Method of and apparatus for improving picture quality

A picture quality improving apparatus which is low in cost and small in circuit scale has a vertical low-pass filter (LPF) for extracting a vertical low-frequency component from an input luminance signal, a horizontal LPF for extracting a horizontal low-frequency component from the output signal from the vertical LPF, a subtractor for subtracting the output signal from the horizontal LPF from the luminance signal which has been compensated for the delay, a gain adjusting circuit for adjusting the gain of an edge signal produced by the subtractor, and an adder for adding the edge signal whose gain has been adjusted to the delay-compensated luminance signal. Each of the vertical LPF and the horizontal LPF comprises an FIR (finite impulse response) filter and an IIR (infinite impulse response) filter which are connected in cascade or parallel to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of and an apparatus for improving the quality of pictures displayed by a picture display apparatus such as a television set, a video projector, or the like.

2. Description of the Related Art

It is known in the art that the quality of pictures displayed by a picture display apparatus such as a television set, a video projector, or the like is reduced by flare. The flare is a phenomenon caused when light leaks from a bright area into a dark area due to light reflections and dispersions in lenses and on illuminated surfaces of a projecting tube or picture tube. The flare results in blurs at edges of displayed pictures across which the luminance difference is large, e.g., a boundary between white and black areas.

FIG. 1shows an original picture from which a picture is projected onto a screen by a video projector. As shown inFIG. 1, the original picture has a central rectangular white area WT and a black area BL disposed therearound, with the luminance difference being large across an edge ED at the boundary between the white area WT and the black area BL.FIG. 1also illustrates, below the original picture, a video signal (i.e., luminance signal) representing the original picture horizontally across a central portion thereof. When the original picture is projected onto the screen by the video projector, light leaks from the white area WT into the black area BL, blurring the edge ED. The flare thus caused tends to reduce the quality of the projected picture.

In order to eliminate the above flare, the video signal to be supplied to the video projector is generally digitally processed to correct the picture data out of edge blurring. Such a digital signal process is referred to as flare correction or flare compensation.FIG. 2shows the concept of flare correction.FIG. 2shows at (a) the waveform of the video signal of an original picture, which corresponds to the video signal of the original picture shown inFIG. 1.FIG. 2shows at (b) the luminance distribution of a picture which is projected and displayed on the screen based on the video signal shown at (a) inFIG. 2.FIG. 2shows at (c) the waveform of a video signal which is produced by effecting flare correction on the video signal shown at (a) inFIG. 2.FIG. 2shows at (d) the luminance distribution of a picture which is projected and displayed on the screen based on the flare-corrected video signal shown at (c) inFIG. 2.

The picture projected onto the screen by the video projector based on the video signal shown at (a) has its edge blurred by flare as shown at (b) inFIG. 2. In order to correct the picture, the video signal shown at (a) may be corrected or compensated at its positive- and negative-going edges thereof to correct (inversely correct) these edges depending on the blurring at the edges, i.e., to emphasize these edges, as shown at (c). By thus correcting the video signal, it is possible to display the picture free of edge blurs on the screen as shown at (d).

One conventional picture quality improving apparatus disclosed in Japanese laid-open patent publication No. 61-296880 (JP, 61-296880, A) carries out the above flare correction based on a correction signal which is generated from a luminance signal among luminance and color signals (wide-band and narrow-band color signals) which are produced from R (Red), G (Green), and B (Blue) signals as primary color signals. Specific details of the disclosed picture quality improving apparatus will be described below.

FIG. 3shows in block form the picture quality improving apparatus disclosed in JP, 61-296880, A. As shown inFIG. 3, the picture quality improving apparatus comprises A/D converters101ato101c, matrix circuit102, compensation delay circuits103ato103c, correction signal generator104, combiners105ato105c, and D/A converters106ato106c. A/D converters101ato101care supplied respectively with luminance signal Y, wide-band color signal C1, and narrow-band color signal C2which are produced from R, G, B signals by an inverse matrix circuit, not shown. A/D converters101ato101cconvert the supplied analog signals into digital signals. A/D converter101asupplies its digital output signal to correction signal generator104and also to matrix circuit102. A/D converters101b,101calso supply their digital output signals to matrix circuit102.

Correction signal generator104comprises correction signal generating circuit106and delay unit107which are supplied with the luminance signal supplied from A/D converter101a, and gain adjusting circuit108which is supplied with output signals from correction signal generating circuit106and delay unit107.

Matrix circuit102converts luminance signal Y, wide-band color signal C1, and narrow-band color signal C2supplied respectively from A/D converters101a,101b,101cinto R, G, B signals as primary color signals. The R, G, B signals output from matrix circuit102are supplied respectively to compensation delay circuits103ato103c.

