Broadcast systems throughout the world create visible artifacts on standard, present day, television receivers and monitors. In broadcast systems such as the 525 line-per-frame, 30 frame-per-second (525/30) NTSC system or the 625/25 PAL system, artifacts are derived from the line-scanning process. These artifacts arise mainly due to the interlaced process as set forth in the standards and as are visible in interlaced display techniques. This process divides the 525-line picture or frame into two successive 2621/2-line fields. The 2621/2 lines of one field are scanned in 1/60th of a second followed by scanning of an additional 2621/2 lines of another field with the second field lines occupying the spaces between the lines of the first field. One subjective effect of this interlaced scan is to create an apparent vertical drift of the lines of the raster as a function of vertical motion. The apparent drift is more easily seen when viewing a wide-screen display at close range. Another visible effect is interline flicker in transitions in the vertical direction that occur within one line time to the next.
Recent interests in the development of high definition television systems (HDTV) has been directed to techniques intended to enhance the subjective performance of present systems within the constraints of existing standards. One approach, a technique referred to as progressive scan, or sequential scan, has been described in patent materials and literature. The incoming signal in a conventional two-to-one vertical interlaced format is stored in an appropriate memory and subsequently displayed in a non-interlaced or progressive line-scan manner. For example, in copending U.S. patent application Ser. No. 526,700 entitled, "Progressive Scan Television Display System Employing Interpolation in the Luminance Channel" filed concurrently herewith in the name of D. H. Pritchard, progressive scan is accomplished by employing line memories with multiple-point interpolation. According to the Pritchard application, in the case of NTSC, 525 lines of the display are displayed in 1/60th of a second wherein alternate "real" and "interpolated" lines are successively displayed at some multiple (generally, two-times) standard horizontal rate. During the next 1/60th of a second a successive set of 525 lines are displayed to complete a total frame time in 1/30th of a second, however, these 525 lines are related to the first 525 lines such that alternate "interpolated" and "real" lines are displayed. In one preferred embodiment of Pritchard a simple two-point (using a 1/H delay) linear interpolator is used to form the interpolated line. Progressive scan results in the elimination of artifacts of "interline flicker" and "line break-up with motion" that exists in conventional two-to-one interlaced displays. The subjective effect is a flicker-free, "smooth" or "quiet", picture presentation that is more pleasing to the viewer.
Advantageously, in accordance with the Pritchard system a simple two-point linear interpolator may be used to effect progressive scanning. Such a system, however, results in some loss of vertical detail on transitions that occur from one line to the next, i.e., at the vertical sampling Nyquist rate. To restore vertical detail some HDTV systems utilize multiple point (i.e., more than two) interpolation processes which require two or more 1-H delay memory elements with appropriate weighting factors in the summation process to provide improved performance by producing a "better" approximation in the interpolated line. The problem with such a scheme is that the vertical detail restoration requires additional lines of memory. For an illustration of a progressive scan system using multiple point interpolation reference may be made to U.S. Pat. No. 4,400,719 in the name of K. H. Powers.