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Compression methods and Picture quality

There are many different compression methods in use by different video recorders. All of these methods are capable of delivering very high picture quality, provided that the hardware or software being used has been implemented with this as an objective, and that the product in question has been programmed correctly.

Conditional systems based upon JPEG and Wavelet are designed to retain the image quality of these systems, but reduce storage and transmission requirements by only storing/transmitting changes in a reference picture. You will often see this type of system referred to as MJPEG. (Motion JPEG)

Building upon this concept, we come to motion predictive methods, such as MPEG2, MPEG4 and H.264. These are the methods used in DVD's, satellite TV (SKY), cable TV (Virgin Media, Telewest etc.) and the BBC iplayer. These systems identify repeated patterns in the video data, and dynamically allocate bandwidth. This is a particularly efficient method of storing and transmitting video, and if used appropriately can provide very high quality images, whilst using less storage space or transmission bandwidth than other methods. IP cameras, and network attached devices in particular are likely to conform to these standards.

The best advice I can offer to anyone in the market for a new recording system, regardless of the technology being used, is simply this, trust your own eyes! Look at the live images being produced by your cameras, and then look at some replay of the same cameras. The replayed images should look just like the live. If the fine detail in the image becomes fuzzy, or the image becomes “blocky” or “pixelated” this is usually a sign of too much compression or insufficient transmission bandwidth.

Many “off the shelf” DVR's, when used with their default programming, apply far too much compression. As a rule of thumb, less compression = better pictures, so it is worth taking the time to make sure that compression ratio can be reduced.

This is particularly true in low light conditions, when the noise generated by a camera trying to produce a usable picture will be difficult to compress. When selecting a high quality recorder you should check that it is capable of producing high quality images in all lighting conditions that your cameras experience.

Beware of anything that lowers resolution to attain recording speed. Reducing the resolution to CIF (352 pixels x 288 pixels) will seriously degrade the picture quality. For high-end applications, you should expect at least 2CIF resolution (704 x 288), or better still 4CIF (704 x 576) or D1 (720 x 576). But remember these numbers alone do NOT tell you about picture quality, because they simply refer to the number of pixels that make up the image. Since the images are compressed AFTER the pixels have been created, the pixels coming out of the recorder (the pictures you see) do not necessarily look the same as the pixels that went in! So whilst lowering the number of pixels is generally bad, a high number of pixels is not necessarily good. This is why it's important to look at the pictures being replayed.

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