In Depth on Image Quality
One way to overcome these issues is to use the PTZ solely as a tool for an operator; his telescope to really zoom in on a far-away incidents. We cover the broader field of view with fixed cameras that provide situational awareness and provide an unchanging source of video for recording. At first blush, this seems to be a good solution. We can point the PTZ anywhere without losing our overall view of the situation. If the PTZ happens to be unmonitored, the fixed cameras will still record an incident. The problem, however, is that the recordings made by conventional fixed cameras are often lacking the detail to be of much value.
To see why, let’s turn to a concept that the industry is just beginning to use: pixels per foot. You can think of pixels per foot as a basic measure of desired image quality. In Figure 1, we can see the effect of varying the PPF. One of the key questions, of course, is how much is good enough? If all you are doing is basic surveillance, and there is no need to identify unfamiliar faces, read license plates or check the denomination of a bill at a point of sale, then perhaps as few as 20 pixels per foot will fill your needs. If, in fact, any of those items are needed, 40 to as much as 80 PPF will be needed. “Image quality is crucial if your application calls for identifying people, vehicles or objects that are unfamiliar to your operators,” says Dave Tynan, vice president of marketing and sales for Avigilon Corp.
The beauty of thinking about image quality requirements in this quantitative way lies in the fact that you are starting by selecting the minimum image quality your application requires. Justifying your equipment selection becomes a lot easier as well when you can clearly show the CFO why 20 PPF will not give you a usable image of the guy taking computers out of the back door. “If a site has enough controls in place that they are not worried about outsiders and just want to catch ‘Billy in the red shirt’ vandalizing the dumpster again, then they may not need the ability to clearly see faces,” says Paul Bodell, chief marketing officer for IQinVision.
The downside to using a high PPF, however, is the limited width a single camera can cover. A conventional VGA camera has an image width of 640 pixels. A 4CIF analog camera is the equivalent of 704 pixels. So, if you decide you need 40 PPF with a VGA or analog camera, you can only cover an area that is 16-17 feet wide. If the back row in your parking lot is 60 feet long, and you want 40PPF, you will need 4 cameras to achieve it (see Figure 2).
Of course, most people’s reaction to the thought of using that many cameras is that it can not possibly be necessary. After all, many of us cover that kind of an area today with one camera. True, but what happens when you take that image from your DVR and blow it up to see a face? A solid and undistinguishable blur. Of course you could go back to a PTZ as long as someone is monitoring it full-time. In this scenario, it makes the cost of four cameras seem reasonable.
Justifying Megapixel Cameras
There is, however, an even better way. You could move to a megapixel camera.
High Definition is not just the latest gadget to appear on the CCTV scene. Anyone that watches an HD TV at home knows that the picture can be gorgeous. For surveillance, however, the benefits of megapixel cameras go well beyond watching the beads of sweat on a quarterback’s face. More pixels in one camera can be used in one of two basic ways; either the increased resolution provides a more detailed and higher-quality image, or all those pixels can be used to produce the same quality image with far fewer cameras. In most cases, the practical answer is some of both.
Megapixel cameras are available in a variety of sizes with 1, 3, and 5 MP being some of the most common. A 5 MP camera has an image width of 2,592 pixels — or four times the width of a conventional VGA camera. We can use that to either provide dramatically better PPF and image quality, or as we see in Figure 2, it can be used to replace as many as 16 cameras at the same resolution. “Most applications seem to end up with a replacement factor of about 8 to 1,” Bodell says.
When used in this way, megapixel cameras provide some easy-to-justify advantages. While still providing the level of PPF you have selected, they do so with significantly fewer cameras in many situations. Fewer cameras translate to a significant cost savings not only in cameras, but in wiring, housings, maintenance and system software costs. “There has been an assumption that you have to pay a huge premium for megapixel systems,” Tynan says. “In fact, high-definition systems now often go in at a price below conventional analog systems.”
The wider field of view also gives the operator a better awareness of the “big picture,” while still allowing individual operators to digitally zoom in on their own areas of interest. In fact, megapixel cameras provide the ability to output multiple video streams which will enable several operators to zoom in on different objects in the image at the same time. Perhaps most importantly, the video contains all of that rich detail which lets you zoom in after the fact, even if no one was watching the live video. “What megapixel brings you is not only picture quality, but the ability to do forensic analysis,” Calderon says.
The PTZ Is Not Dead Yet
Still, issues remain. Digital zoom has some practical limitations. While optical zoom allows ratios such as 32x in common lenses, digital zoom is limited by the resolution of today’s cameras. “Digital zoom as such is a great ability in any digital video,” says Fredrik Nilsson, general manager of Axis Communications. “But with a 5-megapixel camera, you can only do 4x zoom before you start getting lower resolution than VGA.”
A PTZ can provide a full, 360-degree view. While often not required, achieving that kind of wide view with fixed cameras often causes the costs to shift back in favor of panning the camera.
Storage and bandwidth can be an issue for systems with a high PPF requirement. While the requirements do not change whether you use a lot of standard cameras or a few megapixel units, it will be significantly more than a PTZ-based system. Remember, this is a function of making a decision to record enough detail of the entire view to allow you to drill into it later. Proper network design can minimize the bandwidth issue.
Newer compression technologies are also a big help. “H.264 can reduce your bandwidth requirements by a factor of ten, and that also reduces storage,” Calderon says. Clever video software can also help to ensure that full-resolution video is always recorded, but only the minimum resolution needed by the operator is displayed — once again reducing the required bandwidth.
Day/Night performance, while improved, still lags behind a conventional resolution camera. This a function of the reduced pixel size on the camera being able to capture less light. Manufacturers have attacked the problem by improving the sensitivity of the sensor and by increasing the overall sensor size to fatten up those pixels. “CMOS technology has improved dramatically over the last six years,” Bodell says. “You give up very little sensitivity now to gain megapixel performance.”