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Glossary: Interlaced Scan

Interlacing is a way used by IP camera manufacturers to speed up the refresh rate of their cameras.

An IP camera sensor is made up of a mesh of pixels. The colour value of each pixel is transferred to the computer as a series of numbers which the computer then interprets back into colours and outputs on your screen. However, in particular with megapixel cameras, there can be millions of pixels to retrieve, which can require a large amount of processor time, either slowing the camera down, or pushing the price up as more powerful processors need to be used.

Since IP cameras take many pictures per second, the time between images is very small, leaving very little time for objects to move significantly. With this in mind, interlacing was developed whereby instead of reading each line of pixels one after another, only every second line is read. On the first pass the camera will read lines 1, 3, 5, 7 and so on. Once the last line has been read, the camera returns and reads lines 2, 4, 6, 8 etc, meaning that each frame only half the image is read, saving time and bandwdth. This means the camera can achieve a greater frame rate without requiring more powerful hardware.

The downside with this is that since objects are moving between shots, the position of the objects in view will change between shots. With slow moving objects this is fine, but with fast moving objects such as cars or people, a tearing effect occurs along the edges of the object.

Two images one with progressive scan where a face can be seen, the other is a blur

The image on the left is from a progressive scan camera, the image on the right is from an interlaced scan camera. As you can see, the difference with moving images is huge.

Published on November 24th, 2008 by James Drinkwater

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