Product Adopted:Network Cameras
Manufacturers have been promoting surveillance cameras with resolution specifications higher than 540 horizontal TVLs, implying that customers will benefit from enhanced resolution. However, even if a camera can pass a test that validates a resolution specification over 540 horizontal TVLs, the end user is not going to get more than 540 lines out of a typical installation because of system limitations common to all real-world surveillance applications.
Resolution is the ability to look at a video display and delineate detail in an image captured by a camera. In video surveillance, resolution is typically specified in terms of horizontal TVLs (HTVLs).
The 540 HTVLs Limit
Camera manufacturers are advertising specifications of 580, 600, 650 and even 700 HTVLs, and some of these manufacturers are making these claims even though they are using imaging sensors that are not physically capable of capturing more than 540 HTVLs, the practical limit for analog video surveillance systems. There are system limitations that prevent the end user from achieving any benefit from the advertised increase in resolution.
Fundamentally, a composite video signal, the type provided by any analog camera, is limited to 540 HTVLs because of the input circuits built into the monitor. These low-pass filters are present at any analog composite input, whether the display uses CRT or LCD technology.
LCD monitors have to convert the analog signal to digital format in the same manner that a DVR does. If a camera actually has a resolution of more than 540 HTVLs, the increased resolution will not be visible on a typical monitor deployed. Even if users deploy studio monitors, they would only be able to see the additional resolution before the video has been recorded on the DVR. The DVR is going to limit the resolution of the recorded video, and the recorded video is what is going to be used for evidentiary purposes; not the raw output of the camera.
There are no DVRs on the market that can record more than 540 HTVLs. DVRs, like display monitors, have low-pass filters at their video inputs. In order to convert an analog video signal into digital format, there is also what is known as a video decoder. The conversion process samples the analog video and converts it to digital data in an industry standard format. Without going deep into sampling theory, it is important to explain that this limit was not set arbitrarily. It was a result of the sample rate used for converting analog video to a digital format.
The total resolution of an image is simply the horizontal resolution multiplied by the vertical resolution. Until megapixel cameras were introduced, nobody talked about total resolution because certain image capture technologies are inherently limited in terms of their effective vertical resolution.
CCD sensors can capture interlaced video using a process called line-pair summation. This acts as a low-pass filter to minimize image flicker, but has a side effect of decreasing the effective vertical resolution by 25 percent. That is unfortunate for the end user because the DVR and display monitor inputs are very capable of accepting all 480 lines of vertical resolution. It is important to remember that how an image is captured is independent from how it is displayed. Video surveillance systems typically display video in an interlaced format, but that video may have been captured by the camera in either a progressive or interlaced format. If users have a big budget and want to spend a lot of money on studio quality monitors to maximize horizontal resolution, and if they are using CCD-based cameras that capture images in an interlaced format, then they should look for cameras with a minimum of 720 HTVLs to match the total resolution of a camera that uses a 540 HTVLs progressive capture sensor.
Buying Analog Cameras
An analog camera with resolution specified at more than 540 HTVLs does not truly give any additional benefit to the end user. The DVR is not going to record anything higher than 540 HTVLs, and there will be no noticeable benefit when the video is displayed on a security monitor, unless users spend a lot of money on a professional studio monitor and view the video before it is recorded on the DVR. To find the camera that gives the most usable resolution, look for models that have resolution specifications of 540 HTVLs as well as a minimum of 450 HTVLs.