Combiner105ahas an input terminal supplied with the output signal from compensation delay circuit103aand another input terminal supplied with the output signal from gain adjusting circuit108. Combiner105aoutputs a signal which is a combination of the supplied signals to D/A converter106a. Similarly, combiner105bis supplied with the output signal from compensation delay circuit103band the output signal from gain adjusting circuit108, and outputs a signal which is a combination of the supplied signals to D/A converter106b. Combiner105cis supplied with the output signal from compensation delay circuit103cand the output signal from gain adjusting circuit108, and outputs a signal which is a combination of the supplied signals to D/A converter106c.

The picture quality improving apparatus shown inFIG. 3operates as follows: When a digital luminance signal converted by A/D converter101ais supplied to correction signal generator104, correction signal generating circuit106filters the supplied luminance signal to generate a correction signal. Usually, a picture display apparatus such as a television set, a video projector, or the like is supplied with a nonlinear input signal multiplied by a gamma value because of the characteristics thereof, e.g., the characteristics of cathode-ray tubes (CRTs). If such a nonlinear input signal is filtered into a flare correction signal and such a flare correction signal is added, then the linearity of the flare correction signal itself is lost, lowering the sensitivity of the correction filter in dark picture areas where the signal level is low, with the result that the picture quality cannot sufficiently be improved in such dark picture areas. In order to avoid the above drawback, gain adjusting circuit108adjusts the gain of the correction signal generated by correction signal generating circuit106.

The correction signal whose gain has been adjusted by gain adjusting circuit108is supplied to combiners105ato105c. Combiners105ato105ccombine the respective R, G, B signals, which have been delayed by respective compensation delay circuits103ato103cfor a time equal to the delay time required for correction signal generating circuit106to generate the correction signal, with the correction signal whose gain has been adjusted by gain adjusting circuit108, thus effecting flare correction on the R, G, B signals.

Internal details of correction signal generating circuit106are shown inFIG. 4. The picture quality improving apparatus also performs a contour emphasis process for emphasizing the contour of a picture in order to prevent the resolution from being lowered. The contour emphasis process and the flare correction process are concurrently performed in correction signal generating circuit106. Correction signal generating circuit106has a compensation delay unit125for generating and delaying a signal1for a contour correction filter system, a signal m for a flare correction filter system, and a reference signal n, from the input signal applied to correction signal generating circuit106. The contour correction filter system comprises vertical contour correction FIR (Finite Impulse Response) filter121and horizontal contour correction FIR filter122which are connected in cascade, subtractor126afor subtracting a filtered signal from the reference signal n, and coring circuit127aconnected to the output terminal of subtractor126a. The flare correction filter system comprises vertical flare correction composite IIR (Infinite Impulse Response) filter123and horizontal flare correction composite IIR filter124which are connected in cascade, subtractor126bfor subtracting a filtered signal from the reference signal n, and coring circuit127bconnected to the output terminal of subtractor126b. Correction signal generating circuit106also has an adder128for adding the output signals from coring circuits127a,127bto each other and outputting the sum signal as the output signal from correction signal generating circuit106.

Since the present invention is concerned with improving the picture quality based on the flare correction, details of correction signal generating circuit106for carrying out the flare correction will be described below. Vertical flare correction composite IIR filter123and horizontal flare correction composite IIR filter124which are connected in cascade jointly provide a two-dimensional (2D) low-pass filter (LPF).FIG. 5shows vertical flare correction composite IIR filter123in block form, andFIG. 6shows horizontal flare correction composite IIR filter124in block form.

As shown inFIG. 5, vertical flare correction composite IIR filter123comprises two low-pass IIR filters230a,230b, and two field inverters234a,234b. Two low-pass IIR filters230a,230bare identical in structure to each other, and a specific circuit arrangement of filter230aamong two low-pass IIR filters230a,230bis shown in detail.

Low-pass IIR filter230acomprises delay elements231ato231cwhich comprise line memories, coefficient circuits232ato232dconnected respectively to taps and an input terminal, and adders233ato233c. Coefficient circuit232dis supplied with the luminance signal from A/D converter101aand supplies its output signal to an input terminal of adder233a. Adder233asupplies its output signal to field inverter234a. The output signal from adder233ais also supplied via delay elements231ato231c, which are connected in cascade, to coefficient circuits232ato232cwhich are supplied with the output signals from delay elements231ato231c. The output signals from coefficient circuits232ato232care added to each other by adders233b,233c, and the sum signal is supplied to the other input terminal of adder233a. Low-pass IIR filter230athus arranged serve as a recursive filter.

