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MECA Electronics Attenuators

Color TV
October 1953 Radio & Television News Article

October 1953 Radio & Television News
October 1953 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919 - 1959. All copyrights hereby acknowledged.

If the technology futurists of the 1970s had been correct, by now we would be reading historical articles on the early days of holographic television. I have read, though, that any day now a battery-powered flying car with a holographic dashboard / instrument panel display is going on sale. Not. The Sadly, the Ercoupe (pronounced "air coupe," as in automobile coupe of the air) is still the closest thing to "an airplane in every garage." But, I digress. This story from a 1953 issue of Radio & Television News magazine reports on the roll-out of the country's first color television system: The field sequential system (aka the CBS system). It was a fine system, but unfortunately the modulated signal format was not backward compatible with the existing black and white (B&W) system. That meant separate receivers for B&W and color televisions. Even as CBS TV sets were being produced, the National Television System Committee (NTSC) was working on a replacement system that permits co-existence of B&W and color signals. Note the mention of how the DPA (Defense Production Administrator), in 1951, requested that color TV manufacturers halt research and development on the CBS system in order to free engineers for defense-related design and production hardware, it being the death knell of CBS and clearing the way for NTSC (Korean War).

Color TV - Part 1: Clarifying the color TV situation with some predictions on what to expect in color receivers.

Experimental single-tube color TV camera - RF Cafe

Fig. 1. An experimental single-tube color TV camera, not much larger than its black-and-white counterpart.

By William R. Feingold,
Emerson Radio & Phonograph Corporation

On October 11, 1950, the Federal Communications Commission approved the field sequential system (popularly known as the CBS system) as the official color TV standard for the United States. This system had been under development for approximately 10 years, and gave a fairly presentable performance in comparison with competitive methods which were more or less newly conceived. It was an unfortunate choice, however, because the field sequential color system required a vertical field rate of 144 per second (as opposed to the black and white of 60) and a line rate of 29,160 cycles-per-second (as against the black and white of 15,750) to eliminate flicker, thereby making this system incompatible with black and white transmission standards. Finally, when the DPA terminated all color TV activity on November 20, 1951 in the interest of national defense, the CBS system, as used in aerial TV transmissions, died a natural death. For strictly industrial television use, the CBS system is still being employed in closed circuit applications.

In the meantime, the Radio-Electronics-Television Manufacturers Association set up a committee to formulate an improved compatible color signal. The more than 200 engineers and physicists of this National Television System Committee (NTSC) from 91 leading companies in the television industry formulated and tested a color TV signal which could fit into the present 6 mc. channel, and was compatible with the black and white transmission standards.

The NTSC does not own any equipment, neither transmitters nor receivers, and is not interested in the detailed circuitry of either type of equipment. Its interest in equipment stops at the point where it has been established that the signal specifications can be met with readily available gear. The end goal is a set of signal specifications, proven and practical, that can be presented to the FCC for approval. That this goal has been met has been amply attested to by the fact that numerous organized field tests have been arranged by the NTSC and attended by some 15 different manufacturers with their color receivers. These receivers represent the varied outputs of the different engineers all working from the same signal specifications and all receivers are producing excellent pictures.

The NTSC Signal

Although a detailed treatment of this new color signal will be made in the second article of this series, a simplified treatment is in order.

From a welter of data pertaining to the physiological aspects of color vision and a mass of theoretical data regarding the character of the television signal itself, the following color signal has been formulated. To the present monochrome standards as they now exist an additional color signal specification has been added. The resultant effect is not unlike the conventional lithographic technique of printing in three colors plus black to add the detail. In our case, the present monochrome information carries the shades of black and white including all the fine detail, and the color information is added on a color sub carrier to fill in the large areas of color (Fig. 2).

Tests have shown that the eye cannot perceive fine color detail, hence, there is no need to burden the color circuits with wide-band information. The shaded area on Fig. 2 indicates that this color information in the lower sideband is restricted to a bandwidth of 1.3 mc. Note, too, that the upper sideband cannot extend this far since the limits of the channel restrict this area to approximately 0.4 mc. Although this unsymmetrical distribution of sideband energy is not a desirable situation, it has been possible to design the details of the system in such a manner that it causes no extra trouble.

Color signal characteristics - RF Cafe

Fig. 2. Color signal characteristics.

Because the lower sideband of the color information falls well within the monochrome video channel it was necessary, in the interest of compatibility, that this color data be made invisible on a standard black and white receiver. This was done (within the limitations of the linearity of the system) by setting the color subcarrier at a frequency which is an odd multiple of half the line repetition rate. The actual frequency selected by the NTSC is 3.58 mc. This unique feature of adding narrow-band color information on a special color subcarrier to a standard monochrome transmission is the essential characteristic of the signal. When a color signal is transmitted, the conventional monochrome receiver will present the picture in shades of black, gray, and white with a negligible trace of the picture's color signal origin. An interesting point is that these black and white pictures usually have better resolution than that obtained from conventional monochrome reception. The reason for this improvement is simply that the transmitters have to be more carefully adjusted to handle the color data on the 3.58 mc. subcarrier and, as a result, the monochrome information is present in more detail.

