Basic Color TV Part I, January 1954 Radio-Electronics

Convert B&W to Color TV (Wheel) - RF Cafe

Convert B&W to Color TV (Screen) - RF Cafe

Far more than electronics went into the development of color television. Extensive research into how human beings perceive color, combined with the color-producing ability of suitable chemical compounds ultimately determined how color picture tubes would be manufactured, and the electronic circuits which would activate them. Most people are not aware that the very earliest color television schemes were electromechanical contraptions that either spun a transparent color wheel in from of a modulated light beam, or used an oscillating mirror to direct colors to the appropriate position on a display screen. Fortunately, the fully electronic version won. Even so, there were retro-fit kits sold and installed to convert black and white (B&W) sets to color using the spinning color wheel system (upper left). As recently as the late 1960s there were commercials selling a screen to place over the front of your B&W set that was tinted green at the bottom, red in the middle, and blue at the top (lower left).

Fig. 1 - The visible light spectrum, containing the colors from red to violet.

Most readers will recall that back in 1950 the FCC adopted a set of color television broad-casting standards based on the CBS color TV system. Soon after, it became apparent that this system would obsolete millions of black-and-white receivers already in use. This dampened the industry's enthusiasm for the CBS color system, and as a result, little effort was made to develop it into a practical reality.

The general feeling in the industry was that if a color television system was to be universally accepted, it could not afford to obsolete the millions of black-and-white sets already in use.

Fig. 2 - A prism breaks white light up into the spectral colors.

Fig. 3 - Adding primaries to produce color (or white light).

Fortunately, the FCC, in adopting the CBS color system, left the door open for consideration of a better color system should one ever be developed and demonstrated.

Prior to the war, the RETMA, then the RMA, established a committee composed of representatives of most of the major manufacturers, known as the National Television System Committee. The NTSC was given the job of formulating a set of black-and-white television broadcast standards for presentation to the FCC for its approval. These standards were subsequently adopted and are the ones in use today.

Taking a page from past experience, the RETMA again called on the NTSC in November, 1950, and assigned it the job of formulating a set of color television broadcast standards for presentation to the FCC.

As in 1941, the NTSC is composed of the best engineering brains in the country contributed by the members.

On July 23, 1953, the NTSC formally petitioned the FCC for adoption of a set of color television standards.

In studying the NTSC system, you will find it easier to understand if you realize that it is not necessarily the simplest, nor .even the best, color system which could have been designed.

The NTSC system was developed to meet, on the one hand, a set of FCC requirements, and, on the other hand, the economic requirements which its originators felt were a necessity.

2. The quality of color reproduction had to be excellent. The pictures had to be well-defined and free from annoying defects such as line crawl, jitter, or prominent dot structure.

3. Pictures had to be sufficiently bright for adequate contrast under normal lighting conditions, and free from objectionable flicker.

4. Receivers had to be simple enough for the average person to operate, and cheap enough for the average person to buy.

5. The transmitting apparatus had to be reasonable in cost and simple enough to be operated by average station personnel.

6. The system could not be unduly susceptible to interference as compared to black-and-white.

7. The color signals had to be transmittable over existing and future relay facilities.

In addition to the above, the NTSC imposed upon itself other requirements, the most important of which was that the system had to be compatible.

A compatible color television signal is one that can be received by existing black-and-white receivers, in monochrome, without any changes or adjustments.

To accomplish these objectives, a number of new circuit techniques and electrical concepts were necessary. None of these new ideas are basically difficult, but at first they may appear strange and unfamiliar to the service technician.

To understand any color television system, some knowledge of color fundamentals is required:

Fig. 4 - The basic Maxwell triangle. N is the neutral or "white" point.

Fig. 5 - Diagram with artificial primaries, as used by color engineers.

Light is a form of radiant electromagnetic energy, just as is the familiar radio wave. Light waves are of much higher frequency than radio waves, and while radio-frequency energy is not perceptible to our senses, light produces sensation in the brain, via the eye.

Because the wavelengths of the visible frequencies are extremely short, it is not convenient to specify light wavelengths in meters. A much smaller unit, called the "millimicron," is commonly used. A millimicron is equal to one billionth of a meter and is written "mμ".

The frequency range, or spectrum, over which radiant energy is visible extends from 380 to 780 mμ. (See Fig. 1.)

Well, when viewing light of a single frequency, located anywhere within the visible spectrum, we experience a sensation referred to as "color." The particular "color" we see is dependent upon the frequency of the light.