In vertical flare correction composite IIR filter123, the output signal from low-pass IIR filter230ais inverted in each field by field inverter234a, which supplies the inverted output signal to low-pass IIR filter230bthat is structurally identical to low-pass IIR filter230a. The output signal from low-pass IIR filter230bis inverted in each field by field inverter234b. The delay in phase caused by low-pass IIR filter230ais compensated for because the signal is advanced in phase by low-pass IIR filter230bwhen the inverted signal is applied thereto. The filter arrangement shown inFIG. 5provides a good low-pass filter.

As shown inFIG. 6, horizontal flare correction IIR filter124comprises two low-pass IIR filters240a,240b, and two line inverters244a,244b. Horizontal flare correction IIR filter124is of basically the same structure as vertical flare correction composite IIR filter123shown inFIG. 5, but differs therefrom in that line inverters244a,244bare used in place of field inverters234a,234b. Two low-pass IIR filters240a,240bare identical in structure to each other, and each comprises delay elements241ato241cwhich comprise A/D conversion clock registers, coefficient circuits242ato242dconnected respectively to taps and an input terminal, and adders243ato243c. Low-pass IIR filters240a,240bare of basically the same structure as low-pass IIR filters230a,230bshown inFIG. 5except that delay elements241ato241care used in place of delay elements231ato231cwhich comprise line memories.

In the picture quality improving apparatus disclosed in the publications JP, 61-296880, A, etc., each of vertical flare correction composite IIR filter123(FIG. 5) and horizontal flare correction composite IIR filter124(FIG. 6) is of an arrangement for inverting data and requires two frame memories. Therefore, these picture quality improving apparatus cannot easily be reduced in cost and size.

The picture quality improving apparatus disclosed in the publications JP, 61-296880, A, etc. employ a 2D LPF which is made up of the vertical flare correction composite IIR filter and horizontal flare correction composite IIR filter that are connected in cascade. However, there are other picture quality improving apparatus which employ composite FIR filters for vertical and horizontal flare correction. These other picture quality improving apparatus are disadvantageous in that their circuit scale is large though no frame memory is required. For example, the vertical flare correction composite FIR filter has an increased number of multipliers and is large in circuit scale as a delay caused for each line is multiplied as a coefficient.

It is known in the art that the visual effects of contour and contrast can be increased by using the Craik-O'Brien effect with respect to visual perception. There have not been known in the art any arrangements which apply the Craik-O'Brien effect to the flare correction. The Craik-O'Brien effect is also known as Craik-Cornsweet illusion, and will be described in detail later on.

The aspect ratio, that is, ratio of frame height to frame width, of picture display apparatus such as a television set has heretofore been 3:4. With the growing popularity of digital high-definition picture contents, picture display apparatus having a screen whose aspect ratio is 9:16 are becoming more and more general. A screen having such an aspect ratio is referred to as “wide screen”. However, the conventional picture quality improving apparatus described above are not arranged to be compatible with such picture display apparatus having an aspect ratio of 9:16, and cause the following problems if applied to the picture display apparatus having an aspect ratio of 9:16:

When a picture having an aspect ratio of 3:4 is displayed on a picture display apparatus having a wide screen whose aspect ratio is 9:16, an edge is produced at the boundary between a black area and an effective area of the picture, reducing the quality of the picture. If a picture having an aspect ratio of 3:4 is expanded horizontally or both horizontally and vertically in order to be displayed on the wide screen, then the desired improved effect (flare correction or Craik-O'Brien effect) may not be achieved. If a picture having an aspect ratio of 3:4 is expanded nonlinearly in order to be displayed on the wide screen, then the desired improved effect (flare correction or Craik-O'Brien effect) may not be achieved by expanding the picture after the flare correction has been performed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus for improving the quality of pictures, which is low in cost and small in circuit scale.

Another object of the present invention to provide an apparatus for improving the quality of pictures, which uses the Craik-O'Brien effect with respect to visual perception and is low in cost and small in circuit scale.

Still another object of the present invention to provide an apparatus for improving the quality of pictures, which is compatible with wide screens.

Yet another object of the present invention to provide a method of improving the quality of pictures, which can be carried out at a low cost and with a small circuit scale.

Yet still another object of the present invention to provide a method of improving the quality of pictures, which uses the Craik-O'Brien effect with respect to visual perception and can be carried out at a low cost and with a small circuit scale.

A further object of the present invention to provide a method of improving the quality of pictures, which is compatible with wide screens.