The conversion of a good monochrome transmitter from black and white to color is simplicity itself. (See Fig. 3.) If a color video signal is already available, either from a color camera or a network link, no changes are required. To get this video information from a network a minor investment in new terminal equipment will be required. Networks will probably be the main source of nationwide color transmissions until a sufficient number of color studios are constructed.

The color studio gear and the camera equipment are somewhat more complicated than their monochrome counterparts. Present color cameras consist of three pickup tubes mounted side by side with each one masked with a proper primary filter. By the use of properly positioned dichroic mirrors (mirrors which reflect light of one color and pass all other colors) the single viewed image of one lens is made to fall on each of the three photosensitive camera surfaces. Since these images must pass through green, red, and blue filters respectively before they strike these surfaces, the three resultant video outputs. represent the green, red, and blue signals corresponding to these colors in the original scene.

Progress has already been made toward the development of a single camera pickup tube that will put out three primary signals (Fig. 1). There is no doubt that technical advances in studio equipment will be made towards simplification. In this connec­tion it is interesting to note that the compatible nature of the signal allows for testing transmitting and receiving equipment by radiating color signals (with FCC approval) without public announcements of the fact. One of the first such "sneak" transmissions took place late in June in New York City on WNBT during a "Howdy Doody" program.

The Color Receiver

The color receiver is basically a monochrome receiver with additional circuitry. This additional circuitry falls into two groups. The first group is that part required by the color information alone. In Fig. 4 this area is covered by the chroma, decoding. color sync, and matrix networks. This part produces as its end product the green, red, and blue video signals. The second group is that area of circuitry dictated by the requirements of the picture tube (or display device). Since Fig. 4 indicates an RCA tri-color tube, a dynamic convergence network is used, as required by this tube.

Although the interior of the color receiver is somewhat more complex than its monochrome brother, the user's controls are only complicated by the addition of one more knob. This control is marked "Chroma." It allows a customer who takes issue with the mathematically correct ratio of color to monochrome to vary this ratio. The picture color can thus be varied from a light pastel shading to an intense lush overly-colored display.

With regard to the fine tuning control, the public will have to be reeducated. It will be recalled that split-sound receivers of five years ago required a careful setting of the fine tuning control or there was no sound. This careful setting of the tuner oscillator made for optimum picture quality. However, the problem of oscillator drift gave way to the use of intercarrier systems with the net result that sound was always present and tuning the oscillator to obtain the best picture became a forgotten operation.

In order that the color receiver retain the benefits of intercarrier operation and yet force the customer to adjust the tuner oscillator to its proper place to insure good color quality, the designer is forced to use some sort of tuning indicator. In this case what could be better than the face of the picture tube? The sound traps in the receiver are made sharp and deep so that when the oscillator is properly tuned the picture is clean. When the oscillator is mistuned the picture will show annoying sound patterns.

Color TV signal generating equipment used by Emerson - RF Cafe

Color TV signal generating equipment used by Emerson for the design and testing of color TV receivers. Included is a monoscope and flying-spot scanner.

The most publicized aspect of the color TV receiver has been the picture tube. All color tubes presently used or being developed have three color phosphors deposited on the front face of the picture tube in either a dot pattern array or a striped pattern, horizontal or vertical. The RCA tri-color tube" utilizing three gun structures is typical of the former type, while the Lawrence tubes as made by Chromatic Laboratories, containing a single gun, is typical of the latter.

The RCA picture tube contains a phosphor dot pattern consisting of 195,000 dots in each of three colors for a total of 585,000 dots. The three guns are so arranged that each gun will excite only its particular phosphor color. As a consequence it is possible and desirable to excite the three guns simultaneously with their respective color signals and thereby have a simultaneous light output in red, green, and blue. This design does not require any form of sequential color switching and provides a maximum light output roughly three times that which would be available on any sequential system using this tube. The use of three guns carries with it some severe mechanical and electronic circuit requirements. The first is the specification that each gun strike its respective phosphor without contamination from the other two. The second is the problem of registration of the three colored pictures. Improvements in the production control of the picture tube and in the electronic circuitry associated with the tube have reduced both problems to an acceptable level.

