be located anywhere on a network - at the monitoring centre, adjacent to camera clusters, on the edge of a network or collected together in a hardened environment. In use their location is transparent to an operator; the recorded video stream from any camera can be viewed by any operator at any point on the network. NVRs record and replay simultaneously and recordings on any one machine can be remotely viewed by a number of authorised operators spread across the network simultaneously, all totally independently and without affecting each other.

Cost

In many cases the bandwidth needed to transmit CCTV across an existing network can be easily accommodated on the existing corporate network, giving the virtual matrix a distinct cost advantage over analog systems by removing the need for additional cabling. This infrastructure is not only carrying and switching video images, but alarms, control signals, access control data and audio as well. IP network switches are typically 10% of the cost of analog matrices with an equivalent number of ports.

Latency

It really does not matter if the operator witnesses an event one or two tenths of a second after it really happened. The time when latency matters is when an operator is following a target using pan/tilt/zoom controls. If there is any significant delay in the camera image being updated, as the camera is being moved to track the target, it becomes difficult for the target to be followed successfully.

In virtual matrix systems latency occurs when the bandwidth of the transmission channel is insufficient for the volume of data that is being applied to it, and will not happen in a well-designed system. In the event there are two possible remedies - increase the available bandwidth or reduce the data volume, or both. A number of techniques to manage bandwidth can be employed - see below.

Network bandwidth

This is one of the most common arguments employed to denigrate the virtual matrix, and yet it has no real substance. In some circumstances the bandwidth may need increasing, but that is still preferable to laying down a totally new, but already technologically outdated, analog cabling infrastructure.

There are some well-proven techniques for reducing the bandwidth loading on a network, such as:

* Positioning the NVRs near (in network terms) the camera clusters.

* Using Multicast technology - this is where hundreds of operators in different locations can view the same camera feeds but use no more network traffic than one operator.

* Using ACF (activity controlled framerate) at the camera - the video transmission rate is adjusted based on scene activity. This reduces both network traffic and recording space. You would not buy a sports car and expect to get best performance on a dirt track! The same is true for IP video systems: to operate effectively, the network must be correctly specified, configured and maintained.

Summary

The main arguments against the implementation of IP video systems using a virtual matrix have clearly been made redundant. This is borne out by the increasing number of high-profile installations of IP video systems around the world, such as the two most recent Olympic Games in Athens and Turin, where over 2500 cameras were successfully integrated using IP video technology. The flexibility and scalability offered by IP video systems means they can be used for applications that are just not possible or cost effective with analog CCTV solutions, opening up new opportunities for end-users and systems integrators alike.





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