International projectionist (Jan-Dec 1954)

Color Is Catalyst in Battle of the Tubes 24-inch color television tube is already here. So claim the engineers who developed the unpublicized Chromatron. RCA and CBS vie in race for public favor as new medium gets FCC blessing By FREDERICK HODGSON COLOR TELEVISION has arrived officially — and with it comes the start of one of the most exciting races for preferred position since land-hungry settlers in the Old West careened across the plains in the sprint for the Cherokee Strip. The fun began on December 17 when word was flashed from Washington that the Federal Communications Commission had formally approved colorcasting under standards proposed by the National Television System Committee, an all industry group now well known by its initials, NTSC. Thte new colorcast standards, replacing a previous set approved back in 1950 at the behest of the Columbia Broadcasting System with its now outlawed (except for closed circuit colorcasts) field-sequential system, permits reception of color programs on standard black-and-white sets. In this article we are concerned principally with the "Battle of the Tubes," with the receiving end of color television rather than with the now satisfactorily settled issue of how to put the show on the air. This latter PHOSPHOR SCREEN METAL SHELL COLOR GRID POST-DEFLECTION VOLTAGE GUN VOLTAGE 5 KV + — 13 KV This !s a cross section of the Lawrence, singlegun Chromatron tube. Note how the electron stream inside the shell is deflected by the coil magnetic yoke. Note, too, the placing of the wires of the color grid in relation to the phosphor face plate. The 13 KV post deflection voltage varies. matter, of great importance to projectionists because of the increasing use of film both for broadcast and for rear projection in live shows, will be dealt with briefly later on in this piece and will be the subject of future IP attention. The NTSC standards, now a part of the law of the land via FCC regulations, demand that any color television system be compatible, in other words that the 27,000,000 black-and-white sets now in use should not be rendered obsolete. The three types of tubes to be discussed here can receive in either black-and-white or color, a simple clockwise turn of the chroma dial on the receiving set changing a program broadcast in color from the familiar blacks, whites and grays to all the hues of the rainbow. Three picture cathode tubes, or kinescopes, are major entries in the color television sweepstakes. They are RCA's tri-dot, three-gun kinescope, the one-gun Lawrence tube, also known as the Chromatron, and the three-gun CBS-Hytron, or Colortron. Engineers of the three developing companies quite naturally claim very special advantages for their respective brain children. Let's look at this trio of entries in the contest of the cathodes, starting with a brief description of the RCA receiving tube. This kinescope was discussed in some detail by James Morris in an article on "Color TV . . . and How it Works!" in IP for September, 1953. Basically the Same Basically, the three kinescopes are the same. The differences, however, are of extreme importance because they will determine the winner, if any, in the race for public favor — and because the differences will largely determine how much you'll pay for a color TV set and how big a picture you will see. Each of the tubes is of the vacuum type employing one or more cathodes, or "guns," to fire one or more electron Dr. Paul K. Weimer.. RCA engineer, is shown holding an experimental single tube unit for the taking camera in the RCA tri-color television broadcasting system. Called an iconoscope, or orthicon, the tube Is expected to do the work of the three tubes now used. The CBS system uses a single tube for broadcasting color, separating the three colors by means of a color wheel. Circuitry for the single tube is simpler than for three guns, according to both CBS and RCA engineers. beams at a phosphor plate. In the black-and-white phosphor plates the electrons, hitting the phosphor dots at high speed, cause these dots to glow with varying brilliancy. Hence you are able to see a picture. In color television the chemical composition of the dots has been changed so that they glow in color, also with varying brilliancy. In the case of the Lawrence tube, or Chromatron, there are no dots, phosphor strips are used instead of the dots. Mask versue Grid The electron beams, or cathode rays, carry the color and picture information and, in accordance with NTSC demands, utilize black-and-white scanning standards, 520 lines at 30 cycles per second. Two of the kinescopes, the RCA tube and the CBS-Hytron, use aperature masks, each perforation positioned directly behind a phosphor dot. Electrons stream through the tiny holes of the plate, strike the proper dots and so produce the color picture. The Lawrence tube, using but one gun, actually bends the beams by means of an electrostatic "lens," a charged wire grid placed just back of the phosphor face plate. This "lens" system eliminates the perforated shadow mask. The Lawrence tube differs from the RCA and CBS-Hytron in other ways as well. For example, the distance from the cathode to the phosphor plate 14 INTERNATIONAL PROJECTIONIST • JANUARY 1954