UFC 4-021-02NF
27 September 2006
change 1, 23 October 2006
Figure 4-13. Calculation For a Camera in Alarm Mode at 10 fps
At 2 bytes/pixel = 614,400 bytes, or 614.4 kb, per image.
Compression: 614.4kb per second (kpbs/20) = 30.7 kbps per image frame.
Camera rate is 10 frames/sec = 10 x 30.7 kbps = 307 kbps.
Convert bytes to bits (8bits/bytes + 2 control bits = 10 bits/byte)
Bandwidth = 307 kbps = 3.07 megabytes per second (Mbps).
Thus one camera in alarm monitoring transmits video at a bandwidth of 3.07 Mbps.
4-15.3
Compression. Digital images and digital video can be compressed in order
to save space on hard drives and make transmission faster. Typically, the compression
ratio is between 10 and 100. Different encoding/decoding (codec) schemes are used
such as:
4-15.3.1 M-JPEG (Joint Photographic Experts Group, Motion Picture) is a compression
technique for color images and photographs that balances compression against loss of
detail in the image. The greater the compression, the more information is lost. MJPEG
can be provided in any resolution size.
4-15.3.2 MPEG-1 (Motion Picture Experts Group) is the first MPEG format for
compressed video, optimized for CD-ROM. MPEG was designed for transmission rates
of approximately 1.5 Mbps. MPEG-1 encoding can introduce blockiness, color bleed,
and shimmering effects on video and lack of detail on audio. Typical MPEG-1
resolution sizes are 352 x 240 pixels and 320 x 240 pixels.
4-15.3.3 MPEG2 is a variant of MPEG1 that is optimized for broadcast quality video
and high-definition TV. Typical MPEG-2 resolution sizes are 720 x 240 pixels and 640
x 480 pixels.
4-15.3.4 MPEG4 is based on MPEG-1 and MPEG-2. MPEG-4 files are smaller than M-
JPEG files, so they can be transmitted over a narrower bandwidth. In CCTV
applications, MPEG-4 allows a style of transmission where an "anchor" image is
transmitted, and then another image is not transmitted until something in the image
changes. This minimizes the number of images transmitted when there is no movement
in a location. For this reason, MPEG-1 (and other frame-based codecs) can
conceivably be perceived as a better source of forensic evidence since each frame or
image is being sent as opposed to the updates. MPEG-4 offers a wide range of
resolution sizes from 64 x 48 pixels to 4096 x 4096 pixels.
4-15.3.5 Following MJPEG, Wavelet is another frame-based compression codec that
is commonly used. Wavelet operated in the 30Kbps to 7.5Mbps range and 8-30 frame
rate speeds. Typical Wavelet resolution sizes are 160 x 120 pixels and 320 by 240
pixels.
4-15.4
With regard to bandwidth, there are two general system architecture factors:
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