Q: How do I decide how much storage capacity to allow when I’m specifying a digital video recorder?
A: There’s no simple answer – every installation must be assessed individually. Key factors affecting storage are picture quality, frame rate, compression method and the length of time for which images are required.
Q: What are the picture quality options?
A: The lowest resolution now normally adopted is CIF (352X288). CIF is generally the rule of thumb when calculating storage capacity, but higher resolutions, such as 2CIF (704X288), 4CIF (704X576) and D1 (720×576), are now often specified. As a guide, CIF images recorded using MPEG4 compression are around 10Kb, 2CIF images around 20Kb and 4CIF around 40Kb. Megapixel cameras usually produce images between 80 and 200Kb each. A balance must be struck between resolution, archive time and budget.
Q: What about frame rates?
A: Always allocate frame rates appropriate to the application. Live motion is 25 full frames-per-second (fps) but each image can be 40Kb or more (4CIF). This means about 1Mb of storage per second of data from each camera – about 3.6Gb per hour. Using 12.5 fps halves storage requirements and still permits lip-sync audio. Where lip sync isn’t needed, 4 fps is often acceptable, with corresponding savings in storage.
Q: How do compression methods affect the amount of storage?
A: Significantly! The challenge is to reproduce high quality, high-resolution video using the smallest amount of drive space, but remember that there are no free lunches! If a DVR claims much smaller file sizes than comparable machines with the same compression method, beware – reduced file sizes usually mean reduced quality.
Q: How long should recordings be kept?
A: This depends on the application, but don’t automatically adopt the “31-day standard”, a hangover from VHS tape. Digital recording is much more flexible. Discuss the options and costs with your client. In general, look at periods where video data cannot be recovered. If this happens to be 20 days, then 20 to 22 days of archive are appropriate.
Q: Is there no easier solution?
A: When in doubt, seek the advice of several professional suppliers to ensure a balanced view. Also, remember that storage is now much less expensive, so over-specifying a little won’t significantly affect project costs.