In visual monitoring terminology, the noise refers to any interferences randomly generated and located within the image, represented by changes in brightness or colour of individual pixels and groups of pixels. The noise on recorded video images is mainly caused by low lighting conditions which are insufficient for the sensitivity of the image processor, and may pose a serious issue in the security systems.
The image noise may affect not only the brightness of the recorded images, but may also make it difficult or even prevent identification of persons and licence plates of vehicles and may affect performance of the monitoring system in poorly lit office and retail areas. Noise reduction is necessary everywhere, where identification is the main concern.
Two technologies are currently used in CCTV systems to reduce the noise in recorded video images: 2D noise reduction (2D-DNR) and 3D noise reduction (3D-DNR). Each technology has its pros and cons.
2D-DNR noise reduction is based on the analysis of individual video frames, wheras 3D-DNR noise reduction also analyses the differences between subsequent video frames to adapt the pixels and improve image fidelity. 3D-DNR technology provides better noise reduction, however it tends to blur the moving objects.
Fig. 1. Example screenshots with enabled and disabled 3D-DNR noise reduction
Most high-end cameras combine both technologies for improved reduction of interferences. 3D-DNR technology is most commonly used in relation to static areas of the monitored space, whereas 2D-DNR technology yields better results for the moving objects, and is used in the areas where motion is monitored. An algorithm providing a flexible integration of those two technologies gives the best effects of noise reduction and yields sharp images, even under low lighting conditions.
The noise, apart from its negative effect on image quality increases the disk space required for image recording, reduces recording time, increases video stream size and may trigger false alarms. Thus, it is crucial for the CCTV cameras operating under variable and low lighting conditions to feature the noise reduction function to eliminate the interferences.