In the Beginning 53
the luminance or black-and-white information in the picture. Note
that the common symbol for luminance is the letter Y. The lumi-
nance equation is usually expressed to only 2 decimal places as
Y = 0.3R + 0.59G + 0.11B. The letter R is of course representing
red, B representing blue, and G representing green.
Instead of sending luminance (Y) and three full color signals
red, green, and blue, color difference signals are made to conserve
analog bandwidth. The value of green (also Y) is subtracted from
the value of Red (R-Y). The value of green is also subtracted from
the value of blue (B-Y). The result is a color video signal comprised
of luminance Y and two color difference signals, R-Y and B-Y.
Since Y (the luminance signal) is sent whole, it can be recombined
with the color difference signals R-Y and B-Y to get the original
red and blue signals back for display.
PICTURE QUALITY
One of the most important things to a security professional is
picture quality. The effort and expense of capturing video images
will be of little value if, when viewed, the image is unrecognizable.
The fact is that the science of removing redundant information to
reduce the amount of bits that need to be transferred would not
even be necessary if we lived in a world of unlimited bandwidth.
For the present, at least, this is not the case. So we must learn how
to use bandwidth to its fullest advantage. Choices for high quality
or high image rate result in high bandwidth requirements. Choices
for lower bandwidth result in reduced image quality or reduced
update rate or both. You can trade off image rate for quality within
the same bandwidth.
THE BANDWIDTH WAGON
The term bandwidth is used for both analog and digital systems
and means similar things but is used in very different ways. In a
digital system, bandwidth is used as an alternative term to bit rate,
54 Digital CCTV
which is the number of bits per second, usually displayed as kilo-
bits per second. Technically, bandwidth is the amount of electro-
magnetic spectrum allocated to a telecommunications transmitter
to send out information. Obviously, the larger the bandwidth, the
more information a transmitter can send out. Consequently, band-
width is what determines the speed and, in some cases, the clarity
of the information transferred. Bandwidth is restricted by the laws
of physics regardless of the media utilized. For example, there
are bandwidth limitations due to the physical properties of the
twisted-pair phone wires that service many homes. The band-
width of the electromagnetic spectrum also has limits because
there are only so many frequencies in the radio wave, microwave,
and infrared spectrum. In order to make a wise decision about the
Figure 3-8 Traffi c. Courtesy of WRI Features.
In the Beginning 55
path we choose, we need to know how much information can
move along the path and at what speeds. Available bandwidth is
what determines how fast our compressed information can be
transferred from one location to another.
Visualize digital video as water and bandwidth as a garden
hose. The more water you want to fl ow through the hose, the
bigger around the hose must be. Another example can be found
in comparing driving home in rush hour traffi c with transmitting
video signals. If there are 500 cars, all proceeding to the same
destination, how can they make the trip more expediently? A
larger highway would be the obvious answer. See Figure 3-8.
If those 500 cars were traveling over four lanes as opposed
to two lanes, they could move with greater speed and accuracy.
Now imagine those 500 cars on an eight-lane highway. The four
and eight lane highways simply represent larger bandwidths.
Conversely, if there is very little traffi c, a two-lane highway will
be adequate. The same is true for transmitting digital data. The
bandwidth requirements are dictated by the amount of data to be
transferred.
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