Digital video (loop) recorder
Description/objectives
Digital video recorders allow for the continuous recording of video and are the most popular technology for recording surveillance footage. This technology has removed the need for tape cassettes and VCRs, allowing video to be recorded on a computer hard drive, memory card or USB. The use of video cassettes had a number of problems including the degradation of equipment over time and the need to reduce the number of frames per second to facilitate longer recording. Less frames per second meant that events could be missed and footage was less useful, particularly for legal requirements. In modern DVR systems, equipment and image quality does not degrade and a higher number of frames per second can be facilitated, typically 30 per second or more. PC based DVRs are most popular as they are easier to manage. They are also more flexible in that they allow for system updating which extends the usable life of the technology.
Loop recording refers to recording over the oldest footage on the memory unit once capacity has been reached. High capacity memory cards or USBs, wireless transfer as well as high capacity hard drives have meant that this function is not really necessary any longer. In on-vehicle applications wireless LAN networks can be used which automatically download footage onto a central computer system, clearing the remote memory unit and allowing the footage to be kept for a longer period of time, or indefinitely when transferred to a removable memory device.
The role of the DVR unit is to capture analogue video from surveillance equipment, convert it to a digital format, compress and decompress video, and to record it for analysis using diagnostic software. Large scale surveillance systems, such as those used in transport operations, require a lot of processing power to manage the quantity of footage being collected. These can be highly complex systems requiring full-time management. The DVR encoder/video card is the unit component responsible for image capture, compression and decompression. This encoder card includes one or more processors which perform these functions using algorithms. Multiple processors tend to improve performance but this depends on the DVR system used as they can vary widely.
In station or terminal applications, the DVR unit is generally located at a dedicated monitoring/control station. At this station video is received from cameras over a wired network. On-vehicle camera applications tend to have an in-vehicle PC based DVR system which is connected to the on-board computer which provides access to data transmission systems.
Applications
- Surveillance
Benefits and cautions
DVR systems are a relatively young technology which is constantly being improved. As such, it is important when implementing this type of system that it can be updated over time so that the usable lifespan of the system is maximised. There is a wide range of DVR systems available but there can be significant performance differences between these. As such it is critical that the solution chosen is suitable for the intended purpose, specifically in terms of being capable of handling the amount of data required and in compressing video to a usable format.
Case Studies
Dublin, Florence, Prince William County