CA2235105C - Image quality correction circuit - Google Patents

Abstract

An image quality correction circuit having a variable gain amplifier for amplifying an extracted high-frequency component of a luminance signal in such a manner that the gain thereof is increased when detected color density is high, and is reduced when the detected color density is low, and a slice circuit for slicing the outputted amplitude of the variable gain amplifier at a prescribed value, wherein a corrected luminance signal is obtained by combining an image quality correction signal outputted from the slice circuit with the luminance signal to be corrected.

Description

IMAGE QUALITY CORRECTION CIRCUIT

This application is a divisional of Application Ser.
No. 2,121,187, filed April 13, 1994.

BACKGROUND OF THE INVENTION
Field of the Invention The p~esen-t invention relates to an image quality correction circuit for correcting the quality of the images produced by video equipment, such as televisiorl receivers, video cameras, and the like.

Description of Related Art In tlle NTSC, ~AL, and SECAM television signal trans-mission systems, wide-band three primary color signals, R, G, and B, are first subjected to gamma correction, which is necessary to compensate for the equi~ment characteristic at the recei~ing end, and are t~lell converted into a luminance signal Y and color-difference signals, R-Y and B-Y, or chrominanl~e signals, I and Q, for translnission, the bandwidths being limited to about 0.5 to 1.5 MHz for the color-difference signals cr chrominance signals.
The gamma correction and color-difference signal bandwidth limiting performed at the transmitting end, however, result in the introduction of nonlinearity in the transmitted signal, and in the case of a high color saturation image, hig}l-rrequency components contained therein cannot be reproduc~ed satisfactorily which normally CA 0223~10~ 1998-04-17 should be reproduced completely by the luolinance signal alone. [r- other words, the high-frequency componen-t level of the luminarlce signal degrades in the high color sa~ur-ation areas of the ima~e, and fine details of the original scene car1not be displayed sufficierltly. It is also known that blal~k level variations and saturation drops occur in the high-frequency component areas of a high color satur-ation image.
Ima,Je quality correction circuits designed to prevent such ima'5e quality degradations include one such as dis-closed in Japanese Paten~ Ap~lication Laid-Open No. 64-32588 (1989). Fig. 1 shows a bloclc diagralll for ~he inlage quali-ty correction circuit disclcsed therein. The luminance signal Y is inputted to a high-pass filter 63 and also to a delay circuit 65. The high-frequency component of the lunlinance signal p,assed through the high-pass filter 63 is fed -to a variable gain amplifier ~4 whose output signal is supplied to an adder 66. Also inputted to t,he adder 66 is the lunlinance signal Y delayed through the delay circuit 65.
The adder 66 adds together ~he high-frequency component of the luminance signal fed from the variable gain amplifier 64 and the luminance signal Y delayed through the delay circuit 65, and outputs a correct,ed luminance signal Y'.
On the other hand, the color-differerlce signal R-Y is inputte~ to a full-wave rectifier 10, and the full-wave CA 0223~10~ 1998-04-17 rectified color difference signal R-Y is illputted to an adder 13. Similarly, the color-dif'ference signal B-Y is inputted to a full-wave rec~ifier 12, and the full-wave rectified color-difference signal B-Y is inputted to the adder 13, where the color-difference signal ~-Y and the color-difference signal ~-Y are added toge-ther. The color density thus detected is outputted as a color density detection signal which is applied to a control terminal of the variable gain amplifier 6~.
The operation of the a~ove ilnage quality correction circuit will be describecl below. The luminance signal Y is inputted to the high-pasc; fil-ter 63, where the high-frequency component of the luminance signal Y is separated and inputted to the variable gain amplifier 6~. The gain of the variable gain amplifier G~ is controlled irl accordance with the color density detec-tiorl signal ou~putted from the adder 13 by detecting the color density. More specifically, in high color-density areas, the amplitude of the color density detection signal is increased, so that the gain with which to an~plify the high-frequency conlponent of the lulllinance signal is increased; conversely, in low color-density areas, the amplitude Or the color density detection signal is reduced, so that the gain with which to amplify the high-frequency component is reduced.
The delay circuit 65 delays the luminance signal Y

CA 0223~10~ 1998-04-17 before inputting to the ad-1er G6, so that tlle phase of the luminance signal Y to be corrected coincides witll the phase of an im~ge quality correctiorl signal, that is, tlle output of the variable gain amp]Lirier 6~, representing ~he high-frequency component of tlle luminance signal. The adder ~
adds together the lunlinance signal fed from the delay cir-cuit 65 and the image quality correctioll signal fed from the variable gain amplifier ~;4, and outputs the correc-ted lumi-nance signal Y'. Thus, t}le luminallce signal, outputted as the luminance signal Y', is so corrected that the gain of the high-frequency compollellt Or the lumirlarlce signal is increased in the high co:Lor-density areas.
The color-difference signal R-Y is full-wave rectified by the full-wave rectifier circuit 10; when we consider a vector of the color-difference signal R-Y, the amplitude of the rectifier output represellts the length of the R-Y
vector. The color-difference signal B-Y is full-~ave rectified by the full-wave rectifier circuit 12, and the amplitude of its output represents the length of the color-difference signal B-Y vector. The full-wave rectified color-difference signal R-Y outputted from the full-wave rectifier circuit 10 and the full-wave rectified color-differen,ce signal B-Y outputted from the full-wave rectifier circuit 12 are added toge-ther in the adder 13. Although the output s,ignal of the adder 13 does not become equal to the CA 0223~10~ 1998-04-17 length of the resultant of the color-difference sigrlal E~-Y
and B-Y vectors, the output signal can be regarded, for simplicity, as representing the color density; the color-differenre signals have a larger amplitude in the high color-density areas and a smaller alnplitude in the low color-demsity areas. The gain of the variable gain amplifier 6~ is controlled in accordance with the color density detec-tion signal outputted from the adder 13 by detecting the color density.
As described above, accordillg to the prior art image quality correction circuit, when the amount of correction is increased for the ~ligh-frequellcy conlponerlt of the lunlinance signal, the amplitude of tlle high-frequency component of the luminance signal increases in the high color-density areas, but in areas where the high-frequency conlpollent of the luminance signal is large in -the positive side, the ampli-tude of -the luminance signal alone increases while the magnitude of the color signals does not increase. This causes color dropout, resulting in image quality degrada-tion. That is, a saturation drop is exacerbated in the high-frequency areas of a higll color saturation image. This tendency is particularly pronounced in areas where over-shoots and preshoots occur. I1urthermore, in the higil color-density areas, the signal-to-noise ratio decreases since the noise component of the luminance signal is also amplified.

CA 0223~10~ 1998-04-17 That is, as the amount of correction is increased in the high color saturation areas, such problems as poor detail reproductiorl and ~laclc level variation can t)e alleviated correspondingly, but this in turn causes the problem of increased saturation drop and S/N degradatioll, which places a limit to the asnourlt of correction that can be achieved. Therefore, the image quality improvemen-t tllat can be perceived by the eye has not been satisfactory.
Furthermore, in the case Or the color density detection signal produced in the prior art image quality correction circuit, the amoullt of correction is not distributed appro-priately between various colors. As previously noted, in the NTSC, PAL, and SECAM television signal systems, the high-frequency component of the video signal is transmitted by the luminance signal alone, and since the amplitude ratio of the luminance signal contclined in each color is differ-ent, the amount of high-f'requency component reduction is also different. That is, the amount Or high-rrequerlcy component reduction is small in areas of a color containing a large amount of luminance component, while the amount of high-frequency component reduction is large in areas of a color containing a small amount of luminance component. In a specific example, for an image of monochromatic blue consisting of 100% B signal, the amplitude of the luminance signal can be calculated as Y = 0.11 from the equation Y =

CA 0223~10~ 1998-04-17 0.30R + 0.59G + U.11 ~ since N = G = 0 and B = 1. The value is the slinallest of all the colors in the color bars. It is proven t:hat, in this case, the gain of the high-frequency component is reduced to 11% of the gain before transmission at the t:ransmi~tillg end, sup~osing that y characteristic of the television picture tube is 2Ø Accordingly, the gain of the high-frequency con~ponerlt drops down to the amplitude ratio of the luminance signal. If high frequency components are contained in the colors of the color bars, the high-frequenc;y component in each color drops down to the ra-tio shown in Table 1 below.

