Factors in Retention Time
One of the biggest questions we get asked is how long will my footage go back? The three biggest factors that affect retention time are the resolution, the framerate, and the level of motion. Below you’ll find some general considerations and tweaks to help increase retention time, but if you’re looking at an upcoming installation and trying to do some calculations, then the System Design Tool is going to be instrumental in your planning. You can find our KB article about it here: System Design Tool - OpenEye Knowledge Base
The biggest factor to be aware of is a camera’s resolution. While video compression does help quite a bit, at the end of the day an 8MP camera is going to require roughly double the space of a 4MP camera. Lowering the resolution is the quickest and simplest way to increase retention rate; if you have a 12MP fisheye camera for example, if you bump the resolution down from 4000x3000 to 3200x1800, every image that camera sends to the NVR is going to be taking up a little under half of the space it would at full resolution. While effective, end users may have an issue with not getting the full capabilities of the device, which leads us to the next option:
Even if you have a high resolution, you can still see significant improvements by lowering the framerate. This is also why typical installations of video surveillance systems will have low framerates, sometimes as low as 8 or 5. If you think of a surveillance camera as a normal camera that’s taking 30 pictures a second at an 8MP resolution, you’d be filling up the memory card fairly quickly. But if you’re taking 15 pictures a second, that’s only going to take up about half the space.
Level of Motion
The level of motion a camera is recording is going to heavily affect your overall retention rate. Cameras in lobbies, front doors, or other oft-travelled routes will be taking up much more space than a camera in a utility closet. The reason being that most cameras come equipped with two streams, a high- and a low-resolution stream. When the camera/NVR detects a motion event, it will kick the camera from recording in its low resolution to recording in its high resolution. Another option, one could play with the sensitivity and timing settings of motion detection so that the motion recordings aren’t quite as long. If you're unsure about your motion setup or if you're getting a lot of false positives, take a look at our KB article about configuring Motion Setup here: Best Practices for Motion Area Setup - OpenEye Knowledge Base
GOV stands for Grouping of Video, in relation to compression. How H264 works, the camera initially sends an I-frame which is a full resolution picture. Every frame after is going to be a P-Frame, which only contains the pixels which have changed between either the initial I-Frame or the last P-Frame. GOV decides how many P-Frames the camera will send before it sends another full I-Frame. A higher GOV means less I-Frames, and thus less overall space being used. The Gov is either going to match the framerate or be an evenly divisible number of the framerate. We typically recommend it is no more than double the framerate, however it is feasible to higher the GOV even more. Try to keep it at a clean denominator of the framerate and be aware that it can cause stability issues when viewing the camera’s live feed.