Chromatron

The Chromatron is a color television cathode ray tube design invented by Nobel prize-winner Ernest Lawrence and developed commercially by Paramount Pictures, Sony, Litton Industries and others. The Chromatron offered brighter images than conventional color television systems using a shadow mask, but a host of development problems kept it from being widely used in spite of years of development. Sony eventually abandoned it in favor of their famous Trinitron system using an aperture grille.

History

Color television

Color television had been studied even before commercial broadcasting became common, but it was only in the late 1940s that the problem was seriously considered. At the time, a number of systems were being proposed that used separate red, green and blue signals (RGB), broadcast in succession. Most systems broadcast entire frames in sequence, with a colored filter (or "Color gel") that rotated in front of an otherwise conventional black and white television tube. Because they broadcast separate signals for the different colors, all of these systems were incompatible with existing black and white sets. Another problem was that the mechanical filter made them flicker unless very high refresh rates were used. In spite of these problems, the US Federal Communications Commission (FCC) selected a sequential-frame 144 frame/s standard from CBS as their color broadcast standard in 1950. RCA worked along different lines entirely, using the luminance-chrominance system. This system did not directly encode or transmit the RGB signals; instead, it combined these colors into one overall brightness figure, the "luminance". Luminance closely matched the black and white signal of existing broadcasts, allowing it to be displayed on existing televisions. This was a major advantage over the mechanical systems being proposed by other groups. Color information was then separately encoded and folded into the signal as a high-frequency modification to produce a composite video signal – on a black and white television this extra information would be seen as a slight randomization of the image intensity and just appear blurry, but the limited resolution of existing sets made this invisible in practice. On color sets, the signal would be extracted, decoded back into RGB, and displayed. Although RCA's system had enormous benefits, it had not been successfully developed because it was difficult to produce suitable display tubes. Black and white TVs used a continuous signal and the tube could be coated with an even painting of phosphor. With RCA's system, the color was changing continually along the line, which was far too fast for any sort of mechanical filter to follow. Instead, the phosphor had to be broken down into a discrete pattern of colored spots. Focusing the right signal on each of these tiny spots was beyond the capability of electron guns of the era. RCA's early experiments used three-tube projectors, or mirror-based systems known as "Triniscope".

Shadow masks

RCA eventually solved the problem of displaying the color images with their introduction of the shadow mask. The shadow mask consists of a thin sheet of aluminum with tiny holes photo etched into it, placed just behind the front surface of the picture tube. Three guns, arranged in a triangle, were all aimed at the holes. Stray electrons at the edge of the beam were cut off by the mask, creating a sharply focused spot that was small enough to hit a single colored phosphor on the screen. Since each of the guns aimed at the hole from a slightly different angle, the spots of phosphor on the tube could be separated slightly to prevent overlap. The disadvantage of this approach was that for any given amount of gun power, the shadow mask filtered out the majority of the signal. To ensure there was no overlap of the signal on the screen, the dots had to be separated and covered perhaps 25% of its surface. This led to very dim images, requiring much greater power in order to provide a useful picture. Moreover, the system was highly dependent on the relative angles of the beams between the three guns, which required constant adjustment by the user to ensure the guns hit the correct colors. In spite of this, the technical superiority of the RCA system was overwhelming compared to the CBS system, and was selected as the new NTSC standard in 1953. The first broadcast using the new standard occurred on New Year's Day in 1954, when NBC broadcast the Tournament of Roses Parade. In spite of this early start, only a few years after regularly scheduled television broadcasting had begun, consumer uptake of color televisions was very slow to start. The dim images, constant adjustments and high costs had kept them in a niche of their own. Low consumer acceptance led to a lack of color programming, further reducing the demand for the sets in a chicken or the egg situation. In the United States in 1960, only 1 color set was sold for every 50 sets sold in total.

