Introduction
In a previous Tech. Brief (November 2009 – “SURVEILLANCE PLANNING – GETTING THE BEST OUT OF THE HARDWARE [PART II]”), there was a brief discussion on when to use Fixed cameras and when PTZ cameras. This note takes a more detailed look at the issues surrounding the Fixed vs. PTZ camera debate.
Fixed vs. PTZ camera
PTZ cameras are seen as the answer to an organisation’s comprehensive and in-close-detail surveillance needs, by many security managers; thanks, in part, to Hollywood movies (remember the movie “The Bourne Ultimatum”, where there is a scene set in a railway station, with PTZ cameras moving via remote control and closely tracking the suspect).
The fact is that deployment of PTZ cameras raises as many issues as it solves. Security managers will do well to consider these, to ensure that in deploying PTZ cameras they do not degrade, instead of improving, the capability of the surveillance infrastructure.
PROS OF PTZ CAMERAS
More Detail:
| PTZ cameras, thanks to the Zoom capability, are able to provide a more detailed view of a surveillance target. | |
| Though all IP cameras, these days, feature digital zoom, this feature works by cropping the captured image; which means that the final image loses resolution as the zoom factor increases. Overcoming this loss of resolution, by installing higher resolution (more megapixel) fixed IP cameras, is an expensive proposition. |
Flexibility
| The Pan and Tilt capabilities of a PTZ camera allows it to, theoretically, cover a larger area than a fixed camera. |
CONS OF PTZ CAMERAS
Monitoring by Surveillance Personnel
| PTZ cameras need to be monitored in real-time by trained surveillance personnel. Typically, an alert generated by a sensor (either a fixed camera or a different type of sensor) cues the surveillance person to target the source of the alert, and then make a decision on the type of response. | |
| There is video analytics software, these days, that automates the activity of PTZing (as an alert is generated, the software automatically controls the camera to pan, tilt, and zoom towards the source); but, it is not fool-proof. Furthermore, human intervention is still required for the type-of-response decision. |
Surveillance Blind-spot
| In a surveillance set-up consisting entirely of PTZ cameras or consisting of surveillance zones monitored only by PTZ cameras, zooming to the source of an alert renders the rest of the zone unmonitored. This is a surveillance blind-spot until the PTZ camera reverts to its default position. | |
| Patrolling PTZ cameras, which are seen as an answer to the above issue, can easily be studied by sources of threat, to figure out surveillance blind-spots. |
Expensive
| PTZ camera are anywhere between five to ten times more expensive than fixed megapixel IP cameras. |
Higher Failure-rate
| Given the number of moving parts in a PTZ camera, it is more prone to failure than a fixed camera. |
Given the above issues, what could be the architecture for a surveillance camera infrastructure, for organizations that require comprehensive video surveillance coverage?
Comprehensive Video Surveillance – Architecture
The most efficient manner (cost-wise) to leverage the strengths of fixed and PTZ cameras, while ensuring comprehensive coverage, is to design a layered video surveillance set-up.
Layer 1 – Fixed IP Cameras
Layer 1 will consist of fixed IP cameras, and will cover 100% of the area-under-surveillance. These cameras can be of medium resolution (D1 to 2 MP) or high resolution (3 MP or higher), based on the budget available. These cameras will have support for Digital Zoom. The cameras may optionally have built-in intelligence to send out alerts if a threat (pre-defined) is identified.
Layer 2 – Fixed IP Zoom Cameras
Vulnerable points in the area-under-surveillance will be monitored by fixed IP zoom cameras, instead of fixed IP cameras. These cameras feature an optical zoom (remotely managed), but do not have pan and tilt capabilities. Such cameras are significantly less expensive than PTZ cameras, and can assist the PTZ cameras in case there are multiple sources of threat at these vulnerable points.
Layer 3 – PTZ IP Cameras
PTZ cameras will form a layer behind the fixed cameras. The ratio of PTZ cameras to fixed cameras can be decided on, based on the criticality of the infrastructure under surveillance, the budget available, and the threat potential. One thumb-rule frequently adopted, by users, is one PTZ camera for every four fixed cameras.
Layer 4 – Threat-detection Software and/or Surveillance Personnel
Layer 4 is a software and/or human layer. The software automatically detects threats, based on pre-defined threat descriptions. It may reside in the cameras (edge-devices) or may run on a server in a backroom. Accordingly, the threat identification may happen in real-time or near-real-time. This layer is optional, as it increases the level of complexity of the surveillance infrastructure. Also, the robustness of video analytics software in high-traffic, high-movement locations is suspect. Either way, human oversight is mandatory. What the use of threat-detection software can do, is reduce the level of trained human resources required for the monitoring and response activity.
Conclusion
In closing, security managers will do well to plan their video surveillance set-up taking into consideration the strengths and weaknesses of fixed and PTZ cameras described above.
