The advantages of displaying interlaced television signals in progressive scan form are well known and a number of techniques have been proposed for providing interlaced to progressive scan conversions. Generally speaking, the known techniques may be categorized as involving either intra-field processing or intra-frame processing or a combination of the two. An intra-field system derives the extra lines needed for display in a progressive scan system entirely from one field of the currently received lines. Advantageously, such systems are not subject to motion related artifacts. Examples of such systems are described, for example, by Dischert in U.S. Pat. No. 4,415,931 which issued Nov. 15, 1983, by Powers in U.S. Pat. No. 4,400,719 which issued Aug. 23, 1983, by Pritchard in U.S. Pat. No. 4,583,113 which issued Apr. 15, 1986 and by Okada et al. in U.S. Pat. No. 4,451,848 which issued May 29, 1984. In the Dischert system, extra lines for display are obtained by repeating lines of the incoming field. In the Powers, Pritchard and Okada et al. systems the added display lines are obtained by interpolation of adjacent vertical lines of a currently received field.
Although intra-field systems are inherently free of motion related artifacts, they tend to suffer a loss of vertical detail due to line averaging. One solution to this problem, proposed by Pritchard et al. (U.S. Pat. No. 4,558,347 which issued Dec. 10, 1985), is to derive a vertical detail component from the incoming video signal and selectively add the detail component to lines of the displayed progressively scanned signal. Another solution proposed in the aforementioned Powers patent, is to obtain the extra lines for the progressive scan display from the previous field or frame. To solve both the motion and vertical detail related problems, Powers proposed a hybrid system employing both intra-field and inter-field processing. Specifically, the system employs a motion detector which selects a line interpolated signal for display (intra-field processing) when motion is present in a scene and selects a field or frame delayed signal for display (inter-field processing) when motion is not present.
Other examples of hybrid progressive scan systems in which the added lines for display are obtained from the current field and a previous field or frame are described by Lord et al. in U.S. Pat. No. 4,322,750 (issued Mar. 30, 1982), by Casey in U.S. Pat. No. 4,598,309 (issued Jul. 1, 1986) and by Tanaka in Japanese Pat. Appln. No. SHO-58-79379 (Laid open May 13, 1983). In an example of the Lord et al. system a movement detector selects a spatially averaged signal for display when motion is present and a temporally averaged signal otherwise. In the Casey system a frame averaged signal is substituted for a field-delayed and line averaged signal when motion is present. In the Tanaka system extra lines for the progressive display are obtained by combining a low frequency component of a previous field with a high frequency component obtained by interpolation of lines from a currently received field. Since the Tanaka system only uses low frequency components from the previous field, the field memory may be of relatively small storage capacity.
A further example of a progressive scan system is described by Murata et al. in their article "A Consumer Use Flicker Free Color Monitor Using Digital Signal Processing"published in the IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 3, Aug. 1986, pp. 215-227. Advantageously their system does not require the use of a motion detector and so avoids the possibility of motion detection errors producing visible artifacts. The system employs a field memory but is not, in fact, a hybrid system in that all extra lines for the progressive display are obtained on an intra-field basis by "adaptive" interpolation between adjacent lines of a currently received field. Specifically, a field memory is used to store a previously received line which is compared with lines of the currently received field to generate interpolation coefficients. The coefficients are varied depending on the amplitude of the signals from the previous and currently received lines. The field memory thus functions as a controller of the intra-field interpolation and not as a data store for displayed lines. Advantageously, this allows the field data to be stored in a memory of relatively small capacity but the variable intra-field interpolation requires relatively complex arithmetic processing to obtain the desired variable coefficients.