For example, light of a wavelength of approximately 700 mμ is called red; if the frequency is around 550 mμ, it is green; if about 450 mμ, it is blue.

When the light is made up of approximately equal portions of red, green, and blue energy, we see white light. This can be demonstrated by passing white light through a prism, as shown in Fig. 2. As the white light passes through the prism, its individual components separate and produce a series of colors known as the spectrum.

It is apparent from the above that it is quite incorrect to refer to white light as "colorless," since it is made up of all "colors."

A pure color can be described by stating the frequency of its light and by stating its brightness or amplitude.

In nature, light of a pure color seldom occurs. In most cases, it is mixed with, or diluted by, white light. So, to describe the light which we commonly see, we must also state the extent to which the color is free from dilution by white light.

The total amount of light given off by the object determines its brightness. The term used in color television work for brightness is "luminance."

A colored object is either reddish, bluish, greenish, yellowish, etc., depending upon which color predominates. This dominant color or frequency is called "hue."

As stated, most colored objects have a principal or dominant "hue" which is diluted to some extent by white light. For example, if an object were pure red (that is, undiluted by white light), it would be 100% saturated, or have a purity of 1. A "pink" object would be specified as being red in "hue," but incompletely saturated, and its purity would be considerably less than 1. Note: Purity is expressed as a number between 0 and 1, and is the ratio of the intensity of the "dominant color" light to the total light intensity.

You will recall that when using water colors it is possible to mix two colors together and produce a third color. Carrying this idea further, it is possible to mix three selected colors, known as primary colors, and produce a very wide variety of other colors.

In modern printing, the three primary colors are deposited alongside one another in dots of varying size. These dots are so small that the eye blends them together, creating the same effect as the mixed water colors.

In the familiar Kodachrome transparency, three layers of differently colored filters are placed on top of one another. As the transparency is viewed, white light passes through each filter in succession. The eye sees the net result in a wide variety of colors.

Both color printing and color photography achieve their results by using the subtractive method.

In the subtractive method a desired color is obtained by removing a pre-determined amount of red, green, and blue from a source of white light. (You will recall that white light consists of equal amounts of the red, green, and blue primaries.)

By removing portions of the three primaries in varying degrees, we can obtain a wide variety of colors. (If you mix red and blue, for instance, the result is the mathematical difference between the red and blue primaries.)

In a color television receiver, the red, green, and blue light is produced on the face of the C-R tube. On the screen of the receiver the three primary colors are self-luminous - they create their own light and must be added together to produce the desired range of colors. The additive method of producing color is shown in Fig. 3. In this system, we start with the three primary colors (red, green, and blue) and by adding together the required amounts of each of the primaries, we can produce almost any desired color. In both the subtractive and additive systems, we vary the amounts of three primaries to produce the desired color.

You may wonder why red, green, and blue are always used. Actually, red, orange, and green or many other combinations of colors could be used. (Remember that red, yellow, and blue are the common subtractive primaries.) However, red and blue, located at opposite ends of the color spectrum, in combination with green, which is located at the center of the color spectrum (refer to Fig. 1) enable us to obtain the widest possible range of colors.

Years ago, a need for exact color specification became apparent. What one person referred to as "red" might possibly mean anyone of dozens of shades called "red", for example.

Because of these individual variations, tests were conducted with a large number of observers and the results were averaged out. Based on these tests, a set of color standards were adopted in 1931 by the International Commission on Illumination, to apply all over the world in all color activity.

These standards are known as the ICI system, after the commission which adopted them.

The standards specify that the red primary shall correspond to light of the wavelength 700 mμ, green as 546.1 mμ, and blue as 435.8 mμ.

We have seen that almost any desired color can be produced by mixing together appropriate portions of the three primary colors. Therefore, we can say that a particular color consists of so much red, so much green, and so much blue.

A convenient way of illustrating the exact amounts of the three primary colors which are present in a given color is by the triangle of Fig. 4.

Note that the center of gravity of the triangle is the neutral or white point (N) since this represents a "color" to which the three primaries are contributing equal amounts. As the described color departs from the neutral point, it takes on a definite, recognizable hue depending on whether the green, red, or blue frequencies predominate. Also the closer the described color point moves toward the sides of the triangle, the more saturated the color becomes. (Freer from dilution by white.)

When using this type of diagram (known as the Maxwell triangle), an exact color is specified by stating the proportion of each color to the total.

