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SKYROS Corporation VideoNet 8.4 User guide
APPENDIX
А
. VIDEO SURVEILLANCE. TERMS AND CONCEPTS
Video signal format
There are three main formats of video signal: PAL, NTSC and SECAM. Security
video surveillance usually uses cameras supporting PAL format (this format is
called «PAL (B, D, G, H, I)» in the properties of audio/video capture cards). Thus all
the following information is given for this format.
Digital capture
By «digital capture» term we mean analog-to-digital conversion of video, with
which
VideoNet
system works.
Frame and field
Video signal is a sequence of frames. PAL format transmits 25 frames each
second. Maximum resolution (size) of a frame is 768 (horizontal) and 576 (vertical)
pixels. Further we will call image of this size «full frame». Each frame consists of
two fields – even and odd. An even field – image even rows (2, 4, 6 and etc.), odd
field – image odd rows (1, 3, 5, etc.). Fields are captured by camera and transmitted
by turns – odd, even, odd and so on. TV monitor displays fields in the same
sequence. However, computer monitors differ from TV monitors: they can’t display
first even rows and then odd rows. That is why at frames digital capture by a
computer both fields are «added together» in one image and then are displayed
on a computer monitor. If there is any motion in a camera coverage, then during
the fields changing (20 msec) moving object can shift to insignificant, but
noticeable position. This can cause image «dithering» and image ragged edges.
This effect is called «interlacing».
VideoNet
system provides special
VNInterlaceFilter™
,
which enables to get initial image.
If you need to digitize image, which has the height of less then 288 pixels, then it
is enough to use rows of only even or only odd field. In this case, you will see no
interlace effect. Maximum field resolution is 768 x 288 pixels. Such image is called
a «full frame». Full field is a half of a full frame. An image equal to one fourth of a
full frame (384x288), is called CIF. An image equal to one sixteenth of a full frame
(192x144) is called QCIF.
PCI bus
Most of video/audio capture cards, except PowerVN4 Pro3, TitanVN8 Pro, PowerVN8
and TitanVN16, should be installed into PCI bus. In this case,
VideoNet
system
(depending on configuration) can operate with any set of these cards. As a result
the number of real time channels, processed by the system, can exceed 30. If
image of a large size is digitized on each channel, then the large data flow (about
tens of Megabytes per second) will be transferred through the PCI bus. When PCI
bus peak loads there can be data losses, which are reflected in the «skipped»
image rows. To compensate these losses
VideoNet
provides
VNBusFilter™
function.
Adaptive digital capture algorithm
Let’s suppose that
VideoNet
system displays video of not large sizes (for example