Red Green Blue Magenta Cy~n Yellow White 30,~ 59% 11% 41% 70% 89% 100%

Each of the values shown in Ta~le l coincides with the amplitude ratio of the lunlinancé signal contained in each color. .If -the high-frequency component is to be corrected for each color, the amplitude ra~io, 11%, of the lunlinance signal for the blue color, for example, requires that the high-frequency component should be corrected to the ratio of 1/0.11. In the prior art example, no consideratioll is given to the amplitude ratio of the lulninance signal contained in each color. If this factor is to be considered, the color CA 0223~10~ 1998-04-17 density signal obtained in the prior art example nee~s to be divided by the luminance signal.
There is ~isclose~ another prior art WhiCIl proposes an example involving divisioll by the luminance signal, but one shortcolllirlg of this example is that the complexity of cir-cuitry increases because of the inclusior- of a dividing circuit in -the electric circuit.
Another problem is the effect of correction appearing unnatura~l at the boundaries between colors. Fig. 2 shows how a primary color signal is affected whell a color-difference signal is created frolll the lumillallce signal and when the high-frequency colnponent of the lumirlance signal is enhanced, by taking as an example a pattern consisting of successive color bars, i.e., gray, red, white, red, and black arranged in this order from left to right Oll the screen. Solid line 170 :indicates the luminance signal;
dotted lines show the corltours of the portions where high-frequency enhancement are made; 171, 175 indicate the black level; 172 is the R-Y co:Lor-difference signal; 173 is a no-color level; solid line L74 represents tlle R primary color signal; and dotted lines show the portions where the high-frequency correction is nlade to the luminance signal 170, tlle reference signals A, B, C, and D from left to right indicating the waveforms at the respective color boundaries.
As can be seen from Fig. 2, high-frequellcy correction is CA 0223~10~ 1998-04-17 effective in achieving uniforln image quality only in the case of D, but in the cases of ~, B, and C, the high-frequency correction of the luminance signal causes unnatural contours. A, E~, and C are where the slope is reversed between the luminancc signal 170 and the color-difference signal 172. In A, high-frequency correction results in an unnatural step formed in the rising portion of the prinlary color signal. In B and C, the boundary contours which initially were not present on the reverse image side are formed because the lun~inclnce and color-difference signals are transmitted separately in separaLe frequency bands. These contours are further emphasized by the high-frequency correction of the luminance signal. As a result, overcorrection tends to occur in the case of the B and C
patterns, resulting in overelllpllasized contours.
There are other problelns: in the case of an image whose overall S/N ratio is low, if correction is made meticulously on light color portions, the S/N ratio will further degrade, and furthermore, while the appearance of wrinkles in the human skin, a light color area, should be reduced to ob~ain a pleasing image, if correction is made to such light color areas, the image will apF~ear more real, making the wrinkles further noticeable.
A further problem is that aperture correction is per-formed using a separate circuit, requiring the provision of CA 0223~10~ 1998-04-17 a separa.te aperture correction circuit and thus increasing the size of the circuitry required to achieve image im-provements .

SUMMARY OF TIIE INVENTION
An object of the present invention is to provide allimage quality correction circuit that is capable of mini-mizirlg c;olor dropout (sa~uration drop) alld S/N degradation due to t,he positive high-frequency componellt of the lumi-nance signal even when the amount of correction is increased for the high-frequency component of the lulllinance signal.
Ancther object of the present invention is to provide an image quality correct:ion circuit capable of drastically improving the image qual:ity by correcting SUC}l problems as black level variations and poor detail reproduction in high color-density areas.
A f'urther object of the present inven-tion is to provide an image quality correct:ion circuit capable of ob-taining a properly corrected image free rrom excessive correction or insufficient correction depending on colors, by detecting a color density detection signal in which the ratio of the luminance signal contained in each color is considered.
A s.till further object of the present invention is to provide an image quali-ty correction circuit capable of preventing unnecessary con-tour enhancement, and the result-ing image quality degradation, at the boundary between non-CA 0223~10~ 1998-04-17 color and color areas or at the boundary between different colors.
A yet fur-ther object of the present invention is to provide an image quality correc-tion circuit capable of improving the S/N ratio, and also capable of preventillg the image appearance degradation due to empllasized roughness of the human~ skin, by deliberately reducing the effect of correctiorl on low color-density areas.
Yet another object o~ the present inverltiorl is to provide an image quality correction circuit capable of accornplishing aper-ture COrrectiOII, by executing n certair degree of image quality improven~ellt in no-color areas.
According to the present invention, there is provided an image quality correction circuit which comprises color density detecting means for detecting color dellsity, high-frequency component extrac:ting means for extrac-ting a high-frequency component of a luminance signal, a variable gain amplifier for amplifying the extracted high-rrequency COIII-ponent of the luminance signal in such a marlner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color densitY is low and a slice circuit for slicing out the output amplitude of the variable gain amplifier at a prescribed value. The order of lhe variable gain amplifier and the slice circuit may be re~rersed.

CA 0223~10~ 1998-04-17 When the extracted high-frequeIlcy component of the luminallce sigrlal is input into the variable gaill amplifier, and control is performed so tl~at the gain of the variable gain amp].ifier is increased when ~he detected color density is high, and is reduced when it is low, the anlplified higll-frequency componerlt of the lulnillance signal will have a greater amplitude as the color density increases. When the amplified higl--frequency colllporlellt Or the luminunce signal is fed into the slice cir(uit, the positive portions of the high-frequency component of the luminance signal and the ripple component such as noise are removed. After the removal, the negùtive portions of the high-rrequency component of the luminance signal are combined with the luminance signal to be corrected, thus accomplislJing the correction of the luminarlce signal. In high color-density areas, since the luminance signal is corrected only in the direction in which the color density increases, no color dropout occurs at the boundaries between colors. Further-more, removal of the ripple component inlproves the S/N
ratio. It will further be noted -that since the correction is made u,ing the negative portiorls of the high-frequency component, the problems Or black level variations and detail degradation are alleviated. As described, since the high-frequency component of the lumirlance signal to be used for correction of the luminance signal is stripped of its CA 0223~10~ 1998-04-17 positive portions, as wel.l as its ripple component such as noise, by the slice circu:i-t, the amoun-t of correction applied to the luminance sigllal can be increased without causing S/N degradation. This furtller prevents color dropout and other defects due to overshoots and preshoots on the positive side of the lumillallce signal, and tllus minimizes image quality degradation.
According to the present invention, there is also provided an image quality correction circuit wllich, in order to obtain a color density detection signal in which the amplitude ratio of ~he lunlinance signal contained in each color is considered, employs one of -the following three configurations for the col.or density detecting means: the first configuration wherein the color density detecting means includes a multiplier for performing a multiplication by an inverted luminance signal, and a low-pass filter; the second configuration wherein the color density detecting mearls includes a maxilnum value detectioll circuit for detect-ing maximum values for color vectors where the amount of high-freqllency componellt reduction is large; and the tllird configuration wherein the color density detecting mea~ls includes a binary circuit for converting the luminance signal into a binary form. One of the first, second, or third configuration is employed for the color density detecting means so that the,color density can be detected by CA 0223~10~ 1998-04-17 consider:ing the amplitude ratio of the lun~ nce signal contained in each color, i.e., arl o~tilllUIll color density detection sigrlal for image quality correctiorl can be ob tained.
In the first configuration, by multiplying a signal correspondiJI~ to the color density by the inverted signal, the color density detectioll signal is corrected to a relatively low level for colors having a larger luminance component, and to a relatively high level for colors having a smaller lunlinance compollent, tllus renlizing the production of a color density detection signal for optimum control of the image quality correction. In the second configuratiorl, for a number of vectors where the amourlt of high-frequency component reduction is large, signal waveforms are synthe-sized by taking pOilltS where the colors along the designated vector directions are at their respective maxilllulll values, and then, maxilnum values among the signal waveforms are detected to obtain the color density signal for optimum control of the image quality correction. In the third configuration, the luminance signal is converted into a binary form with a low voltage for image areas of high luminosity and a high voltage lor image areas of low luminosity, and a signal corres~onding to the color density, in which ~he amplitude ratio of the luminance signal has yet -to be reflected, is multiplied by the binary luminance CA 0223~10~ 1998-04-17 signal; as a result, the color density detection signal is corrected to a relatively low level for relatively light colors containing a larger luminance compollent, and to a relatively high level for relatively dark colors contairling a smaller luminar-ce component, tllus realizirlg t~le production of a color density detection signal for optimulll control of the image quality correction.
Another image quality correc~ion circui-t, according to the invention, includes a boundary reduction circuit for reducing the amplitude of -the color density detection signal at the boundary between colors. The color density detection signal detected by the color dellsity detecting mealls is supplied to the boundary reduction circuit where the edges of the signal are detected and the amplitude of the signal is reduced in areas near the edges to reduce the effect of correction at the boundary betweell no-color and color areas or at the boundary between different colors. When the resulting signal is used as a new color density detection signal for image quality correction, the effect of image quality correction can be reduced at the boundary between colors, i.e., at the edge portions of the color density detection signal detected by the color density detecting means. This is effective in alleviating the problem of unnatural contours.
Another image quality correction circuit, according to CA 0223~10~ 1998-04-17 the invention, includes a smal.l-alnplitude removal circuit for removing small-amplitude portions of the color density detection signal to suppress the effect of image correction in low color-density areas. In small-aJnplitude'portions of the color density detection signal, its reference voltage is raised to a level higher than the no-color voltage level, and the signal portions below tlle reference voltage are relnoved. By performing conversion with the reference voltage level as the minilllum value, tlle snlall ampli~ude portions are removed, which has the effect of preventing the overall S/N perception from substan~ially droppillg ill a low S/N image, and also preventing the wrinkles, stains, etc. on the human skin from being emphasized and affecting the image appearance.
Another image quality correction circuit, according to the invention, includes an offset addition means for adding a DC component to the color density detection signal so that a certain degree of high-frequency correct,ion can be made in no-color areas. By deliberate.ly adding the DC offset to the color density detection signal, a certain degree of high-frequency correction effect can be obtained in no-color areas, thus serving -the function of an aperture correction circuit.
As described above, since the color density detection signal is created by taking int,o considera-tion -tlle ratio of ' 16 CA 0223~10~ 1998-04-17 the luminance signal contained in each color, no such phe-nolllena as excessive correction or insufficierlt correction occur depending on colors, and a properly corrected image can be obtained.
The above and further objects and features of the inventioJI will more fully be apl>ulcn-t from the following detailed description with acconlpanying drawings.

