Image quality is one of the most important features of any camera. This is especially true in security, surveillance and remote monitoring applications, where lives and property may be at stake. While analog cameras are often thought to have higher image quality than network cameras, this is a myth. Advancements have been made in the past few years that have allowed network cameras' image quality to equal - and in some cases surpass - that of analog technology.
When comparing network and analog cameras, it is best to look at professional, high-quality network cameras. Professional network cameras should not be confused with lower-end network or Web cameras used for Web attraction applications. These cameras cannot deliver the same image quality required for security and surveillance applications. However, even in professional network cameras, image quality can vary considerably and is dependent on several factors such as the choice of optics and image sensors.
A good image sensor and optics are the most important factors in providing high quality images. Network cameras now have image sensors and optics that are the same as or better than those used in analog security cameras, and network cameras now can make use of progressive scan and megapixel sensors that are not available to analog technology.
The image sensor of the camera is responsible for transforming light into electrical signals. When building a camera, there are two possible technologies for the camera's image sensor: a Charged Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS). Analog cameras utilize only CCD sensors, while network cameras can be produced with both types of sensors. This provides further flexibility for optimizing the network camera to fit the installation.
CCD sensors use a technology developed specifically for the camera industry. They are more light sensitive than other sensors, which means they produce better images in low-light conditions. CCD sensors typically are more expensive and more complex to incorporate into a camera because they produce an analog signal that needs to be converted into a digital signal.
CMOS sensors are based on a standard technology that is already used extensively in memory chips, such as those inside of PCs. Recent advances in CMOS sensors bring them closer to their CCD counterparts in terms of image quality. CMOS sensors tend to cost less than CCD sensors and contain all the components needed to generate digital signals. Eliminating the digital conversion process has also made it possible to produce smaller network cameras because fewer components are required.
The Interlacing Issue
At a high 4CIF resolution, the clarity of rapidly moving objects - such as a person running or speeding car - has long been problematic in security and surveillance applications. In an analog environment, a rapidly moving object will appear blurry. This is because an analog video signal, even when connected to a DVR, interlaces to create the images. Interlaced images use techniques developed for analog TV monitor displays, made up of visible horizontal lines across a standard TV screen. Interlacing divides these into odd and even lines and then alternately refreshes them. The slight delay between odd and even line refreshes creates some distortion - only half the lines keep up with the moving image while the other half waits to be refreshed. This causes moving objects to blur (see Illustration 1).
A network camera, on the other hand, uses progressive scan technology to capture moving objects. Progressive scan captures the whole image at one time, and scans the entire picture line by line every 1/16th of a second. This eliminates the delay between odd and even line refreshes and prevents the picture from being split into separate fields. Images from network cameras are also displayed on computer monitors. Unlike TV screens, computer monitors do not interlace. They display images one line at a time in perfect order, so there is virtually no "flickering."