This is illustrated graphically in Fig. 4. Note that the proportion of red is indicated by the length of the line drawn perpendicularly from the side of the triangle opposite the red apex point. Similarly, the proportion of blue and green is specified by the perpendicular lines drawn from the sides of the sides of the triangle opposite the blue and green apex points.

Thus by knowing the numerical proportions of the three primaries to the total, we have sufficient information to reproduce any color. These proportions are always stated as a fraction of 1 to provide a standard method for color specification.

Since the total is always arbitrarily taken as 1, no information is furnished as to the real brightness of the colored object being described. If we were to reproduce a color using only the information given in the triangle, its color would be correct, but its brightness might be quite different from the original. Therefore, if we are to match the original colored object exactly, we must also be given information concerning the brightness. We see that we can specify the color of any object on the one hand, and its brightness as a separate quantity on the other. This principle is used in the NTSC color television system and will be described in the next article.

The triangle shown in Fig. 4 is seldom actually used. The basic reason for this is that it requires that three fractional proportions be specified while only two are necessary.

You will recall that the proportions are always stated as fractions of 1. If we know two of the proportions, the third can be found by simply adding the two and subtracting their sum from 1.

Since only two numerical quantities or proportions are required, it is more convenient to plot a color or "chromaticity" diagram in the form of a right triangle as shown in Fig. 5.

The three apex points of the triangle in Fig. 5, labeled X, Y, and Z, represent what can be considered imaginary primaries as compared to the real primary colors shown in Fig. 4. This is done for the sake of convenience in specifying all of the colors in the spectrum, including those which cannot be achieved by mixing the three standard ICI primaries.

Using the standard color diagram, a color is specified by stating its vertical (Y) and horizontal (X) co-ordinates. For example, the color C, corresponding to X = 0.6 and Y = 0.35, is seen to be a not quite fully saturated orange. In other words, color C consists of 60% imaginary red primary color X, 35% imaginary green primary color Y, and 5% (1.0 minus 60% plus 35%) imaginary blue primary color Z.

Using these figures, plus information on brightness, the skilled colorimetrist is able to specify any given color. The method is not simple, and uses rather involved mathematics, therefore it would be unprofitable to attempt to go further into the details at this point.

The three ICI primaries which are used in color television are marked on the XYZ diagram. The area enclosed by. the triangle connecting these points represents all of the colors which can be achieved with these primaries. Note that some of the highly saturated blues and greens, indicated by the shaded portion of the diagram, fall outside of this area and thus cannot be reproduced with the ICI primaries. These colors seldom occur in nature and very little is lost by not reproducing them in color television. The ICI primaries permit us to reproduce the saturated reds, yellows, and oranges, which are very common. With this compromise we can still obtain very pleasing color reproduction.

From the above, we see that it is possible to specify, or describe accurately the color of any object in terms of its hue (principal color) and saturation (freedom from dilution by white), by simply stating its X and Y coefficients on the color diagram. This in-formation, plus brightness or "luminance" information, tells us all we need.

We are all familiar with that peculiar behavior of the human eye known as persistence of vision. The brain's ability to retain an image for a fraction of a second after it has actually dis-appeared forms the basis of motion pictures and television.

Less commonly known is the fact that people become progressively more color-blind as the size of the object they are looking at gets smaller. Another way of stating this is to say that the color of a large area looks different than the same color confined to a small area.

Most of us have had the experience of selecting paint of a certain color from a small sample and perhaps being disappointed in the results when the entire room was painted. We are sometimes inclined to question the skill of the painter and tend to blame him for improper mixing of the color. In many cases, the actual colors are identical - it is simply the fact that the difference in size has the effect of producing a mismatch.

This color-blindness phenomenon follows a definite pattern which has been established as a result of extensive tests conducted with a great many people having normal color vision. The NTSC color television system takes advantage of this fact by transmitting only that amount of color information that can be appreciated by the eye. Full color information is transmitted for large-area portions of the picture; restricted color information is supplied for smaller areas of the picture, and only brightness information (no color at all) is furnished for the tiny areas (fine detail) of the picture.

This results in a substantial reduction in the bandwidth necessary to transmit a color television picture, and is one of the reasons it was possible to sandwich the color signal into the existing 6-mc channels. (To Be Continued)

In the next article we will discuss the basic requirements of a color television system, and how the peculiarities of color vision have been used to advantage in the design of a compatible system utilizing a 6-mc transmission channel.

Basic Color TV Part IJanuary 1954 Radio-Electronics

Basic Color TV Part I
January 1954 Radio-Electronics