BRIEF DESCRIPTION OF TI~E DI~AWINGS
Fig. 1 is a block diagram showing the configuration of a prior art image quality correction circuit;
Fig. 2 is a wavefornl diagraln for explaining problems of the prior art.
Fig. 3 is a block diagranl showing the configuration of an image quality correction circuit ( a first embodilllen-t) according to the present inventiorl;
Fig. ~ is a blocl{ diagralll showing Lhe conriguration Or an image quality correction signal generator;
Figs. 5A - F are wavefornl diagrams of signal voltages;
Figs. 6D - F are waveform diagrams of signal voltages when circuit constants are challged;
Figs. 7D - F are waveforlll diagrams of signal voltages when circuit constants are change~;
Fig. 8 is a block diagram showing the configuration of an image quality correction Cil'CUi t (a second embodilllent) CA 0223~10~ 1998-04-17 according to the present inventioll;
Fig. 9 is a block diagram showing the configuration of an image quality correction circuit (a tllird embodilllent) accordirlg to t}-le present invenlion;
Fig. 10 is a block diagram showing t}le configuration Or a contour detector;
Fig. 11 is a block diagranl showing the configurAtion Or an image quality correction circuit (a fourth embodiment) according to the present invention;
Fig. 12 is a block diagram showing the configuration of an image quality correction circuit (a rifth emboditllent) according to the present inventiorl;
Fig. 13 is a block diagram showing the conrigur~tion of an image quality correction circuit (a sixth embo~iment) accordillg to the present inventioll;
Figs. l~A - H are output waveform diagrams for various p~rts of a color density detector in the sixth embodilllent when color bars are inputt,ed;
Fig. 15 is a block diagram showing the configuration of a modified example of the SiXt]l embOdilllellt.
Fig. 16 is a block diagrarn showing an alternative con-figuration for a high-frequency component detector in the sixth embodiment;
Fig. 17 is a block diagram showing the configuration of a color density detector in an image quality correction CA 0223~10S 1998-04-17 circuit (a seventh embodiment) accor~ing to the present invention;
Fig. 18 is a block diagram showing the configuration of a color densi-ty detector in an inlage quality correction circuit (an eighth embodiment) according to the present invention;
Fig. 19 is a diagram showing vector orientations where compollerlt signals in -the color density detector Or the eighth embodiment are at their nlaximum values;
Figs. 20A - D are output waveform diagrams for various parts of the color density detector in the eighth embodiment when color bars are inputted;
Fig. 21 is a circuit diagram showing the configuration of a maximum value detection circuit;
Fig. 22 is a blocl~ diagram showing the configuration of a color density detector in an image quality correction circuit (a ninth embodiment) according to the present in-vention;
Fig. 23 is a diagram showing vector orientations where component signals in the color density detector of the ninth embodiment are at their maximum values;
Figs. ~4A - E are output waveform diagrams for various parts of the color density detector in the ninth embodiment when color bars are inputted;
Fig. 25 is a block diagram showing the configuration of CA 0223~10~ 1998-04-17 a color density detector in an image quali-ty correction circuit (a 10th embodiment) according to the present inven-tion;
Fig. 26 is a diagram showing vector orientations where conlponellt signals in the color density detector of the 10-th embodiment are a-t their maximu-n values;
Figs. 27A - E are output waveform diagralns for various parts of the color density detector in the 10-th embodilllent when color bars are inputted;
Fig. 28 is a block diagram showing -the configuration of a color density detector in an image quality correction cir-cuit (an 11th embodiment) according -to the present inven-tion;
Figs. 29A - D are outyut waveform diagrams for various parts of the color density detector in the 11th embodimen-t when color bars are input;
Fig. 30 is a block diagranl showing the conriguration of a color density detector in an image quality correction cir-cuit (a 12th embodiment) accordil-lg to the present inventiorl;
Fig. 31 is a block diagram showing the configuration of an image quality correction circuit (a 13th embodimen-t) according to the present invent,ion;
Fig. 32 is a block diagram showing the configuration of a modified example of a color density detector of the 13th embodiment;

CA 0223~10~ 1998-04-17 Fig. 33 is a block diagram ShOwillg the configuration of another modified example of the color density detector of the 13th embodiIllent;
Fig. 34 is a block diagrclllI showing the configuratioIl of a modified exalIlple of the first embodiment;
Fig. 35 is a block diagraIll showing the configuration of a modified eY~ample of the sixth embodiment;
Fig. 36 is a block diagram showing the configuration of an image quality correction circuit (a 14tll embodiment) according to the present invention;
Fig. 37 is a blocl~ diagram showing the configuration of a modified exanlple of the 14th enIbodilllent;
Fig. 38 is a waveforIn diagram showirlg signal waveforms at various portions of the circuit shown in Fig. 37;
Fig. 39 is a blocl~ diagram showing the configuration of an image quality correction circui-t (a 15th embodimeIIt) according to the preserlt inventiorl;
Fig. 40 is a waveform diagraIll showing signal waveforms at various portions of the circuit shown in Fig. 39;
Fig. 41 is a blocl~ diagram showing the configuration of an image quality correction circuit (a 16th embodilIlent) according to the present invention;
Fig. 42 is a circuit diagram showing the configura-tion of a snlall-amp1itude reIlloval circuit;
Fig. 43 is ~ signal waveform diagr~m for explaiIling the CA 0223~10~ 1998-04-17 operation of the small-amplitude renloval circuit showrl in Fig. 42;
Fig. 44 is a blocl~ diagralll showing tlle configuration of an image quality correction circuit (a 17th embodinlent) according to the present invention; and Fig. 45 is a signal waveforlll(1iaglam for explaining the opera-tion of an offset additiorl circuit shown in Fig. 4~.

