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.
