How Big is Your Hard Disk
Digital Video Recorder (DVR) is fast replacing VCR. Utilizing hard disks instead of video tapes, images from security cameras are digitized and stored. Retrieval is fast and time saving as you can specify particular date and time. But how big should the hard disk be?
Understand your requirement
To determine the required size, we need to ask the following questions:
- how many cameras do you have
- what is your require image resolution
- how smooth the movement determined by frame rate
- how long do you need to keep the records before it can be overwritten
- what compression algorithm is used
Your answers will affect our computation.
How many cameras
Images collected by each camera is individually digitized and stored. Hence, the more cameras you have, the more images will be generated, the bigger the hard disk required. Typical DVR supports 4, 8 or 16 cameras.
Image Resolution
Like a digital camera, you can select the resolution of the image recorded. Depending on the DVR, the image file size can vary from 10KB to 25 KB. The bigger the file size, the higher its resolution.
Frame Rate
Digitization involves breaking the video stream into frames and capturing each frame. Frame rate, normally depicted as frame per second (fps) is the number of frames captured per second. Typical range is 1, 2, 3, 6, 12, 25 frames per second. A DVR that is set at 1 fps means that only 1 picture will be recorded in a second, resulting in jerky movement. Typical setting for surveillance ranges from 3 to 6 fps.
How Long before Overwritten
How long does it take before you discover an incident or an irregularity? That time lapse is often a good guide to how long you need to keep the record for. Idealistically we may like to keep the data forever, but that is a practical impossibility. Hence we need to consider how much time will lapse before we discover an incident. For example, retail cashiers close the accounts everyday and if there were any shortfall, it will show up by end of business day. Any investigation can be conducted and completed within 3 days, making a week long enough.
What compression method
Depending on the compression algorithm, the size of video can differ. Common compression methods used by DVR are MJPEG and MPEG.
So how big the hard disk
Assuming that you have 4 cameras recording at normal resolution (say 10 KB) at 3 fps, the hard disk spaced used to record for 1 second would be 4 x 10 x 3 = 120 KB.
If you are to record continuously for 24 hours and would like to keep the data for a month, the total hard disk space used would be 30 (days) x 24 (hr) x 60 (min) x 60 (sec) x 120 KB = 311,040 KB, approximately 300 MB.
This is a simplistic calculation as it does not take into consideration the effect of data compression. With compression, the requirement would be lower than the 300 MB. Hence the 300 MB represent a higher but reasonable estimate.