DESCRIPTION OF TIIE PI~EFER~ED EMBODIMENTS
The present invention will be described in detail below with reference to the drawirlgs illustrating the preferred embodiments .
(Enlbodi~nent 1) Fig. 3 is a block diagralll showiIlg tlle con~iguration of an image quality correction circui t according to one elllbodi-ment (a first embodiment) Or the present invention. A
luminance signal Y is applied t,o a negative terminal - of a subtractor 4 tllrough a low-frequency pass filter 2 (here-inafter called the low-pass filter). The luminance signal Y
is also inputted to a delay circuit 3 through which -the signal is supplied to an adder 7 as well as to the subtrac-tor 4. An output signal fronl the subtractor 4 is supplied as an inputted to a variable gain amplifier 5 whose output signal is fed -to a slice circuit G. An output signal from the slice circuit 6 is supplied to the adder 7.
The adder 7 outputs a corrected luminance signal Y'.

CA 0223~10~ 1998-04-17 On the other hand, a chrominance signal I is inputted to an adder 13 through a full-wave rectifier 10, while a chromi-nance signal Q is inputted to the adder 13 through a full-wave rectifier 12. An output signal frolll tlle adder 13 is applied to a control terminal of the variable gain amplifier 5. Ilere, the chrominance signa1s, I and Q, are inputted after a delay of a predetermined time so that they are in phase with the output signal frolll the su~tractor 4 when fed into the variable gain amplifier 5. The subtractor 4, the variable gain amplifier 5, the slice circuit 6, and the adder 7 together constitute an image quali~y correction signal generator 14 for genera~ing tlle corrected lunlinallce signal Y'.
The ope1ation of the above-configured image quality correction circuit will be described below.
When the luminance signal Y is inputted to the low-pass filter 2 and tlle delay circuit 3, the low-pass filter 2 with a prescribed high frequency cutoff characteristic transmits only a low-frequerlcy component 16 of the luminance signal.
The delay circuit 3 outputs a l.uminance signal 15 with such a delay that its phase coincides with the phase of the low-frequency component of the luminance signal passed through the low-pass filter 2. Then, in the subtractor 4, the low-frequency component 16 of the luminance signal fed from the low-pass filter 2 is subtracted from the luminallce signal 15 CA 0223~10~ 1998-04-17 which is fed from the delay circuit 3 and contains the hig~l-frequency componellt; as a resu:lt, only the high-frequency component of the luminarlce signal is output from the sub-tractor 4. The variable gain anlplifier 5, as will be described in detail later, amp:Lifies the high-frequency componellt of the luminance signal in accordance with the amplitude of a color density detection signal 26 outputted from the adder 13. To describe specifically, control is performed for amplification so that the gain is increased in high color-density areas and is reduced in low color-density areas.
The high-frequency comporlent of the lumillance signal, with its gain correlated to the color density by the vari-able gain amplifier 5, is inputted to -the slice circuit 6 where those portions Or the higll-frequellcy comporlent of the luminance signal which exceed a prescribed value are cut off, thus removing the positive portions, as well as the ripple component such as noise, from the high-frequency component, and the image quality correction signal results.
The image quality correction signal outputted from the slice circuit 6 is added in the adder 7 to the luminarlce signal to be corrected, i.e., the luminance signal containing the high-frequency component and delayed tllrough the delay circuit 3. The adder 7 outputs the result as the corrected luminance signal Y'.

CA 0223~10~ 1998-04-17 On the other hand, the chrominance signal I is full-wave rectified by tlle full-wave rectifier 10 and inputted to the adder 13, while the chrominal-lce signal Q is full-wave rectified by the full-wave rectifier 12 and inputted to the a~der 13. The full-wave rectified chrolninance signals I and Q are added toge-ther in the adder 13, which therl outputs the color density detection signal 26 correspondillg to the color density. This color density ~etection signal, whose ampli-tude increases as the color density increases and decreases as the density decreases, is used to control the gain of -the variable gain amplifier 5; that is, the anlplitude of ~he high-frequency componellt of the lulninarlce signal that the variable gain amplifier 5 outputs is increased in high colol~-density areas alld is reduced in low color-density areas. Color-dii'ference signals ~-Y and B-Y
may be used instead of the chrominance signals I and Q, respectively.
Fig. 4 is a circuit diagram showing the configuration of one example of the image quality correction signal generator 14. Power supply V is grounded tllrough a series circuit consisting of a resistor 33, a transistor 32, a transistor 23, a transistor 17, and a constant current source 19, and is also grounded through a series circuit consisting of a resistor 29, a transistor 28, a -transistor 24, a transistor 18, and a constant current source 20. The CA 0223~10~ 1998-04-17 base and collector of the transistor 32 are connected together, and the base and col:lec-tor of tlle transistor 28 are also connected together. The collector of -the transistor 23 is connected to tl~e collector of ttle transistor 25, while the emitter of the transistor 25 is connected to the emitter of a transistor 24. Further, the collector of the transistor 2~ is connected to the collec~or of a transistor 22 whose emitter is in turn connected to the emitter of the transistor 23.
The bases of the transistors 23 and 24 are coupled together and connected to the positive terminal of a constant voltage source 27 whose negative terminal is grounded. The base Or the transistor 25 is conrlected to the base of the transistor 22. The emitter of the transistor 17 is connected to the enlitter of the transistor 18 via a resistor 18. Thus, the transistors 22 and 23 form a differential amplifier, and the transistors 2~ and 25 also form a differential amplifier. Further, the transistors 17 and 18 form a differential alnplifier. The powel supply V is grounded through a series circuit consisting of a resistor 35, a transistor 30, a transistor 36, and a resistor 37, and is also grounded through ~ series circuit consisting of a constant current source 40, a transistor 38, and a resistor 39.
A transistor 34 is connecl,ed in parallel to the CA 0223~10~ 1998-04-17 transistor 30 whose base is connected to the ~ase of the transistor 28 and is grounded via a capacitor 31. The base of the transistor 34 is connected to the base of the transistor 32, so that the transistors 32 and 34 fornl a mirror circuit. Similarly, the base of the tr~nsistor 36, which is connected to its collector, is conllected to the base of the transistor 38, also forming a nlirror circui-t. A
node between the constant current source 40 and the tran-sistor 38 is connected to the base of the transistor 17 via a resistor 41 WhiC}l acts as the load for the transistor 38.
The color density detection signal 26 outputted from the adder 13 (Fig. 3) is applied to the bases of the transistors 22 and 25. The transistor 17 is su~plied at its base with a voltage Vy that represents tlle lumillance signal 15 containing the high-frequency coml~onent outputted from the delay circuit 3 (Fig. 3).
The transistor 18 is suppLied at its base with a voltage VyL that represents the luminal-lce signal 16 consisting of the low-frequerlcy comporlent outputted from the low-pass filter 2 (Fig. 3). The image quality correction signal, which is outputted from the node between the constant current source 40 and the transistor 38, and the luminance signal 15 containing the high-fre~uency component, which is output through the resistor 41, are combined together -to produce the corrected luminance signal Y'.

