Q: What is important in evaluating frame rates?
A: Frame rate is one of the most important and one of the most misleading areas when evaluating a DVR. Some DVRs specify fps in increments of 30, 60, 120 and 240. Typically, this refers to the number of frames per second (fps) the video capture board can capture. This is not necessarily the same as the number of fps recorded.
Further, the fps number is meant to be divided by the number of channels available. For example, you might expect a 120 fps video capture board with 12 channel inputs to provide (120/12) or 10 fps on each channel. Often the actual recorded fps number captured (let alone recorded) is less than 10. The reason is that the capture board may spend some time switching between one channel and the next, resulting in a capture frame rate closer to 8 or 9.
Commonly observed frame rates are:
Please note, when viewing a moving vehicle, particularly when the camera is placed at 90 degrees to the motion, it is hard to capture without noticeable jerkiness unless using a high frame rate (25 fps or better). Testing frame rates is not difficult if there is an option to playback video one frame at a time. Simply record using the desired configuration for one minute and then count the frames for an entire minute. Be aware that if there is motion on all channels the system may not perform as well as if there is no motion on any of the channels.
Todd Radermacher is co-founder of Presidio Technology, San Francisco, CA. Presidio provides a managed Internet based video surveillance service.