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MULTI-CAMERA
SYSTEMS
Multi-camera systems using one
recorder for two or more
remote cameras require careful analysis of the following parameters:
A. Are
the subjects to be viewed
by the cameras close enough together to allow reasonable cable lengths
from the
cameras to the recorder? Will
the same
be true of future projects for the system or will you have to buy
additional
recorders for future projects?
B. Are
the levels of activity
at each site sufficiently similar that the same temporal interval can
be used
for all cameras?
C.
Do you plan on using the recording
system for analysis playback? If
recording and analysis are to be done simultaneously at different
location, additional
hardware is
required for the analysis location.
Some exceptions to this are possible if a
networkable digital recorder
has internet or data network (T1, LAN, etc.) access, since certain
digital
recorders can be remotely downloaded to offsite computers while the
recorder is
actively recording new images.
Multi-camera
recording technology and terminology
Multiplexing
Sequential
digital switching, compression and recording of the images from two or
more
cameras onto the recording media that is synchronized with the
recording
interval chosen at up to 60 images per second per channel. As the images are passing
through the
multiplexing system, a flag is placed on each image going to the
recorder to
indicate the camera of origin. During
recording or playback, the
images from
each camera can be placed in a specific location on the monitor or the
images
can be displayed sequentially or any individual camera can be played
back at
full screen size at full resolution unlike quad processor units which
have
highly degraded images when a single camera is displayed full screen
size. Certain
digital multiplexer recording systems allow remote monitoring by
internet /
local network (dialup , DSL, LAN, WAN, T1 etc.) of live video or
previously
recorded video while the multiplexer continues to record.
It
is very important to remember that the recording
interval is not the Temporal Interval (TI).
For example if the recording interval is 1 second
and 4 cameras are
being recorded, the TI is 4 seconds.
If
subjects are moving quickly, data
could
easily be lost in that interval. Because
of this potential data loss factor, we rarely build systems
using multiplexers with more than 4 channels even though units from the
security industry can have as many as 32 channels
PIP
Picture in picture is sometimes
useful when only two cameras
are required and a second monitor is not wanted since both images can
be
displayed on the same monitor. It
is
also the technique of choice when the images from each camera must be
recorded
simultaneously. Simultaneous
imaging is
usually required for fast moving subjects that require orthogonal or
stereo
imaging.
There
are two technologies available to create PIP.
A digital technique which digitizes the images from
both cameras and
allows the insertion of the full image
of either camera into a small image (usually) at the bottom of the
monitor
screen. A switch
allows image
choice. This
technology can be used
with virtually any two cameras but after recording the images cannot be
interchanged nor can one image alone be displayed.
The
alternate technology uses only certain types of
cameras that can be synchronized or genlocked.
A master camera is chosen to synchronize a slave
camera. When this
is done, either camera can be adjusted
to fill all or part of the monitor image.
However they are not the full image of each camera
but only a that
portion which you display on the screen.
Again after recording the images cannot be
interchanged nor can one
image alone be displayed.
Quad
processor image systems
A
technology that allows the images from up to four camera to be
simultaneously
recorded as a single image on the media. The problem is that 3/4s of
the pixels
from each camera image is not recorded and that the image quality if
you try to
enlarge the 1/4 image to full size is very poor. After recording one
image can
be displayed but the quality is very poor.
This technology is not recommended.
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