CA 0223~10~ 1998-04-17 The transistor 28, the resistor 29, the transistor 30, the capacitor 31, the transistor 32, the resistor 33, the transistor 3~, and the resistor 35 together constitute the slice circuit ~. The resistors 29 arld 35 are chosen to have the sume resistan~e value, and the resistarlce value of the resistor 33 is chosen to be slightly smaller than the individual resistance value of the resistors 29 and 35.
Further, the reslstors 39 and 37 are chosen to have the same resistance value. The resistance value of the resistor 41 is denoted by R2.
The operation of the above image quality correction signal generator l~ will ~e described below.
Let i represent a differential current proportional to a voltage differential, Vy - V~L, such that i = (Vy -VyL)/R1, where R1 is the resistallce value of the resistor 21, Vy is the voltage of the luminance signal 15 containing the lligh-frequency component and being applied to the base of the transistor 17, and VyL is the low-frequency lunlinance signal 16 stripped of the high-frequency colnponent and being applied to the base of the transistor 18. Now suppose that the differential current i is flowing from the e~nitter of the transistor 17 toward the emitter of the transistor 18 in the dlfferential amplifier formed from the transistors 17 and 18. In this SitUatiOII~ if we consider that the collec-tor current of the transistor 17 is approxilllately equal to CA 0223~10~ 1998-04-17 the emitter current, the collector current can be considered to be equal Lo Io ~ i which is the sum of the currenL Io that the constant current supply 19 flows to ground and the differential current i.
Similarly, if we consider that tlle collector current of the transistor 18 is approximately e~ual to the emitter current, the collector current can be considered to be equal to Io - i which is the difference between the current Io that the constant current source 20 flows to ground and the differcrltial current i. 'rherel'ore, tlle differential currellt i corresponds to tlle high-freqllency conlporletlt of the lumi-nance signal. Further, in connection with the voltage differential VcLR between the reference voltage that the constant voltage source 27 applies to t,he bases of the trnnsistors 23 and 24 and the voltage (includirlg the bias) of the color density detection signal 26 corresponding to the color density arld applied t,o the bases of tlle tran-sistors 22 and 25, a voltage for controlling the gain in accordance with the density of a specific color is applied to the differential amplifier formed from the transistors 22 and 23 and also to the differential amplifier formed from the transistors 24 and 25. First, we will describe the operation of the differential amplifier formed from the transistors 24 and 25.
It is assumed here that the capacitor 31 is not connected in the circuit. Whell we consider the situation where the differential amplifier formed from the transistors 2~ and 25 is in a ~erfect equilibrillnl condition, i.e., VcLR

= O, it can be considered that the base-enlitter voltage VBE24 of the transistor 24 is equal to tlle base-emitter voltage VBE25 of the tran~istor 25.
Let this situation be expressed as VBE24 = VBE25 = VBE (1) Suppose that the emitter current of the transistor 24 is approximately equal to the collector current, and let this current be delloted by IC24. Also suppose that the enlitker curreirlt of the transistor 25 is approxinlately equal to the collector current, ~nd let tihis current be deno-ted by IC25.
In the equilibrium condition IC24 = IC25 = ICA ~ IEA (2) where ICA alld IEA are the collector current and emitter current, respectively, in the equilibrium condition. For differential voltuge VcL~, if -the change is such that VBE24 VBE ~ VBE and VBE25 = VgE + ~ VgE~ the following equation hold~.

VCLiR = VBE25 ~ VBE24 = 2A VBE (3) With the voltage differential VcLR, the collector currents of the transistors 24 and 25 change as follows 24 = ICA ~ ~ IC (4) 25 = ICA + ~ IC (5) where ~ IC is the amount of change of the collector current.
On the other hand, the re1atiol~s~-lip between ICA and VBE
is expressed as CA ~ IEA = IS eXY[VgE/hJ ... (6) h - 1~T~q where lS ~ ~ Sa~uration current = l x 10 llA (Approx.) q ... Electron cllarge = 1.6 x l0 l9C (Approx.) k ... Boltzmalln constant = l.38 x l0 23J/K (Approx.) T ... Absolute temperature If the change due to the voltage differential VcL~ is considered, the following equation is given IC25 = ICA + ~ IC = IS eX~[vBE25/ll] = IS eXP~VBE+~ VBE/~I]
= IS exp[VBE/h]exp[~ VgE/h] = ICA exp[~ VBE/II] ~-- (7) Considering ~hat ~ VBE/h < l, if the above is approxinlated by the Taylor expansion, we have IC25 = ICA exp[~ VgE/~lJ ~ ICA [l + ~ BE
This can be simplified as ~ IC ~ ICA [~ VBE/h]
Similarly Ic24 ICA ~ ~ IC ~ ICA [1 - ~ VgE/h] ... (l0) In the differential amplifier formed from the transistors 24 and 25, the sum of the emitter current IC24 of the transistor 24 and the emitter current IC25 of the transistor 25 becomes equal to the collector current of the transistor 18. Hence CA 0223~10~ 1998-04-17 ICA = 1/2 (Io - i) ... (11) Rearranging for Ic24 alld IC25' we have IC24 = 1/Z (Io - i) [1 - ~ V~E/h] ... (12) IC25 = 1/2 (Io - i) [1 + ~ VBE/h] ... (13) 'rhe differential amplifier forllled from the tlallsistors 22 and 23 is considered in like manner, and rearranging for the collector current IC22 of the trarlsistor 22 and the collector current IC23 of the l.ransistor 23, we have lC22 = 1/2 (Io + i) [1 + ~ VgE/h] ~-- (14) IC23 = 1/2 (Io + i) [1 - ~ VBE/h] ~-- (15) The collector current IC32 of t,he transistor 32 is equal to the sum of IC23 and IC2s~ and using Equation (3), is expressed as 32 = IC23 + IC25 = 1/2 (Io + i) [1 - ~ VBE/h~
+ 1/2 (Io - i) [1 + ~ VBE/h]
Io ~ VBE/h i = Io - VCLR/2h i ... (16) Likewise, if it is assumed that the capacitor 31 is not connected in the circuit, the collector current IC28 of the transistor 28 is equal to the sum of IC22 and IC24. Hence 23 = IC22 + IC24 = 1/2 (Io + i) L1 + ~ VBE/h]

+ 1/2 (Io - i) [1 - ~ VBE/h]