According to a first aspect of the present invention, there is provided an apparatus for improving the quality of a picture, comprising a two-dimensional (2D) low-pass filter (LPF) for being supplied with a luminance signal obtained from a video signal and extracting, from the supplied luminance signal, low-frequency components in vertical and horizontal directions of a picture displayed based on the video signal, a subtractor for subtracting the low-frequency components extracted by the 2D LPF from the luminance signal thereby to generate an edge signal, and an adder for adding the edge signal generated by the subtractor to the luminance signal, the 2D LPF comprising a vertical LPF for extracting the low-frequency component in the vertical direction and a horizontal LPF for extracting the low-frequency component in the horizontal direction.

Each of the vertical and horizontal LPFs has an FIR (finite impulse response) filter and an IIR (infinite impulse response) filter which are connected in cascade, or an FIR filter and an IIR filter which are connected parallel to each other.

According to a second aspect of the present invention, there is provided a method of improving the quality of a picture, comprising the steps of extracting low-frequency components in vertical and horizontal directions of a picture displayed based on a video signal from a luminance signal obtained from the video signal, and producing extracted low-frequency signals, subtracting the extracted low-frequency component signals from the luminance signal thereby to generate an edge signal representing an edge corresponding to a contour of the picture and having a level and a slope of predetermined magnitudes, and adding the edge signal to the luminance signal.

With above arrangement of the present invention, each of the vertical and horizontal LPFs of the 2D LPF comprises an FIR filter and an IIR filter which are connected in cascade or parallel to each other. The impulse responses of the FIR and IIR filters are combined with each other to provide an LPF of phase linearity. The 2D LPF thus arranged dispenses with frame memories which have heretofore been employed. Since a single FIR filter is used in each of the vertical and horizontal LPFS, the 2D LPF is smaller in circuit scale than the conventional LPF having a composite FIR filter for vertical flare correction.

Of the edge of the edge signal, the level and slope of the edge corresponding to the contour of the picture displayed based on the video signal are set to given magnitudes, i.e., conditions to achieve the Craik-O'Brien effect, for thereby increasing the visual effect of contour and contrast. The Craik-O'Brien effect can be accomplished by coefficient units and gain adjusting circuits of the FIR and IIR filters of each of the vertical and horizontal LPFs, without the need for any special arrangements.

If the edge signal held to a certain value in a black area is output, then any edges responsible for a reduction in the picture quality at boundaries between black and effective areas are not produced.

If the horizontal LPF which is arranged to have response characteristics shorter by a predetermined picture magnification ratio than the response characteristics to be provided if the picture is not expanded, is used to extract the low-frequency component in the horizontal direction, sufficiently improved effects with respect to flare correction and the Craik-O'Brien effect are obtained even when the picture is expanded horizontally. Similarly, if the vertical and horizontal LPFs which are arranged to have respective response characteristics shorter by predetermined picture magnification ratios than the response characteristics to be provided if the picture is not expanded, are used to extract the low-frequency component in the vertical and horizontal directions, sufficiently improved effects with respect to flare correction and the Craik-O'Brien effect are obtained even when the picture is expanded both vertically and horizontally.

According to the present invention, furthermore, by nonlinearly expanding the picture displayed based on the video signal before the edge is detected, sufficiently improved effects with respect to flare correction and the Craik-O'Brien effect are obtained even when the picture is expanded nonlinearly.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment: Flare Correction

FIG. 7shows a picture quality improving apparatus according to a first embodiment of the present invention. The picture quality improving apparatus shown inFIG. 7generates a flare correction signal from a luminance signal among luminance and color signals which are produced from R, G, B signals as primary color signals, and adds the flare correction signal to the original luminance signal for performing flare correction. The picture quality improving apparatus comprises vertical low-pass filter (VLPF)1, horizontal low-pass filter (HLPF)2, delay circuit3, subtractor4, gain adjusting circuit5, and adder6.

VLPF1has two output terminals, i.e., an output terminal “Main out” for directly outputting a luminance signal YINwhich is obtained from R, G, B signals and supplied thereto, and an output terminal “LPF out” for extracting a vertical low-frequency component of the supplied luminance signal YINand outputting the extracted vertical low-frequency component. The output signal in the output terminal “Main out” is supplied to delay circuit3, and the output signal in the output terminal “LPF out” is supplied to HLPF2.

Delay circuit3delays the supplied signal for a time corresponding to the time required for VLPF1and HLPF2to filter the signal. Delay circuit3supplies the delayed output signal to a “plus (+)” input terminal of subtractor4, an input terminal of gain adjusting circuit5, and an input terminal of adder6.

HLPF2extracts a horizontal low-frequency component from the output in the output terminal “LPF out” from VLPF1, i.e., the vertical low-frequency component of the luminance signal YIN. VLPF1and HLPF2jointly make up a flare correction two-dimensional (2D) low-pass filter (LPF). The output signal, i.e., the extracted horizontal low-frequency component, from HLPF2is supplied to a “minus (−)” input terminal of subtractor4.