The Lawrence tube contains a series of red, green, and blue stripes approximately 0.015 inch wide with a built-in switching grid to allow the single electron beam to scan anyone color depending on the switching potential present. Because of the single gun construction, a sequential display is essential. This means that this tube cannot suffer from any registration problems. However, it does suffer from a light output loss of two-thirds because only one phosphor is in use at one time. There is the additional requirement of substantial switching energy to the switching electrodes.

The test receivers used by the various manufacturers during NTSC field trials have utilized the RCA tri-color tube exclusively. Although limited numbers of the Lawrence tube have been released to the industry, a comparative appraisal is not possible at this time. It is probable that the first production color receivers will contain the RCA tube.

It should be quite obvious by now that the color receiver will be somewhat more complex and considerably more expensive than the present monochrome receiver. The picture tube alone is expected to cost from 150 to 200 dollars. The tube complement of the receiver will fall between 40 and 50 tubes. These two factors alone make a cost estimate of the first color receivers fall in the $750 to $1000 class. In addition, these receivers will produce a small picture, judged by present-day standards. The most successful RCA tri-color tube developed to date contains an exterior shell similar to the old 16AP4 and produces a 12 1/2" pumpkin-type picture. Although intensive developmental work is now going on toward a 16" picture there is no indication when this larger tube will be available or how much it will cost.

Receiver Production

Simplified block diagram of a typical color television transmitter - RF Cafe

Fig. 3. Simplified block diagram of a typical color television transmitter.

Simplified block diagram of a typical color television receiver - RF Cafe

Fig. 4. Simplified block diagram of a typical color television receiver.

Obviously there will be no rush to buy the first color receivers. Very few prospective customers will buy a 12 1/2" color picture at $800 in preference to a 21" monochrome picture at $300. However, the novelty is expected to appeal to some, and first production schedules will cater to this rather meager demand.

How soon manufacturers will put color receivers in the field, after FCC approval, is entirely up to the manufacturer himself. There will probably not be a repeat of the 630TS experience where RCA released complete data on this black and white receiver to their licensees, covering chassis layout, component specifications, alignment procedure, troubleshooting, etc. It will be necessary for each manufacturer to build up a nucleus of engineers familiar with the color TV problem. Fortunately, a number of companies have already done this and have also accumulated a fair amount of specialized test and alignment equipment.

The next step, however, is actual production. The single bottleneck here is the test and alignment equipment for line use. Delivery of this material, from only a few sources, is quoted at from six months to a year. Realizing this, some manufacturers have already placed orders for production test equipment prior to FCC approval. Assuming FCC approval early in 1954, it is safe to say that the first color receiver will be released in the late spring of 1954 with a number of manufacturers in the field by the fall.

The Service Technician

The conscientious service technician who understands the workings of a black and white receiver will not fear the complexities of a color receiver. The transition is not as dramatic as his previous transition from a 5-tube radio to a 25-tube TV receiver. The additional 15 to 25 tubes required by a color receiver are, in some circuits, just more of the same. However, in other circuits there are new concepts, an understanding of which is essential to the proper diagnosis of trouble. At the present stage of color receiver development, color adjustments are extremely critical, requiring a broad knowledge of color circuitry. Service technicians will have to study such circuits conscientiously to be successful in this field.

At this point the writer would like to utter a small prayer on the service technician's behalf. Knowing that more service test and alignment equipment will be required in working with a color receiver let us hope that the designers of this test gear (it has not yet been designed) will keep down the size and weight. In the same breath let us pray that the designers of these new color receivers allow for troubleshooting in the home and in the cabinet. These larger chassis are considerably heavier than black and white chassis with half their tube complement. A combination of lighter test gear and a chassis which could be "troubleshot" in the cabinet will make for fewer ruptured service technicians, fewer chassis dropped down the stairs, and what is not a negligible factor in service technician-customer relations, smaller service bills.

This article has attempted to dispel the fog that has shrouded the color TV picture. Considerable progress has been made in the formulation of an improved compatible color signal. Two applications for a "Change of Rule" have been filed with the FCC. The first was made on June 25, 1953 by RCA, and the second by the NTSC one month later on July 23, 1953. In each case a request was made to have the present monochrome standards amended to provide for color TV transmissions in accordance with the new NTSC signal. FCC approval is expected by the first of the year or shortly thereafter.

Receiver production should start slowly in the fall of 1954 with a price range of $750 to $1000. This price range is expected to fall as developmental work continues. One laboratory has already produced an experimental receiver which requires. only 34 tubes.

Part 2 of this series will discuss the color signal in detail and examine a typical color receiver.

References

1. Buchsbaum, Walter H.: "The RCA Tri-color Tube," Radio & Television News, November 1951.

2. Dressler, Robert,: "The PDF Chromatron - A Single or Multi-Gun Tri-Color Cathode-Ray Tube," Proceedings of the IRE, July 1953.

(To be continued)

 

 

Posted December 3, 2018

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