Io + ~ VBE/h i = Io + VCLR/2h i ... (17) As it is, since the capacitor 31 is connected in the circuit CA 0223~10~ 1998-04-17 of Fig. 4, the AC component (VcLl~/21l) i in Equation (17) is absorbed into the ground potential side via the capacitor 31, and therefore, the collect,or current of the transistor 28 becomes equal to Io.
The transistors, 28 and 30, and tlle transistors, 32 und 34, are respectively paired to form a mirror circuit, but since the resistance value of the resistor 33 is slightly lower than that of the resistor 29, the vol-tage applied to the base of the transistor 30 is lower than the bias applied to the base of the transistor 34. When the voltage applied to the base of the transistor 3~ is higller than the voltage applied to the base of the transistor 30, the transistor 34 is off, which Ineans -that when the base voltage of the tran-sistor 34 is approximately equal to t}-le bias voltage, the transistor 34 is in the off state, so that alnlost all the current flowing tllrough the resistor 35 flows into the emitter of the transistor 30.
The only time that the transistor 34 is turned on is when the voltage applied to the base of the transistor 34 falls below the voltage applied to the ~ase of the tran-sistor 30 because of the AC component. If the current flowing through the resistor 35 is appro~imately equal to the current flowing from the interconnected collectors of the transistors 30 and 34 into the collector of the tran-sistor 36, the DC component of the current has precedence CA 0223~10~ 1998-04-17 since, as described above~ the voltage applied to the base of the transistor 30 is lower than the bias applied to the base of the transistor 34, and because the resistors 29 and 35 have the same resistance value, tlle DC compollellt flowing through the resistor 35 is equal to Io, so that the DC
comporlent flowing into the collector of tl-le trallsistor 3G is equal to Io.
The AC component flowing into the collector of the transistor 36 is given as -(VcLR/2h)i', which corresponds -to the difference between the DC component Io and the current that flows from the power supply V to the emitter of the transistor 34 via the resistor 35 when the voltage applied to the base of t}le transistor 3~ has fallen equal to or below the voltage applied to t~e ~ase of the transistor 30 because of t~le AC component. This AC component -(VcLR/2h)i' is the same as t~le AC component of the collector current of the transistor 32, -(VCL~/2h)i, except that a portion of its amplitude is cut off.
Ilence, the collector current of the transistor 36 is given by Io -(VCLR/2h)i'. The transistors 36 and 38 form a mirror circuit, and the resistors 37 and 39 have the same resistance value. Therefore, the collector current of the transistor 38 is approximately equal to the collector current of the transistor 36, which is given by Io -(VCLR/2h)i'. The collector of the transistor 38 is CA 0223~10~ 1998-04-17 connected to the constallt current source ~0 w~liCll flows the DC component current Io from the power supply V toward the collector of the transistor 38; therefore, the current flowing from the collector of tlle transistor 38 towurd the resistor ~1 is given by (VcLR/~h)i'. Since (VCLR/2h)i' is the current portion of the image quality correction signal, the voltage of the corrected lulllinallce signal Y' is obtained by adding the correction voltage (VCLR/2h)i'R2, where R2 is the resistance value of the resistor 41, to the lunlinance signal 15 to be corrected, whi(,h contains the high-frequency compoIIent. Fronl the expressiorl (VcLR/2~l)i'R2, i-t can be seen that the correction voltage is obtained by ~licing the voltage differential between the voltage Vy of the luminance signal and the voltage VyL Or the low-frequency compollerlt of the luminance signal alld then Illultiplying the result by a voltage whose gain is controlled by the voltage differential VcLR relating to the color density.
Referring now to Yig. 5 illustrating waveforms for various signal voltages, we will describe the operation of the correction signal generator 1~ where the color density detection signal, the high-frequency component of the luminance signal, and the luminance signal containing the high-frequency component are inputted. In Fig. 5, the time is plotted along the abscissa and the voltage along the ordinate. Fig. 5A shows the voltage variation of the color CA 0223~10~ 1998-04-17 density detection signal 26 in the direction of decrensing density, with the numeral 42 indicatirlg t,he level Or the reference voltage of the constant voltage source 27. Fig.
5B shows the voltage variation of the luminance signal 15 containing the high-frequency colllpollellt, while Fig. 5C shows the voltage variation of the low-frequency component lG of the luminance si~nal stripped of its hig~l-frequency com-ponent. Fur~her, Fig. 5D showc; signal voltage varicltiorls at the bases of the transistors 30 and 34, the numeral 43 indicating the base voltage variation for the transistor 34 and 44 for the transistor 30.
The numeral 45 indicates t,he ripple comporlent, such as noise, ap~earing near the bias areas of the high-frequency component of the lun~inance signal being applied to the base of the transistor 34. Fig. 5E shows a signal voltage variation at the emitter of the transistor 34; this AC
waveform is the voltage waveform of the correction signal.
Fig. 5F shows the voltage variation of the corrected luminance signal Y' produced by- combining the correction signal and the luminance signal containing the high-frequency component. The signal voltage 43 shown in Fig. 5D
is produced by subtracting the high-frequency-compollent-stripped luminance signal of Fig. 5C from the high-frequency-containirlg luminance signal of Fig. 5B and then controlling and anlplifying the gain of the resulting signal CA 0223~10~ 1998-04-17 in accordance with the color density represented by the color density de~ection signal 26 of Fig. 5A.
The portions where the voltage level of the signal voltage ~3 is higher than the voltage level of tl~e signal voltage ~ ShOWIl in Fig. 5D, i.e., tlle positive portions, are cut orf in accordance with the operation Or the tran-sistors 30 and 3~ illustrated in Fig. 4, as a result of which the image quality correction signll is obtairled which consists only of the nega-tive ~ortions of the high-frequency component of the luminance signal, as shown ir- Fig. 5E. The ripples, such as noise, appearing near the bias portions of the signal voltage 43 are also cut off. The image quality correction signal shown in Fig. 5E is combined with the luminance signal shown in Fig. 5B, to ~roduce the corrected luminance signal Y' shown in Fig. 5F. In the corrected luminance signal Y' shown in Fig. 5F, the positive portions of the high-frequerlcy conlponent of -the luminance signal are not amplified, but only the nega-tive portions of the high-frequency component are amplified, with the ripples such as noise removed. Furthermore, the negative portions of the lu~ninance signal are corrected in accordance with the detected color density.
In this embodiment, the s:Lice circuit is so constructed as to cut off the ripples such as noise as well as the positive portions of the high-frequency component of the CA 0223~10~ 1998-04-17 luminance signal, but it will be noted that by charlging the resistance ratio of the resistors 33, 29, and 35, i-t is possible to change the voltage value over which the high-frequency componel-lt of the lumillance sigrlal is cut off.
Also, if it is desired to cut off only ~he positive portions Or the high-frequency componerlt of the lulllinallce signal and correct for the ripples such as noise, the resistors 33, 29, and 35, for example, should only be chosen to have the same resistance value. In that case, the variations of the signal voltages will be as showrl in Fig. 6.
Figs. 6D, E and F show ~he signal voltage variations corresponding to those showrl in Figs. 5D, E and F, and -the other signal voltage variations are not shown since they are the same as those ShOWII in Figs. 5A, B and C, respectively.
In Fig. 6D, a signal voltage 47 is the base voltage of the transistor 30 shown in Fig. 4; as shown here, -the level of this voltage is equal to the bias applied to the base of the transistor 34. The correctiorl signal showrl in Fig. 6E is the same as the negative portions extracted from the high-frequency component of the lumirlance signal shown in Fig.
6D. Furthermore, as can be seen from Fig. 6F, the amount of correction applied to the luminance signal in accordance with the color density can be ma~e larger than that shown in Fig. 5F, also for the ripple compollents such as noise con-tained in the high-frequency component of tlle luminance CA 0223~10S 1998-04-17 signal.
Fig. 7 shows the waveforms of tlle various signal voltages for a case in which the resistors 33 and 35 are chosen to have the same resistance value while the resist-ance value of the resistor 29 is made slightly snlaller -than the individual resistance value of the resistors 33 and 35.
Similarly to the foregoing example, Figs. 7D, E and F, correspond to Figs. 5D, E an~ I, and Figs. ~D, E and F, respectively. The signal voltage ~6 shown in Fig. 7D
indicates the base voltage of the transistor 30, which, as shown, is set at a slightly higher level than the bias of the signal voltage 43 applied to the base of tlle transistor 34. The result is the waveform shown in Fig. 7E where the positive portions of the high-frequellcy componellt of the luminarlce sigrlal are suppresse(l. As can be seen from the corrected luminance signal Y' shown in Fig. 7F, the amount of correction applied to the luminance signal can be made l~rger, in accordance with the color density, for the negative portions of the high-fre~uency conlponent of the luminance signal and also for those portions of the positive portions thereof nearer to the negative portions.
(Embodiment 2) Fig. 8 is a block diagram showing the configuration of an image quality correction circuit according to ano-ther elnbodiment (a second embodiment) of the present invention.