Subtractor4subtracts the horizontal low-frequency component (the output signal of the 2D LPF), which is supplied from HLPF2to the “−” input terminal thereof, from the original luminance signal which is supplied from delay circuit3to the “+” input terminal of subtractor4, and supplies an edge signal, which represents the difference, to gain adjusting circuit5.

Gain adjusting circuit5serves to prevent the sensitivity of the flare correction filter from being lowered in dark picture areas where the signal level is low, when a nonlinear signal multiplied by a gamma value is supplied thereto, as with the conventional gain adjusting circuit shown inFIG. 3. The flare correction filter is the 2D LPF made up of VLPF1and HLPF2. The output signal from gain adjusting circuit5, i.e., the edge signal whose gamma characteristics have been compensated into the linear characteristics, is supplied to the other input terminal of adder6.

Adder6adds the edge signal whose gain has been adjusted by gain adjusting circuit5to the original luminance signal supplied from delay circuit3, and outputs the sum as a luminance signal YOUT.

With the picture quality improving apparatus thus arranged, the luminance signal YINis supplied to VLPF1, which extracts the vertical low-frequency component from the luminance signal YIN, and further supplied to HLPF2, which extracts the horizontal low-frequency component from the luminance signal YIN. The luminance signal YINis also supplied to delay circuit3, which delays the luminance signal YIN.

The extracted low-frequency signal, which represent the vertical and horizontal low-frequency components extracted by VLPF1and HLPF2, is supplied to the “−” input terminal of subtractor4. The original luminance signal which has been delayed by delay circuit3is supplied to the “+” input terminal of subtractor4. Subtractor4subtracts the extracted low-frequency signal supplied to the “−” input terminal from the original luminance signal supplied to the “+” input terminal, thus producing an edge signal.

FIG. 8Ashows the waveform of the edge signal, andFIG. 8Bshows the waveform of the original video signal. The edge signal shown inFIG. 8Acorresponds to the horizontal edge ED of the original picture shown inFIG. 1, and the video signal shown inFIG. 8Bcorresponds to the video signal of the original picture shown inFIG. 1. When the edge signal and the original video signal are added to each other, the signal waveform shown at (c) inFIG. 2which has been corrected for flare is obtained. The luminance signal YOUToutput from adder6and the color signal (wide-band and narrow-band color signals) which are produced from R, G B signals as primary color signals are supplied to a known matrix circuit (not shown), which re-converts them into R, G, B signals as primary color signals. A picture display apparatus then displays a picture based on the re-converted R, G, B signals.

The present invention resides in VLPF1and HLPF2of the picture quality improving apparatus. VLPF1and HLPF2will be described below in detail below.

VLPF1and HLPF2may comprise an FIR (finite impulse response) filter and an IIR (infinite impulse response) filter which are connected in cascade, or may comprise an FIR filter and an IIR filter which are connected parallel to each other.FIG. 9Ashows the impulse response of an FIR filter,FIG. 9Bthe impulse response of an IIR filter, andFIG. 9Cthe impulse response of an LPF. If an LPF comprises an FIR filter and an IIR filter which are connected in cascade, then the impulse response shown inFIG. 9Cis obtained by convoluting the impulse response (FIG. 9A) of the FIR filter and the impulse response (FIG. 9B) of the IIR filter. If an LPF comprises an FIR filter and an IIR filter which are connected parallel to each other, then the impulse response shown inFIG. 9Cis obtained by adding the impulse response (FIG. 9B) of the IIR filter to the impulse response (FIG. 9A) of the FIR filter.

Specific details of VLPF1and HLPF2will be described below.

(1-a) VLPF Comprising FIR Filter and IIR Filter Connected in Cascade:

FIG. 10shows by way of example VLPF1used in the picture quality improving apparatus shown inFIG. 7. As shown inFIG. 10, the VLPF comprises 12-tap FIR filter10and 1-tap IIR filter20which are connected in cascade.

FIR filter10comprises eleven cascaded delay elements (line memories)11supplied with the luminance signal YIN, twelve coefficient units12supplied with the luminance signal YINand the output signals from delay elements11, and eleven adders13for adding the output signals from coefficient units12. InFIG. 10, each of the delay elements which comprise line memories is represented by “H”. The output terminal of eleventh delay element11is branched into a line connected to eleventh adder13and a line connected to the output terminal “Main out”. The output signal from eleventh adder13serves as the output signal from FIR filter10.

