Determining bandwidth is accomplished by simply calculating the size of each image and keeping the compression for all cameras the same. Once you have the image size, you multiply that by the images per second to get your bandwidth requirements. To be fair, let's use the compression guidelines given by a popular NVR supplier, Milestone. They publish compression guidelines on their website at www.milestonesys.com/?cid=419. Milestone's specs indicates that a 640 x 480 image with medium-low compression has a file size of approximately 50kB. For higher resolution images we can use IQinVision's comparison matrix for megapixel cameras (in PDF format), which shows a typical 3.1 Mpix image will have a 225kB file size.
The following examples show what it would take to cover the parking lot with the desired resolution. Option #1 (show in photo column above as Image 1) would require 35 cameras operating 640 x 480 resolution (a.k.a. 4 CIF). If each of those 35 cameras was using 50kB of bandwidth, the total bandwidth needed for this option would be 1.75MB.
The second option is to pursue a megapixel application (Image 2 in the photo column). Four cameras operating at 2048 x 1536 (3 Mpix) would cover the scene. If each of those cameras is using 225kB of bandwidth, the total is 900kB.
So, an apples-to-apples comparison reveals that megapixel cameras can deliver the same image quality for about half the bandwidth. But let's face it; most people won't employ cameras to cover the entire area. To save money, they are more likely to only use cameras at critical "choke points," so the 35-camera-to-4- camera comparison isn't really practical. Let's take a quick look at a choke point example and see how Megapixel cameras stack up. First, let's define the choke point area; this is the area in which something you want to record is likely to happen (see Image 3).
Then, we run the numbers again. Option #1 (Image 4) is to use six cameras running 640 x 480 resolutin (4 CIF). At 50kB each, the bandwidth requirement would total to 300kB. The second option (Image 5) is to use a single megapixel camera at 2048 x 1536 (3 Mpix). The bandwidth requirement in that design would be 225kB.
This example demonstrates that covering our choke point with a megapixel camera would use 33 percent less bandwidth than using 4CIF, 640 x 480 cameras.
As a side note, some of today's megapixel cameras incorporate smart features such as digital image cropping that allow the user to optimize a camera for a given application. Essentially they can crop out areas of the image that are not needed and save the corresponding bandwidth. In the above example, the choke point is 1920 x 960, so a 3.1 Mpix camera with digital image cropping could be cropped to 1920 x 960 and still cover the area. This should yield a file size 135kB, which would be about 56 percent less bandwidth than what what would be used if we were to cover the same area using 640 x 480 cameras.
In the final installment of this series on the realities of megapixel surveillance, we turn to a comparison of digital pan, tilt and zoom versus mechanical pan/tilt/zoom cameras. Look for that update on March 20th, on SecurityInfoWatch.com.
About the author: Paul Bodell is vice president of sales and marketing at IQinVision, a U.S.-based vendor of high resolution/megapixel surveillance cameras. Bodell has been with the company since 2002, and has worked in the electronic security industry since 1994.