CA 0223~10~ 1998-04-17 The luminarlce signal Y is inpu~ted to a subtractor 4 through a low-pass filter 2 and also through a series circuit con-sisting of a low-pass filter 48 and a delay circuit 49.
Furthermore, the lulninance signal Y is inputted to an adder 7 through a delay circuit 3. Tlle low-pass filter 48 is chosen to have a higher cut-oI'f' frequency tllarl that of the low-pass filter 2. The delay circuit 49 delays tl1e lumi-nance signal inputted to it so that the phase of the luminance signal passed through the low-pass filter 48 coincides with the phase of the lumillance signal passed through the low-pass filter 2. In other respects, the configuratioll of tllis embodilllerl~ is the same as that shown in Fig. 3, and the same componellts are designated by the same reference numerals as those used in Fig. 3.
In -~he image quality correction circuit of this embodimen1, the luminance signal supplied through the low-pass filter Z is subtracted in the subtractor 4 from the luminance signal that is passed tllrough the low-pass filter 48 having a higher cut-off frequency than the low-pass filter 2 and that is delayed throug1l the delay circuit 49 to achieve phase synchronization to the luminance signal inputted through the low-pass filter 2. A desired high-frequency component of the luminance signal is thus obtained. This makes it possible to amplify a designated band component of the luminance signal in accordance with CA 0223~10~ 1998-04-17 the color density detection signal.
By limiting the band in the high-frequency area in this~
manller, when a carrier chrominance signal and -the luminance signal are separated from the composite video signal, any chrominance subcarrier frequency components remaining in the luminance signal can be removed before creating the correc-tion signal. Furthermore, without the high-frequency band limiting, the amount of correction applied to the luminance signal would become excessive for portions where the lumi-nance signal changes abruptly, and the resul-ting image would appear rather unnatural. Lim:itillg the lligh-rlequency band has the effect of appropriately limiting the amount of correction applied to the luminance signal.
(Embodiment 3) Fig. 9 is a block diagram showing the configuration of an image quality correction circuit according to another embodiment (a third enlbodimellt) of the present invention.
The luminance signal Y is inputted to ~ contour detector 50.
A contour signal 56 outputted from the contour detector 50 is supplied to a variable gain amplifier 5. A delayed luminance signal 57, also outputted from the contour detector 50 but delayed to achieve phase synchronization to the contour signal, is inputted to an adder 7. In other respects, the configuration of this embodilllent is the same as that shown in ~ig. 3, and ttle same components are ~1 CA 0223~10~ 1998-04-17 designated by the same reference numerals as those used in Fig. 3 Fig. 10 is a block diagram sllowing the configuration of the contour detector 50. The luminance signal Y is inputted to a subtractor 53 and also to a delay circuit 51 which introduces a delay of a very small amount of time, for example, of the order of 100 ns. The delayed luminal1ce signal 57 outputted from the delay circuit 51 is outputted from the contour detector, while the same delayed luminance signal is inputted to subtractors 53 and 54 and also to a delay circuit 52 that has a similar delay charac~eristic to that of the delay circuit 51. The luminance sigr1al delayed through the delay circuit 52 is inputted to the subtractor 54. Output signals from the subtractors 53 and 54 are supplied to an adder 55, from which the contour signal 5G is outputted.
In the contour detector 50, the inputted luminance signal Y is passed through the delay circuits 51 and 52, while the subtractor 53 subtracts the lumillance signal Y
directly .inputted to it from the luminallce signal passed through the delay circuit 51. The output signal of the subtractor 53 represents preshoots which are outputted as positive at the right edge of an image and as negative at the left edge of the image. On the other hand, in the subtractor 54, the luminance signal passed through the delay CA 0223~10~ 1998-04-17 circuit 5~ is subtracted from the luminarlce signal fed from the delay circuit 51. The output signal of the subtrac-tor 54 represents overshoots which are outputted as positive at the left edge of an image and as negative at the right edge of the image. In the adder 55, the preshoots outputted from the subtractor 53 and the overslloots outputted from the subtractor 54 are summed, to obtain the contour signal 56.
The phase of the contour signal 56 coincides with the phase of the luminance signal 57 outputted from the delay circuit 51. With this image quality correction circuit also, the image quality can be corrected, as with the image quality correction circui-ts o~ the foregoing embodilllerlts.
In this embodilnent, a contour detection method used in an aperture correctioIl circuit is used to detect the high-frequency component of the luminance signal. Alternatively, a circuit for obtaining the second derivative of the lumi-nance signal and extracting the resulting values may be used as the contour detector shown in Fig. 9. It is also possible to use a circuit in which the high frequencies of the luminance signal are directly extracted using a high-pass filter.
(Embodinlent 4) Fig. 11 is a block diagram showing the configuration of an image quality correction circuit according to anotller embodiment (a fourth embodiment) o~ -the present invention.

CA 0223~10~ 1998-04-17 A carrier chrominallce signal C, which is delayed so that its phase coincides with the phase of the lligh-frequency component of the luminance signal outputted from the subtractor 4, is inputted to a full-wave rect,ifier circuit 59. The carrier cllronlinance signal C outputted from the full-wave rectifier circui~ 59 is passed ~llrOUgh a low-pass filter 69 where the chrominance subcarrier frequency com-ponents are removed before it is supplied to the variable gain amplifier 5. In other respects, the configuration of this embodiment is the same as that shown in Fig. 3, and the same components are designated by the same reference numerals as those used in Fig. 3.
According to this image quality correction circuit, after full-wave rectificatioll through the full-wave rectifier circuit 59, the carrier chrolllinance signal C is stripped of the subcarrier wave components by the low-pass filter 69, and a color density detection signal proportional to -the color density is obtained. Then, in accordance with the color density detection signal fed from the low-pass filter 69, the variable gain amplifier 5 controls the gain with which to amplify the high-frequellcy component of the luminance signal. This operation is the same as that of the image quality correction circuit illustrated,in Fig. 3.
Extraction of the high-frequency component of the luminance signal and correction of the luminance signal are also done 4~