IIR filter20comprises two coefficient units21,24, adder22, and delay element23which is a line memory. Coefficient unit21is supplied with the output signal from FIR filter10, and supplies its output signal to an input terminal of adder22. Adder22has its output terminal branched into a line connected to the output terminal “LPF out” and a line connected to delay element23. Delay element23supplies its output signal via coefficient unit24to another input terminal of adder22.

(1-b) HLPF Comprising FIR Filter and IIR Filter Connected in Cascade:

FIG. 11shows by way of example HLPF2used in the picture quality improving apparatus shown inFIG. 7. As shown inFIG. 11, the HLPF comprises 17-tap FIR filter30and 1-tap IIR filter40which are connected in cascade.

FIR filter30comprises sixteen cascaded delay elements (registers)31supplied with the luminance signal YINwhich is output from the VLPF described above in (1-a), seventeen coefficient units32supplied with the luminance signal YINand the output signals from delay elements31, and sixteen adders33for adding the output signals from coefficient units32. InFIG. 11, each of the delay elements which comprise registers is represented by “D”. The output signal from sixteenth adder33serves as the output signal from FIR filter30.

IIR filter40comprises two coefficient units41,44, adder42, and delay element (register)43. Coefficient unit41is supplied with the output signal from FIR filter30and supplies its output signal to an input terminal of adder42. The output terminal of adder42is branched into a line connected to the output terminal “LPF out” and a line connected to delay element43. Delay element43supplies its output signal via coefficient unit44to another input terminal of adder42.

(2-a) VLPF Comprising FIR Filter and IIR Filter Connected Parallel to Each Other:

FIG. 12shows another example of VLPF1used in the picture quality improving apparatus shown inFIG. 7. As shown inFIG. 12, the VLPF comprises 12-tap FIR filter50, 1-tap IIR filter60, and adder70for adding the output signals from FIR filter50and IIR filter60to each other.

FIR memory50comprises eleven cascaded delay elements (line memories)51supplied with the luminance signal YINtwelve coefficient units52supplied with the luminance signal YINand the output signals from delay elements51, and eleven adders53for adding the output signals from coefficient units52. The output terminal of eleventh delay element51is branched into a line connected to eleventh adder53and a line connected to the output terminal “Main out”. The output signal from eleventh adder53serves as the output signal from FIR filter50.

IIR filter60comprises two coefficient units61,64, adder62, and delay element63which is a line memory. Coefficient unit61is supplied with the output signal from FIR filter50, and supplies its output signal to an input terminal of adder62. Adder62has its output terminal branched into a line connected to the output terminal “LPF out” via adder70and a line connected to delay element63. Delay element63supplies its output signal via coefficient unit64to another input terminal of adder62. Adder70adds the output signal of the eleventh adder53of FIR filter50to the output signal of adder62of IIR filter60.

(2-b) HLPF Comprising FIR Filter and IIR Filter Connected Parallel to Each Other:

FIG. 13shows another example of HLPF2used in the picture quality improving apparatus shown inFIG. 7. As shown inFIG. 13, the HLPF comprises 17-tap FIR filter80, 1-tap IIR filter90, and adder100for adding the output signals from FIR filter80and IIR filter90to each other.

FIR filter80comprises sixteen cascaded delay elements (registers)81supplied with the luminance signal YINwhich is output from the VLPF described above in (2-a), seventeen coefficient units82supplied with the luminance signal YINand the output signals from delay elements81, and sixteen adders83for adding the output signals from coefficients units82. The output signal from sixteenth adder83serves as the output signal from FIR filter80.

IIR filter90comprises two coefficient units91,94, adder92, and delay element (register)93. Coefficient unit91is supplied with the output signal from FIR filter80and supplies its output signal to an input terminal of adder92. The output terminal of adder92is branched into a line connected to the output terminal “LPF out” via adder100and a line connected to delay element93. Delay element93supplies its output signal via coefficient unit94to another input terminal of adder92. Adder100adds the output signal of the sixteenth adder83of FIR filter80to the output signal of adder92of IIR filter90.

Second Embodiment: Use of Visual Illusion

The Craik-O'Brien effect with respect to visual perception can be used to increase the visual effects of contour and contrast for improving the quality of pictures. The Craik-O'Brien effect represents a visual illusion that a wide edge provided in an area free of any luminance difference makes the viewer see a luminance difference. Details of the Craik-O'Brien effect are described in, for example, Japan Television Society Technical Reports (Nov. 24, 1977, VVI 24–2, pp. 7–13). The picture quality improving apparatus according to the present embodiment relies upon the Craik-O'Brien effect to increase the visual effects of contour and contrast. The picture quality improving apparatus according to the present embodiment is of basically the same construction of the picture quality improving apparatus according to the first embodiment. Principles and arrangements for achieving the Craik-O'Brien effect will be described below.

