Many people who don't fully understand CCTV promote real-time DVR recorders as being a good thing. The single biggest mistake you can make when it comes to DVRs, particularly High definition DVRs is selecting real-time.
First of all, what is real-time? When you go to the cinema what you are actually seeing is a series of still images shown so quickly that you think the subject is moving. Cinemas project at 24 frames per second, this is quick enough to trick the brain, and you can no longer make out the individual stills. The problem with using 24 frames per second with CCTV is that it generates too much data for the hard drive to store. This is particularly relevant when it comes to HD CCTV.
You don't need anything like 24 frames per second. We recommend between 3 and 6 frames per second for CCTV use. This takes up less space on the DVR hard drive without costing much in terms of evidence gathering. To put frames per second into perspective we've made a little video.
A 2TB hard drive in a 4 channel HD 1080P DVR recording at 6 frames per second gives about 25 days of footage. Setting the DVR to record in real-time would lower this down to just 6 days which is obviously not enough.