Improved Television Systems: NTSC and Beyond
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• Improved Television Systems: NTSC and Beyond By William F. Schreiber After a discussion ofthe limits to received image quality in NTSC and a excellent results. Demonstrations review of various proposals for improvement, it is concluded that the have been made showing good motion current system is capable ofsignificant increase in spatial and temporal rendition with very few frames per resolution. and that most of these improvements can be made in a second,2 elimination of interline flick er by up-conversion, 3 and improved compatible manner. Newly designed systems,for the sake ofmaximum separation of luminance and chromi utilization of channel capacity. should use many of the techniques nance by means of comb tilters. ~ proposedfor improving NTSC. such as high-rate cameras and displays, No doubt the most important ele but should use the component. rather than composite, technique for ment in creating interest in this sub color multiplexing. A preference is expressed for noncompatible new ject was the demonstration of the Jap systems, both for increased design flexibility and on the basis oflikely anese high-definition television consumer behaL'ior. Some sample systems are described that achieve system in 1981, a development that very high quality in the present 6-MHz channels, full "HDTV" at the took more than ten years.5 Orches CCIR rate of 216 Mbits/sec, or "better-than-35mm" at about 500 trated by NHK, with contributions Mbits/sec. Possibilities for even higher efficiency using motion compen from many Japanese companies, im sation are described. ages have been produced that are comparable to 35mm theater quality. The noncompatibility of that system, he existing television broadcast ing much of the following discussion and the very large required band T ing systems - NTSC, PAL, and were dealt with in an earlier paper and width, have sparked developments in SECAM - have spawned large and will only be summarized here. I a number of other countries, mostly profitable industries; indeed they may directed,·to improvements that would fairly be said to have had a profound Why Now? be easier to implement.6 influence on modern society. Never As equipment has improved and Limits to Quality in Present-Day theless, they are far from perfect. professional viewers have become Most of the knowledge exists to im more critical, the shortcomings of Television prove picture and sound quality so as NTSCa have become more obvious, at The image quality actually ob to equal or exceed that of 35mm mo least to TV practitioners. What they served on home receivers is affected tion pictures. If we want it enough. we accepted 30 years ago. in the full flush by every physical element of the sys can have higher resolution. better mo of the successful design ofa color sys tem, as well as by the system design. tion rendition, a wider field of view. tem that did not obsolete existing re Since it is spectrum space that is the worldwide compatibility, and free ceivers, is no longer acceptable. There limited resource, an argument can be dom from most defects. certainly is no grass-roots demand for made to ensure that only the band Many, if not most, of the issues improvements, and there is little evi width limit the quality. As a practical involved in improving existing sys dence that these perceived shortcom matter. this is not entirely possible. tems or introducing entirely new ones, ings are significant to the commercial However, the extent to which other, are primarily political or economic health of TV. Nevertheless. there correctable. factors degrade the qual and not technological. The main so may well be commercial opportuni ity at present is much larger than cial value of a television system. eco ties in the provision of significantly seems reasonable. nomics aside, certainly resides in the improved television services. programming and not the image qual Rapid progress in semiconductor Interlace and Line Structure ity. Interesting as these issues are. development, driven primarily by the The choice of lines/frame and they lie outside the subjcct of this pa demands of the computer industry, frames/sec for a given bandwidth per. We are concerned here exclusive has led to more powerful chips and trades offspatial and temporal resolu ly with means by which sound and lower prices. This presents the possi tion. All modern TV systems use 2: 1 picture quality can be improved. and. bility of much more "intelligent" re interlace in order to raise the large to a lesser extent, with the'means by ceivers and more efficient use ofchan area flicker rate that results from this which such improvements can be nel capacity. The existence of such trade-off. Alternatively, the use of in brought to the marketplace. capability. and its utilization in relat terlace can be considered an attempt The psychophysical phenomena ed areas. creates a pressure to make to raise the vertical resolution for a and signal-processing ideas underly- applications to TV, as exemplified by given field rate and number of lines/ digital receivers. field. The effectiveness of this strata ~resenled althe 21st Annual S\1PTE TeleviSIOn Con Another factor creating a favorable gem is. at best, limited. 