Interlaced scanning was originally introduced as a way to improve the image quality of a video signal without consuming additional bandwidth. The method soon became ubiquitous in traditional, analog television sets. To put it simply, the technique splits each frame into two so-called fields. The scanning starts at the top-left corner and sweeps all the way to the bottom-right corner, skipping every alternate row on the way. Interlaced video thereby reduces the signal bandwidth by a factor of two, allowing for a higher refresh rate and, thereby, reducing flicker and improving the portrayal of motion.


There are, however, some downsides to interlaced video. For instance, if objects are moving fast enough they will be in different positions when each individual field is captured. This may cause what is called motion artifacts. Normally these are not visible but can appear if the video is displayed at a slower speed than it was captured or when presented as a still frame. Another potential problem is called interline twitter. It is an effect that shows up when the subject being shot contains very fine vertical details that approach the horizontal resolution of the video format.


These limitations can be avoided using progressive scanning. This technique captures, transmits and displays all lines in the image in a single frame. Scanning is done line by line, from top to bottom. In other words, captured images are not split into separate fields like in interlaced scanning so there is virtually no “flickering” effect.


In a surveillance application, this can be critical for viewing details within a moving image such as a person running or a vehicle moving. Another benefit of this technique is that single frames can be used to make paper copies with almost photographic quality. This can be crucial if the material is, for instance, to be used as evidence in a court of law. Of course, these potential gains must be weighed against pro- gressive scanning’s requirement for somewhat more bandwidth.


At left, a full-sized JPEG image (704x576 pixels) from an analog camera using interlaced scanning. At right, a full-sized JPEG image (640x480 pixels) from an Axis network camera using progressive scan technology. Both cameras used the same type of lens and the speed of the car was the same at 20 km/h (15 mph). The background is clear in both images. However, the driver is clearly visible only in the image using progressive scan technology.


Frame rate Frame rate is defined as the number of image frames per second (fps). For interlaced systems, the number often implies the field rate, which means the number usually is twice as high since there are two fields to every frame.


Historically, one of the thornier issues had to do with a suitable frame/field refresh rate. Countries were divided into two camps – for reasons that very much depended on the frequency of the mains electrical supplies, which in turn affect image stability – favoring either 25/50 fps or 30/60 fps. Nonetheless both systems are compliant with HDTV and consequently also meet the full frame rate requirements of video surveillance.


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