Considering “Repeatability” of P/T/Z Cameras

Yes… another detail to consider when reviewing P/T/Z cameras; “Repeatability”. This may be of little concern when the intended P/T/Z camera application uses very low or no zoom within the presets of a tour. High end P/T/Z cameras with large optical zoom ranges tend to (and should) have higher repeatability ratings than other cameras. WHY?

First…
Repeatability, with regard to a P/T/Z camera specification, can be defined as the accuracy within which the camera position of the pan AND tilt axes are able to recall the previously set (aka: preset) position. Repeatability is typically given in degrees: 0.05°, 0.08°, 0.25°, etc.

Second…
The numbers given for repeatability are degree based. In effect, the repeatability specification references the ability of the camera to return to an exact position specified within the circular rotation of an axis. In the case of P/T/Z cameras… both the pan and tilt axes.

With these definitions in mind…
In a wide angle shot (with zoom at or near zero), any factor of repeatability found in today’s P/T/Z cameras is likely negligible. The image shift on either pan and/or tilt of even 0.25° when viewed after recalling a preset position will be very hard to notice.

The difference in repeatability becomes noticeable when a zoom setting has been associated with the preset position. A high zoom setting amplifies the image shift that occurs when the preset is recalled. The 0.25° image shift now becomes visible.

Repeatability can have a distinct impact on the resulting video image provided by the camera for real-time viewing as well as for what may be getting recorded into a DVR/NVR. The positioning of the viewable image provided by the recall of a preset should allow for variations due to the camera repeatability. This means that the user should test the presets several times to ensure the viewable image will still provide the desired video data even if the image has shifted.

Unrelated to the repeatability of the camera itself, yet related to the repeatability of image acquisition from a preset… stability of the camera mount location. The characteristics of the camera mount should be considered when establishing presets and expected image shifts. Mounting to the top of a pole is very common along highways, intersections, etc. Consider the suns effect on the pole material throughout the course of a day. One side of the pole is exposed to the sun during the morning hours while the other side is not; one side “hot” and one side “cold”. The material expansion coefficient, coupled with the height of the pole, can create an angular shift much greater than 0.05°. This angular shift may reverse throughout the course of the day as the sun changes position. Oh yes… and don’t forget wind!

In general, every P/T/Z camera has a number that specifies repeatability. A lower number will tend to indicate a higher quality of manufacturing, and is more likely to be associated with units incorporating a higher optical zoom or specialty video (thermal) capability.

~ David Scales, Chief Technology Officer


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WTI (Wireless Technology, Inc.)

Video Surveillance Products Dedicated to the Broadcast, Global Security and Transportation Markets!

http://www.gotowti.com/ ● http://www.wirelesstech.com/

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