Television digest with AM-FM reports (Jan-Dec 1950)

tions there was a most noticeable line crawl or jitter, which seriously marred the quality of the transmitted picture. The line crawl apparently results from the fact that there is a relatively low line repetition rate in each of the primaries and the eye thus tends to follow each line down. As was pointed out above, line crawl is inherent in a line sequential system where the shift in initiation of colors is uniform. The purpose of the various shifts developed by CTI w'as to eliminate the problem, but they did not accomplish this purpose. Although the last shift demonstrated— the interlaced shift — may have reduced line crawl somewhat, it still remains to a prominent degree. This line crawl or jitter was noticeable even with the very low level of picture illumination that CTI was able to produce on its projection receiver. Since line crawl is a flicker phenomenon, it can be expected to be intensifled with increased brightness. This turned out to be the case at the San Francisco demonstration where the CTI color transmissions utilizing the interlaced shift were received on a 3-tube, dichroic mirror type of receiver. This receiver produced higher brightnesses than the other CTI receivers and, as was to be expected, the line crawd was more noticeable. It can be expected that the development of tubes with long persistence phosphors would reduce the line crawl but to what extent is not known since no demonstration has been held and line crawl may very w'ell be an inherent defect of the CTI line sequential system. Moreover, it should be noted that if the phosphor was of long enough persistence to eliminate the problem of line crawl, the effect on color fldelity and the portrayal of objects in motion would have to be carefully evaluated. 64. So far as continuity of motion is concerned, the CTI system produces no more than 10 color pictures per second. No problem was evident at the demonstrations as to continuity of motion but such demonstrations involved only pictures of low illumination. 3. The CBS System 65. The CBS system has a field repetition rate of 144 per second. However, the primary color repetition rate is 48 per second. It is this latter figure that is utilized in assessing the susceptibility of the system to large area flicker in color pictures. Why this is so can be illustrated by the situation where a scene is being scanned that has large areas of gi’een in it — or any other single primary. When the field containing the green components is on the screen, the tube will be illuminated. The tube face will be relatively dark thereafter until the green field appears again in 1/48 of a second. Thus, in effect there is a repetition rate of 48 per second so far as flicker is concerned under the circumstances described above.’" 66. CBS testified that the flicker in its disc type receiver became noticeable at 24 foot lamberts ” (for a 7 to 1 viewing distance) and that flicker did not become objectionable until a higher figure of brightness was reached. Flicker is a physiological phenomenon that varies from person to person and hence is not capable of exact measurement. However, on the basis of the record, the Commission is able to find that flicker is not objectionable on the CBS disc type receiver up to highlight brightnesses of from 20 to 30 foot lamberts. The present black and white television system with a field rate of 60 has a flicker threshhold many times that of the 24 foot lambert figure mentioned above, although there is serious doubt as to whether such highlight brightnesses are used by the public.'* If When a black and white picture is shown, the susceptibility to flicker would probably be less than for a color picture. I'A foot lambert is the unit for measuring the brightness of light reflected from a surface. i^The only testimony on this subject was given by John V. L. Hogan, Vice-Chairman of JTAC, testifying as a witness for CBS, and T. T. Goldsmith of DuMont. Hogan’s testimony was based upon a survey in stores of 75 new receivers of various makes which showed that only 0.7 of 1% of such sets had a highlight brightness reading of between 28 and 30 foot lamberts; the largest percentage was between 18 and 20 foot lamberts; and the average was 17. Goldsmith testified on direct examination that he had measured DuMont receivers and that 120 foot lamberts were typical. On cross examination, he testified that the average was 50 foot lamberts. The Commission does not believe that there is sufficient evidence in this record upon which to base a finding as to the level of brightnesses at which sets are operated in the home. Nor tubes with long persistence phosphors were utilized, it would be possible to increase brightness several fold wdth no flicker problem. A CBS witness testified that brightness could be increased to more than 100 foot lamberts without flicker, but it is not possible to predict the exact extent of such improvement without further testing. It should be noted, however, that there is a limitation on the use of very long persistence phosphors with a disc type receiver. To illustrate, when a field is scanned in red, the red filter is in front of the tube. In the next 1/144 of a second, the blue filter is in front of the tube. If there is still any substantial illumination left, it will merge with the illumination from the blue signal. 67. No problem of small area flicker was observed at any of the demonstrations on the record. However, if horizontal interlace were utilized, a certain amount of small area flicker might appear which can best be described as dot motion or twinkle. Since it is a flicker phenomenon, tubes with long persistence phosphors should minimize the problem. The magnitude of this flicker cannot be ascertained without further study. 68. As to continuity of motion, no problem was experienced at any of the demonstrations on the record. 4. The RCA System 69. The RCA system has 60 fields per second and its performance as to large area flicker is the same as the present system, so far as black and white pictures are concerned. So far as color pictures are concerned, insufficient evidence was offered as to whether the flicker characteristics are the same as for black and white pictures. There was no flicker observed at any of the demonstrations on the record, but it should be noted that the pictures had very low illumination. 70. At the demonstrations on the record small area flicker in the form of dot motion or twinkle has been observed. How serious a problem this is cannot be entirely ascertained at this time since the RCA demonstrations produced only a dim picture. With increase in brightness of the picture to the level necessary for home use, the effect v.dll be accentuated, although it is possible that this effect may be overcome or minimized by the utilization of tubes with long persistence phosphors. The extent of such improvement cannot be determined without further testing. If the phosphor is of long enough persistence to reduce substantially the above defects, the effect on color fidelity and the portrayal of objects in motion would have to be carefully evaluated. 71. As to continuity of motion, no problem was observed at any of the demonstrations on the record with the low level of illumination present in the picture. C. Brightness-Contrast 1. General 72. Under the preceding heading we discussed the problem of brightness as it related to the question of flicker. In this part brightness will be considered from the point of view of adequacy — is the picture bright enough so as to give a sufficient contrast range “ and so as to be capable of being viewed under normal home viewing conditions? There is no precise ratio for satisfactory contrast; it is a matter of choice with the individual viewer. In general, the wider the contrast range the better, since there is more flexibility for reproducing shades of gray in black and white pictures and shadings of color in a color picture. Based upon the demonstrations on the record, a contrast ratio of 30 to 1 for color pictures produces a very satisfactory picture.''’ However, with receivers operating in is it necessary that we do so because, as will be pointed out below (Paragraph 74) 20 to 30 foot lamberts are sufficient for disc type receivers and long persistence phosphors make higher brightnesses without flicker possible on other types of receivers. The contrast range is determined by the ratio of brightness between the brightest and darkest portions of a picture. The brightest portion of the picture is determined by the amount of illumination which is produced by the scanning beam. The darkest portion is determined by the amount of light which the tube surface will reflect; the light in general comes from illumination in the room where the picture is being viewed. So far as black and white pictures are concerned, it is possible that a higher contrast range is required since color by its nature gives contrast to a picture. 10