7 Interline ference in San Francisco (paper NO.2 1- 21) on Febru ary 7. 1987. Dr Schreiber IS affiliated With \1IT. climate for change in TV is the im flicker at the frame rate is produced Cambridge. \1A 02 139. This article was received Jan proved understanding of TV signal along with artifacts due to vertical uary D. 1987. and also appears In Tflft'won Tfrh processing. Such processing has al motion. The former becomes disturb nology: A Look Toward the 21s1 eentun'. pubhshed 1987. SMPTE. Copyrighl 'i:! 1987 by the Sociely of ready been applied to graphic arts, ing according to the vertical resolu- \101ion Picture and Television Engineers. Inc. generally at low data rates, but with . tion of the camera, which, typically, is 734 SMPTE Journal, August 1987 4.5.1 barely half what it might be with 525 Performance ofCameras and transmission. Especially in analog lines/frame.8 Ironically, if the cam Displays systems, there is no simple remedy to era resolution were raised to the alias Ideally, the response of cameras this problem, except to advocate a free Nyquist rate of 262.5 cycles/pic and display should be 100% through much higher standard of operation ture height (cph), the interline flicker out the Nyquist baseband. Because of and maintenance of every part of the would be unbearable.b While the arti some peculiarities of camera opera TV plant, from studio to the home. facts could be avoided by using pro tion, the vertical response is much Should digital transmission ever real gressive scanning, that would also poorer than that. The temporal reo ly become the norm in delivery ofsig permanently enshrine the lower verti sponse is determined by camera inte nals to the home, the job would be cal resolution. As discussed below, a gration. This blurs moving objects, immeasurably simplified. Perhaps solution lies in separating camera and but does not limit the bandwidth that is one good reason to go digital. display scan patterns from that of the enough to avoid temporal aliasing. Important as this problem is, it is not channel. Use of a shutter to keep moving ob the subject of this article. jects in focus makes the aliasing Interference Between Luminance and worse. What is needed is a camera ImprOl'ed NTSC Cbrominance: Resolution with high and fully controllable reso We firmly believe that the full ca Compromises lution in all three dimensions. a result pabilities of the NTSC system have The ingenious band-sharing princi that can only be obtained by operat not yet been exploited. While it might ple9 that made NTSC backward com ing a camera of inherently high reso not be capable of "theater quality," if patible does not, in fact, work perfect lution at a line and frame rate well the defects are removed and certain ly with the kind of processing above that of the channel. other changes made in the studio and envisaged by the advocates of that Displays have somewhat different in the receiver, then pictures much system in the 1950s.< This is some problems from those of cameras. The better than we have ever seen with this what puzzling. since comb filters were line structure, which tends to mask system should be produced. well known for other purposes at that fine detail. cannot readily be removed There are many advantages to im time,lO and, in fact, had been dis without reducing sharpness. The fact proving TVeon the basis of the present cussed by Gray in his 1929 patent. the that this seems not to be a serious systems. This evolutionary approach, earliest mention of frequency inter problem in contemporary HDTV sys in which existing receivers may con leaving. 11 The pioneers seemed much tems is further evidence that the cam tinue to be used, albeit without all of more concerned with making the sub era and the display, and not primarily the benefits of whatever improve carrier less visible on the existing the system parameters. limit the verti ments are made at the transmitter, monochrome receivers (that had the cal response. A corresponding effect ensures the presence of the audience, fulI4.2-MHz bandwidth called for by is present along the time axis. Phos an element necessary for manufactur the existing standard) than on making phor persistence short enough to ers, producers, and broadcasters alike color receivers work properly. 12 avoid temporal blurring produces a to be willing to make the required Whatever the history.