Commercial developments

The University of California, Berkeley set up "Chromatic Television Laboratories" to commercially develop the system, in partnership with Paramount Pictures who provided development funding. They started producing the PDF 22-4 22-inch prototype tubes in 1952 and 1953, with a display area of 14 by 11 inches. In practice the design proved to have serious problems. Since the focusing system had to quickly move the beam to generate the correct colors, very high voltages and powers had to be used, leading to arcing problems and radio frequency (RF) noise. Around 50 watts of RF power had to be applied to the wires to deflect the electron beam, directing it to the right colored stripe. The latter was particularly annoying when used as the basis of a television, as the noise interfered with the radio receivers that picked up the broadcasts. The university eventually abandoned their interest in Chromatron, but Paramount continued development as a system for displaying film during editing, which meant that the RF noise did not present a problem. Development was still continuing in the early 1960s when their work was bought by Sony. In spite of these problems, the promise of the Chromatron system was so great that a number of companies continued development of the system throughout the 1950s. The radiation problem was solved by placing the wires further back from the phosphor stripes which reduced the capacitance and significantly reduced the driving power requirements. Later the wire diameters were reduced to accommodate the increased phosphor pitch. In addition, the output of the three video amplifiers switched the color information to hit each RGB stripe at the precise moments required. In other words, a keyed operation of the three color stages. Later, the General Colornetron adopted this approach. Successor Chromatron tubes experimented with different color phosphors to obtain the correct combination of brightness and persistence. Finally a rather large copper box or cage was mounted externally to the bell to reduce the remaining radiation from the grid coil. No interference was observed and the pictures produced were described as fairly decent and viewable with no visible streaking or striping. The Chromatron design was also licensed for a variety of other uses; Litton Industries used the Chromatron with a two-color display (blue-red) as the basis for an Identification Friend or Foe system.

Limited sales

Prior to sale of the first Trinitron televisions in the United States, a limited number of 7-inch Chromatrons were built and offered for sale in the United States starting in April 1968 as the KV 7010U. These were replaced about three months later by the KV 7010UA Trinitron tube. The Sony KV 7010U CRT used the newly invented Trinitron gun combined with the Chromatron PDA wire grid instead of a shadow mask or aperture grill.

Description

The basic concept that defined the Chromatron was the near-screen focusing system, which provided the beam resolution needed to accurately hit the individual colored phosphor strips. The grid both focused the signal as well as guided it to the correct colors. The phosphors were silk screened onto the back of the tube in strips 2 mils wide with 2 mil wide gaps between them, and then coated in aluminum to make the screen conductive. Since the grid had to be charged to relatively high voltages, the aluminum coating was fairly thick, which dimmed the image to some extent. The phosphors were patterned in an RGB-BGR-RGB pattern. The focusing grid was aligned so the beam would normally focus down onto the green strips in the middle of each pair of wires. To produce different colors, say blue, the beam would have to be pulled to the right for one pixel, and then to the left for the next. Since the adjacent stripes of phosphors shared one of the wires, this meant that a single voltage setting would produce the blue color on two adjacent pixels. Since a single frame of color television does not consist of a single color, the deflection system had to be continually varied as the beam moved across the screen.

References

Citations

Bibliography

John Nathan, "Sony: The Private Life", Houghton Mifflin Harcourt, 2001, Sergey Shewchuck, "Summary of Research Progress Meeting of September 27, 1951", UCRL-1563, Radiation Laboratory, University of California, 14 November 1951, pp. 2–4 Milton Kaufman and Harry Thomas, "Introduction to Color TV", Taylor & Francis Edward W. Herold, "History and development of the color picture tube", Proceedings of the Society of Information Display, Volume 15 Issue 4 (August 1974), pp. 141–149

Patents

Cathode Ray Focusing Apparatus", Ernst O. Lawrence, University of California/Chromatic Television Laboratories (original Chromatron patent) "Symmetrical to Asymmetrical Signal Conversion Circuit", Hoffman Electronics "High Voltage Power Supply System for Cathode Ray Tubes Employing Protective Time Delay Means", Sony

Names in other languages