FIG. 14is illustrative of the visual effect of contrast based on the Craik-O'Brien effect. Assuming that an edge signal as shown inFIG. 14is displayed on the screen of a CRT or the like, the human vision perceives a step pattern of bright and dark areas as indicated by the dotted lines when a certain condition is satisfied. This visual perception is caused by an illusion according to the Craik-O'Brien effect. The illusion occurs if the contrast (degree of modulation) of the edge is of a low value in the range of 10 to 20% of the average luminance of the screen. The Craik-O'Brien effect is also governed by the size of the slope of the edge as well as the low contrast. Using an edge signal representing a wide edge having a large slope, it is possible to effectively achieve the visual effect of contrast according to the Craik-O'Brien effect.

In order to obtain the visual effect of contrast according to the Craik-O'Brien effect, the signal produced by subtracting the extracted low-frequency signal (representing a blurred picture) output from the 2D LPF made up of VLPF1and HLPF2from the original video signal (original picture) output from delay circuit3, i.e., the edge signal (the output signal from subtractor4), is to satisfy the conditions for the visual illusion, i.e., the low contract condition and the size of the slope of the edge. When a picture is displayed based on video signals (R, G, B signals), the level and the size of the slope of the edge represented by the edge signal output from subtractor4and corresponding to the contour of the displayed picture are set to such values as to cause visual illusion according to the Craik-O'Brien effect.

The low contrast can be achieved by adjusting the gain with gain adjusting circuit5. The slope of the edge is determined by the coefficient units (i.e., filter coefficients) of VLPF1and HLPF2. In the present embodiment, the edge signal whose gain has been adjusted by gain adjusting circuit5is established to satisfy the low contrast condition that the contrast of the edge is of a low value in the range of 10 to 20% of the average luminance of the screen, and the slope of the edge is established so as to be large, for thereby increasing the visual effect according to the Craik-O'Brien effect.

In the present embodiment, gain adjusting circuit5operates to adjust the gamma characteristics into linear characteristics, in the same manner as described above with respect to the first embodiment.FIG. 15shows a circuit arrangement of gain adjusting circuit5in the present embodiment. As shown inFIG. 15, gain adjusting circuit5has multiplier5asupplied with an edge signal E1output from subtractor4. Multiplier5ais supplied with the original luminance signal Y output from delay circuit3, as a control signal for determining which level the output signal from subtractor4is to be set to. Multiplier5athus changes the level of the output signal (edge) of subtractor4depending on the brightness represented by luminance signal Y. Though not shown inFIG. 15, the luminance signal Y and the edge signal E1are compensated for delays. The circuit arrangement of gain adjusting circuit5changes the level of the edge of the edge signal depending on the brightness. Specifically, when the level of the luminance signal Y is high, i.e., the brightness is high, gain adjusting circuit5lowers the level of the edge as indicated by the broken-line curve shown inFIG. 16, and when the level of the luminance signal Y is low, i.e., the brightness is low, gain adjusting circuit5increases the level of the edge.

In the present embodiment, as described above, the picture quality improving apparatus can operate to carry out the flare correction according to the first embodiment and simultaneously achieve the visual effect of contrast according to the Craik-O'Brien effect.

Third Embodiment: Compatibility with Wide Screen

There are several display modes available for displaying a picture signal having an aspect ratio of 3:4 on a wide screen having an aspect ratio of 9:16.FIGS. 17A to 17Eshow those several display modes for displaying pictures on a wide screen.FIG. 17Ashows an original picture having an aspect ratio of 3:4 which is to be displayed on a wide screen. The left portion ofFIG. 17Aillustrates a screen having an aspect ratio of 3:4 and the right portion illustrates a wide screen having an aspect ratio of 9:16. The various display modes will be described below with respect to an example in which a picture having a circle at its center at an aspect ratio of 3:4 is to be displayed on a wide screen having an aspect ratio of 9:16. The display modes include a normal display mode, a full display mode, a zoom display mode, and a nonlinear display mode.FIG. 17Billustrates the normal display mode,FIG. 17Cthe full display mode,FIG. 17Dthe zoom display mode, andFIG. 17Ethe nonlinear display mode.

