Home' RTCA Documents for Review : DO-230H FRAC Contents 184
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The first issue to be addressed is operational performance - what level of target detail is required at specific
distances from each camera over the full range of operating conditions, especially lighting conditions.
Multi-imagers, by definition, have several cameras, usually housed inside a dome. Selecting a multi-imager
or fisheye camera begins with the number of cameras per unit; their lens parameters (some may be fixed
while others are variable); their angular coverage and imaging performance under variable lighting
conditions; the effectiveness of the manufacturer's imaging software to remove distortions and integrate
multiple camera images; their video stream transmission bandwidths (bit rates), which will vary with
lighting conditions and with encoding (video compression) schemes; the computation power provided in
the servers, etc. For example, using three 4MP cameras, for a unit rating of 12MP, may or may not be
better in a particular situation than using four 2MP cameras depending on how they are integrated,
differences in camera sensitivity and their ability to cope with changes in scene conditions (known as wide
dynamic range), and other factors.
Detailed specifications for multi-imagers cameras cannot fully depict the resulting image quality or PPF
performance. On-site testing is the only way to resolve these issues and expect to get satisfactory results.
It is also essential for measuring the video streams under variable motion and lighting conditions, data
needed to determine network bandwidth and storage requirements.
For airport applications, multi-imager cameras are best used where wide angular coverage is required and
targets of interest are at relatively short distances, Candidates for ceiling-mounted multi-imaging cameras
include terminal concourse junctions and the monitoring of baggage belts. Wall-mounted imaging cameras,
with 180 degree or less angular coverage, will find applications in concourses and hallways.
Encoding involves the digitizing of an analog image (as part of a video stream) and then applying
compression to minimize the bit rate required to represent the video. Minimizing the bit rate reduces
network bandwidth if the video is to be transmitted or storage requirements if it is to be recorded. For both
Standard Definition as well as High Definition and Multi-Megapixel cameras, the most prevalent CODEC
algorithm is H.264, which also has multiple levels of implementation offering different degrees of
The H-264 compression standard dominates video security applications because it reliably reduces video
transmission bandwidth and storage costs. The newer H.25 compression codec is not so widely adopted
and is being challenged by proprietary coding schemes of camera manufacturers. Testing in the actual
surveillance environment, over a full range of lighting and motion conditions, is necessary to assess the
merits of a particular compression scheme.
The most common protocols used to encode digital video streams are constant bit rate (CBR) and variable
bit rate (VBR), which function just as the names imply. The method of encoding can impact both image
quality and transmission bandwidth requirements. Selecting between CBR and VBR, or VBR with a cap,
depends on scene conditions as well as the amount of information needed (image quality) and the available
network bandwidth. Scenes with a lot of motion, such as the entrance to an airport terminal during peak
traffic periods, or night scenes with point light sources, can require more bandwidth than simple scenes
such as viewing parked aircraft.
Digital video streams vary in bandwidth depending on several factors including the content of the stream
(which can spike with motion or when strong light sources appear), how the stream is encoded, and the
method of compression used. When specifying hardware take great care to match the hardware to the exact
mode of operations – i.e. how many cameras, what resolution, what frame rate, what lighting, how many
simultaneous replays etc.
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