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Primed for color.

GO OVER TO YOUR TELEVISION set, turn down the color control, sit back in your chair, and take an objective look at the picture. How interesting is it when all the various colors can only be distinguished by shades of gray? How easy is it to identify objects? You can clearly see that an individual is wearing an article of clothing, but to carry the identification process any further is impossible. overall, the picture is dull, and after a quick scan of its elements you would probably turn your attention elsewhere.

We have become a nation of color. Since the early 1960s color television has become a fixture in our living rooms. The penetration of color television is so great that recent statistics published by the Electronic Industries Association indicated black-and-white television would shortly become extinct.

The security world has long recognized the value of color. Its presence can enhance every aspect of video security surveillance. In traditional applications, the use of color results in more accurate identification and a higher degree of apprehension and conviction. Those results have led banks to increase their use of color equipment, particularly in high traffic and VIP areas. Color has also increased the use of video for such nonsecurity applications as medical diagnostics and industrial testing.

The path from black-and-white to color is not easy. It is laced with pitfalls, especially in price, resolution, sensitivity, and system interface. Any CCTV manufacturer would quickly make the transition to an all-color system provided those obstacles could be removed. The inability to do so is not based as much on a lack of desire as it is on the limitations of the current US television standard (NTSC, named after the National Television Standards Committee). Many of the problems in converting video security systems from black-and-white to color originate in standards set for commercial television transmission in the late 1940s and color transmission in the early 1960S.

Television signals are transmitted over the air along a 6 megahertz (MHz) bandwidth of frequencies. For example, channel two in any given city can be found at frequencies between 54 and 60 MHz, channel three 60 to 66 MHZ, and so on. Within this 6 MHz band of frequencies, the range between 0 and 4.2 MHz is reserved for video information.

These numbers relate two important qualities that affect the way one views a video picture. First is the amount of separation between the lowest and the highest frequencies. This is known as the signal's bandwidth. In video, the black or darkest areas of the picture are represented by the lowest frequencies, the white areas by the highest frequencies. The combination of the two determines the contrast quality of the picture. The greater the bandwidth, the greater the picture contrast, and the easier it is to see details separated by various shades of gray. The smaller the bandwidth, the less the picture contrast, and the more shades of gray run together, making picture details harder to distinguish.

The second picture quality is determined by the frequency of the signal. In video, the higher the frequency, the greater the resolution. A signal at 5 MHz contains more details than one at 4 MHz. When the designers of NTSC set these standards, they were in fact setting the limitations on picture resolution and quality, The range of these frequencies is known as luminance and holds all the information one sees as picture details.

When color was introduced, it was located at a frequency point of 3.58 MHz within the 0 to 4.2 MHz video range. Its basic criterion for acceptance was that it had to be compatible with the existing population of black-and-white television sets. In other words, a color television picture had to have the ability of being viewed on a black-and-white set with no distortion.

What does all this television technology have to do with converting a security system to color? In a word, everything. To start, closed-circuit television systems, those systems that transmit and receive signals over wires and not over the air, must conform to the same technical standards as commercial, over-the-air television transmission requirements.

The inclusion of color at 3.58 MHz means information must be taken away from the luminance signal. That loss of information results in a loss of picture details. The signal must be altered to accommodate the introduction of color.

The introduction of color affects the basic components of a video security

system as follows:

The color camera. Black-and-white video cameras deal only with the element of incoming light. Color cameras must not only produce light in the form of luminance but also create the three primary colors of red, green, and blue along with their various combinations to create the complete spectrum of color.

Video cameras used for television production allocate a single pickup element for each of the primary colors. For security applications, the cost and space requirements for a three-element camera would be prohibitive. Instead, colors must be created using only a single image device.

To understand how the single-chip camera operates, one must first recognize that it is basically blind to incoming light. Thus the process of creating red, blue, and green signals must be done by some external component.

Color signal creation is the function of a physical dichroic matrix positioned on the surface of the chip. It extracts the red, green, and blue frequencies from the incoming light. After passing through the optical filter, the signal can be electronically processed to create all the spectrum colors.

While the exact theory of creating color is beyond the scope of this article, suffice it to say an optical filter severely limits camera performance. The function of the camera must be split between processing the incoming light and focusing on the dichroic filter. Sensitivity is lost as light is diffused by the filter. That sensitivity loss requires that color cameras operate under much higher light than black-and-white models.

Also, the range of lighting is limited. Under extremely low light, not enough light may be available to resolve the filter, and color may be lost. Under extremely bright light, blooming can wash out the picture, resulting in color loss.

Light levels are not the only consideration. The color of the light source adds to the color of the object. The resulting color is a product of both. For example, a white object illuminated by a red light takes on the tint of the red light by reflecting it. The object as seen by the eye is neither red nor white-it is a tint of red.

Eyes maintain the consistency of color under various lighting sources so well that the beholder may not realize how much the natural color of light varies over the course of a day. Color cameras are developed under a greenish-yellow light, as human eyes perceive green as having the greatest detail. Such light exists only under controlled conditions. Bright sunlight has a bluish tint, and twilight has a red tint. All these color changes require that a color video camera maintain a color balance regardless of the color of the light source.

As for resolution, in standard black-and-white cameras increasing resolution is directly related to increasing the number of pixels. Although in color cameras a similar equation would apply, the function of each pixel is split between producing resolution and creating color. A color and black-and-white camera containing the same number of pixels do not produce the same resolution.

Switching systems. In a black-and-white system the switcher is thought of simply as a device for getting a signal from point A to point B. In color, the switcher must also be able to pass the color signal without introducing any distortion.

Time-lapse video tape recorders. As time-lapse recorders were developed directly from consumer technology, they contain most of the limitations described earlier. As a result, first-generation recorders produced a resolution of only 250 horizontal lines in color and 300 lines in black-and-white. The difference between color and black-and-white resolution is due to having to, account for the 3.58 MHz color information.

A second consideration takes place when a recorder senses the difference between a color and black-and-white signal. Most standard consumer recorders detect the loss of color when the 3.58 MHz signal is no longer present. Then the video signal is internally routed through a different path to increase the resolution.

The switching between the two paths causes the video monitor to flash. That is not a great problem for consumer units, but it is significant for time-lapse recording, where the flash can mean the loss of critical frames of video information.

The total system. When several color cameras feed a time-lapse recorder through a switcher, further recorder stability problems can result. In such systems, the recorder can reproduce clear, stable playback pictures only if the recordings were made at a constant speed.

All cameras in a system must share a common reference point. Camera manufacturers have provided that reference by tying a camera's vertical sync reproduction rate to that of the incoming AC power. Both have a rate of 60 cycles per second.

In color cameras, however, the vertical sync rate must be changed from 60 to 59.94 cycles per second. Although that may not seem like a great change, the resulting effect makes it impossible to synchronize color cameras in a system by using the same method applied to black-and-white cameras.

Every security user can benefit from the addition of color. The future is in color. Be ready for it.

About the Author . . . Wayne Kennedy is national sales manager of Sanyo Fisher (USA) Corporation's Industrial Video Division in Compton, CA.
COPYRIGHT 1990 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Lights! Camera! Action! supplement; closed-circuit TV in security systems
Author:Kennedy, Wayne
Publication:Security Management
Date:Mar 1, 1990
Words:1611
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