In the normal display mode, as shown inFIG. 17B, the original picture shown inFIG. 17Ais directly displayed centrally on the wide screen in a display area which serves as an effective area. Areas shown hatched on both sides of the effective area are black areas. In the full display mode, as shown inFIG. 17C, the original picture shown inFIG. 17Ais displayed as a picture expanded horizontally at a given magnification ratio. In the zoom display mode, as shown inFIG. 17D, the original picture shown inFIG. 17Ais displayed as a picture expanded both vertically and horizontally at the same magnification ratio. In the nonlinear display mode, as shown inFIG. 17E, the original picture shown inFIG. 17Ais displayed as a picture which is not magnified in a certain central area and is magnified at a magnification ratio as it becomes progressively higher outwardly from the central area.

When a picture is displayed on a wide screen, the picture is displayed in either one of the above display modes. In order to adapt the picture quality improving apparatus according to the first and second embodiments to the above display modes, it is necessary to process video signals in those display modes as follows:

(1) Normal Display Mode

When the normal display mode is selected, the picture quality improving process is carried out on the original picture which is the picture with the black areas in its opposite sides, as shown inFIG. 17B, for example. The original picture having an aspect ratio of 3:4 has upper, lower, left, and right black areas (blanking areas) therein. Since the picture quality improving apparatus according to the first and second embodiments are arranged to detect an edge, when they process the above original picture, they detect edges at the boundaries between the black and effective areas, and the detected edges tend to cause a reduction in the picture quality. The reduction in the picture quality is avoided as follows:

FIG. 18shows the waveform of a video signal in a horizontal direction in the normal display mode. InFIG. 18, zone al represents a black area (blanking area) on the left side of the picture, zone b represents an effective area for displaying the original picture, and zone a2represents a black area (blanking area) on the right side of the picture. In HLPF2of the picture quality improving apparatus shown inFIG. 7, value PS at the start of the picture is set in all the registers when the effective area b begins, the values of the registers are shifted to update the picture during effective area b, and value Pc at the end of the picture is set (held) in the registers without updating the picture in the black area a2. Since there are usually blanking areas in the vertical direction of picture, VLPF1also performs the same process as with HLPF2.

(2) Full Display Mode

When the full display mode is selected, if the original picture is expanded horizontally at a given magnification ratio after the picture quality improving apparatus shown inFIG. 7has performed the flare correction and achieved the visual effect of contrast according to the Craik-O'Brien effect, the correction values are also expanded, failing to sufficiently achieve the desired corrective effects (the flare correction and the Craik-O'Brien effect). In this case, it is necessary to change, i.e., shorten, the response of HPLF2depending on the magnification ratio at which the picture is expanded. Specifically, the impulse response (filter coefficients) of HPLF2is set to reduce the width of the edge of the edge signal output from subtractor4by a value commensurate with the expansion of the picture.

(3) Zoom Display Mode

When the zoom display mode is selected, the effect of the picture expansion in the full display mode on the correction occurs also in the vertical direction. In the zoom display mode, it is necessary to change, i.e., shorten, the response of VLPF1and HPLF2depending on the magnification ratio at which the picture is expanded. Specifically, the impulse responses (filter coefficients) of VPLF1and HPLF2are set to reduce the width of the edge of the edge signal output from subtractor4by a value commensurate with the expansion of the picture.

(4) Nonlinear Display Mode

When the nonlinear display mode is selected, if the original mode is nonlinearly expanded after the picture quality improving apparatus shown inFIG. 7has performed the flare correction and achieved the visual effect of contrast according to the Craik-O'Brien effect, the correction values are also expanded, failing to sufficiently achieve the desired corrective effects (the flare correction and the Craik-O'Brien effect). In this case, it is necessary to expand the original picture nonlinearly before the picture quality improving apparatus shown inFIG. 7performs its corrective process. Specifically, a nonlinear processing circuit for displaying the original picture in the nonlinear display mode may be connected to the input side of the picture quality improving apparatus shown inFIG. 7.

In each of the above embodiments, a picture displayed based on a video signal includes an effective area where picture information is represented and black areas (blanking areas) free of picture information around the effective area. With VPLF1of the picture quality improving apparatus shown inFIG. 7, all areas including the black areas are stored in memories. However, only the effective area may be stored in memories. Since the line delay is to be effected on only the effective area, the memory capacity required for the line delay may be small.

A coring circuit may be provided in the output stage of subtractor4of the picture quality improving apparatus shown inFIG. 7.FIG. 19shows the characteristics of such a coring circuit, the horizontal axis representing an input level and the vertical axis an output level. The coring circuit removes edges responsible for noise, i.e., edges below a given level, from the edge signal output from subtractor4, for thereby increasing the S/N ratio.

A video signal captured by an imaging device having an optical system already contains flare caused by the optical system. As the amount of such flare can be adjusted by the gain adjusting circuit, the picture quality improving apparatus according to the present invention is also applicable to the processing of video signals containing such flare.