CA 0223~10~ 1998-04-17 in the same manner as previously de~scribed. In this embodiment, the means for detecting the color density from the carrier chrominance signal C is used in combination with the me~ns for e~tracting the higll-frequency component of the luminance signal as used in the inlage quality correction circuit shown in Fig. 3. Alternatively, -the mealls for detecting the color density from the carrier chrominance signal C may be used in combination with the means for extracting the high-frequency component of the luminance signal as used in the image quali-ty correctioll circuit shown in either Fig. 8 or Fig. 9. Further, a half-wave rec-tifier circuit may be used instead of the full-wave rectifier circuit 59.
(Embodimerl-t 5) Fig. 12 is a block diagram showirlg the configuration of an image quality correction circuit according to another embodiment (a fifth embodimerlt) of the present invention.
The chronlinance signal I is inputted to a variable gain amplifier 60, whose output signal is fed to a full-wave rectifier 10. The chromillance signal Q is inputted to a variable gain amplifier 61, whose output signal is fed to a full-wave rectifier 12. A microcomputer 62 supplies a control signal 70 to control the gain of the variable gain amplifiers 60 and 61. In other respect, the configuration of this embodiment is the same as that shown iIl Fig. 3, and CA 0223~10~ 1998-04-17 the same components are designated by ~he same reference nulllerals as those used in Fig. 3.
A signal representing the value of all output voltage of a digital/allalog converter or a signal representing the value of a voltage created by snloothing a pulse width modulation output is used as the con-trol signal 70 supplied to the variable gain amplifiers 60 and 61. In the image quality correction circuit of -this embodinlent, the variable gain amplifier 60, whose gain for amplificatiorl is controlled in accordance with the voltage value of the control signal 70 supplied from the microcomputer 62, amplifies the chrominance signal I and supplies the amplified signal to the full-wave rectifier 10. The varia~le gain amplifiet 61, wllose gain for amplification is controlled in lilce manner, amplifies the chromirlallce signal Q and supplies the amplified signal to the full-wave rectifier 12. The full-wave rectifiers 10, 12 and the adder 13 operate in the same manner as in the image quality correction circuit shown in Fig. 3. In this enlbodiment also, the adder 13 outputs a color density detection signal corresponding to the color density. This color density detection signal corresponds to the sum Or the absolute value of the chrominance signal I and the absolute value of the chrominance signal Q; therefore, by controlling the amplitude of the chrominance signals I and Q by the CA 0223~10~ 1998-04-17 microcomputer 62, the amplitude of tlle color density detection signal can also be controlled at the same time.
This means that the gain of the higll-frequency com-ponent of the luminance signal amplified by the variable gain amplifier 5 is controlled by the microcomputer 62, which further mealls that the amouIlt of correction itself, by which the corrected lumirlallce signal Y' is produced, is controlled by the microcolllput~--r G2.
~ ith this configuration, the allloullt of correction applied to the luminance signal can be changed using the microcomputer, according to the kind of the picture signal or to the preferences of the viewer. It will also be no~ed tllat the circuit section consisting of the variable gain amplifier 60 and full-wave rectifier 10 and the circuit section consisting of the variable gain amplifier 61 and full-wave rectifier 12 can be constructed easily by using a four-quadrant variable gain amplifier.
Further, instead of using the microcomputer 62, the control signal 70 used to control the gain of the variable gain amplifiers 60, 61 may be created using a voltage settable by a variable resistor. In this case also, the amount of correction applied to the luminance signal can be changed according to the preferences of the viewer.
(Eolbodiment 6) Fig. 13 is a block diagram showing the configuration of ~7 CA 0223~105 1998-04-17 an image quality correction~circuit according to another embodiJnent (a SiXt~l embodiment) of the preserlt invelltion.
In the figure, the reference numeral 1 designates a color density detector, and 78 indica~es a high-frequency component detector for detecting the high-frequency component of the lunlinallce signal. The lulllinance signal Y
is inputted to an inversion circuit 11 as well as to the low-pass filter 2 and the delay- circuit 3. The output signal 79 of the subtractor 4 rel~resents the high-frequency component of the luminance signal and is inputted to the variable gain am~lifier 5. On the other hand, the output signal of the adder 13 is sup~lied as an input to a multi-plier 9. The output signal of the inversion circuit 11 also is supplied to the multiplier 9, whose output is thell fed to a low-pass filter 8. The output signal of tlle low-~ass filter 8 is applied to the control terminal of -the variable gain amplifier 5. In other respect, the configuration of this embodimellt is the same as that showll in Fig. 3, and the same components are designated by the same reference numerals as those used in Fig. 3.
In this embodiment, a signal corresponding to the color density is outputted from the adder 13, but this output signal of the adder 13 cannot be said to be optimum as the color density detection signal contemplated under the present invention. This is because the amplitude ratio of CA 0223~10~ 1998-04-17 the luminance component; is not reflected in the output signal, as described above. To address this, in the present embodiment the output signal of the adder 13 corresponding to the color density and the inverted luminallce signal outputted frool the inversion Cil'CUit 11 are multiplied together in the nlultiplier 9. The output of the multiplier 9 is supplied as a color density detection signal 26 -to the variable gain amplifier 5 via tlle low-pass filter 8. The color density detection signal 26 is created by correcting the output signal of the adder 13 corresponding to thé color density to a relatively low level for a color of high lumi-nosity and to a relatively high level for a color of low luminosity. The low-pass filter 8 is ~rovided to remove the high-frequency com~onent of the inverted luolinallce signal contained in the output signal of the multipliel 9, thereby preventing the gain of the high-frequency componerlt from dropping or distortion from being caused as a result of the gain control of the varia~le gain amplifier 5.
Fig. 14 shows the waveforms of the various signals for explaining the operation of the color density detecttor 1.
In Fig. 14, 14A shows the inverted luminance signal outputted from the inversion circuit 11, wherein 82 indicates a reference voltage for the inverted luminance signal to be inputted to the multiplier 9, i.e., the difference between the invert,ed luminance signal voltage and ~g CA 0223S10~ 1998-04-17 the reference voltage 82 is used ill the multi~lication operation performed by tlle mult:iplier 9; 14B shows the chrolllinance signal I, whereill 8:3 indicates a reference vol-tage for the chromirlance signal I to be inputted to the full-wave rectifier 10; 14C shows the cllronlinarlce signal Q, wherein 84 indicates a reference volLage for the chrolllinarlce signal Q to be inputted to the full-wave rectifier 12; 14D
shows the waveform of the full-wave rectified chrolllillance signal I outputted from the full-wave rectifier 10, wherein 85 indicates a reference vol~a~e for the full-wave rectified chrominance signal I to be inputted to the adder 13; 14E
shows the waveform of the full-wave rectified chrolllinance signal Q outputted from the full-wave rectifier 12, wherein 86 indicates a reference vol-tage for the full-wave rectified chrominance signal Q to be inputted to the adder 13; 14F
shows the output signal of the adder 13, i.e., the signal representing the color density given as the sum of the full-wave rectified chrominarlce signals I and Q wherein 87 indicates a reference voltage lor the signal corresponding to the color density to be inputted to the multiplier 9; 14G
shows the output signal of the multiplier 9, wherein 88 indicates a reference voltage for the waveform 14G, the reference voltage level indicating the absence of color; and 14H shows the color density detection signal 26 outputted from the low-pass filter 8, wherein 89 indicates a reference CA 0223~10~ 1998-04-17 voltage for the color density detection signal 26 to be inputted to tlle variable gain amplifier 5, the reference voltage Level indicating the absence of color.
The operation of the color density detector 1 will be described below with reference to Fig. 14. Fig. 14 shows the waveforms obtained when a color ~ar signal- is processed in the color density detector 1 in the image quality correc-tion circuit of the present embodinlent. When the chronli-nance signal I of 14B is full-wave rectified by -the full-wave rectifier 10 with reference to the reference voltage 83, the waveform of 14D results. Likewise, when the chrominance signal Q of 14C is full-wave rectified by the full-wave rectifier 12 with reference to the reference voltage 84, the waveform of 14E results. The waveform 14D
of the full-wave rectified chromillance signal I and tlle waveform 14E of the full-wave rectified chrominance sigl~al Q
are added -together in the adder 13, and the result is the waveform 14F. It is assumed here that the voltage level representing the reference voltage 87 is outputted when the voltage levels of the reference voltages 83 and 84 are inputted to the adder 13. When the inverted luminance signal of 14A as the reference voltage 82 and the signal waveform of 14F multiplied together in the multiplier 9, the waveform 14G results. The referellce voltage 82 is the zero level of the inverted luminarlce signal 14A for multipli-CA 0223~10~ 1998-04-17 cation, while the reference voltage 87 is the zero level of the signal 1~ for multiplicatiorl. T~le reference voltage 88 is the zero level for the result of the multiplication and represen-ts t}le voltage level wllell no color is present. The waveform 141I results when tlle wavefor~ G is stripped of its high-frequency colnponent through the low-pass filter 8.
The reference voltage 89 represents the voltage level when no color is present. From the observation of the waveform 141~, in conjunction with the previously given Table 1, it can be seen that tl~e amplitude is increased in the direction of decreasing higll-frequency com~onent and is reduced in the direction of increasing high-frequency component.
Fig. 15 shows a modified example of tlle sixth embodi-ment. In Fig. 13, the lunlinance signal Y is inputted directly to the inversion circuit 11, while in Fig. 15, the low-frequency component 1~ of the lunlillallce signal passed through -the low-pass filter 2 is inputted to the inversion circuit 11. Furthermore, in Fig. 13, tlle n~ultiplier 9 and the variable gain amplifier 5 are coupled together via the low-pass filter 8, but in the configuration of Fig. 15, the low-pass filter 8 is eliminated. The configuration of Fig.
15 is effective when the cut-oEf frequency of the low-pass filter 2 is sufficiently low to ensure that the high-frequency component of the inverted luminance signal that can adversely affect the processing for variable gain ; CA 0223~10~ 1998-04-17 amplification, as previously mentioned, does not remain in the color density detectiorl signal 2~ outputted from the multiplier 9. The resultant color density detection signal 26 is substantially the same as that shown in Fig. 14H.
In the configurations of Figs. 13 and 15, the chromi-nance signals I and Q nlay be replaced ~y the color-differ-ence signals R-Y and B-Y; in that case also, substantially the same eI'fect can be obtained.
Fig. 16 shows a modified example of the high-frequerlcy component detector 78 shown in Fig. 13. The luminance signal Y is inputted to a delay circuit 81, and also to a high-pass filter 80 which outputs a high-frequency signal 79. The delay circuit 81 outputs a luminance signal 75 contalning the high-frecluency conlporlent. The high-pass filter 80 extracts only the liigll-frequency colnponent from tlle luminance signal Y, and outputs i-t as the high-frequency signal 79 which is inputted to the variable gain amplifier 5 shown in Fig. 13. On the other hand, the uncorrected luminance signal is delayed t,hrough the delay circuit 3 so as to achieve phase synchroniza~ioll to the high-frequency signal 79 delayed due to the processing through the high-pass filter 80, and is then inputted to the adder 7 of Fig.
13 as the luminallce signal 15 containing the higll-frequellcy component.
(Embodiment 7) CA 0223~10~ 1998-04-17 Fig. 17 is a block diagram showing the configuration of a color density detector in an image quality correction circuit according to another embodiment (a seventh embodi-ment) of the present invention. The numeral 90 designates a full-wave rectifier where the inputted carrier chro~ ance signal C is full-wave rectified to produce a signal corre-sponding to the color density. This signal is inputted to the multiplier 9. The configuration other tharl the color density detector is the same as that shown in Fig. 13.
In Fig. 17, tlle carrier chrominance signal C is inputted to the full-wave rectifier 90 which then outputs a signal corresporlding to the color density. This signal corresponds to the outpu~ of ~he adtler 13 in the example of Fig. 13; when the color bar signal is inputted, the resulting output sigrlal is essentially the same as the signal of Fig. 14F but contains harlllonics Or the chrominance carrier frequency. The signal output from t}-le full-wave rectifier 90 is fed to the mul1iplier 9. Subsequent processing is the same as that illustrated in Fig. 13. It will be noted, however, that not only the high-frequency component of the inverted lulllinallce signal but also the harmonics of the chronlinallce carrier frequency are removed by the action of the low-pass filter 8 so that the signal outputted from the low-pass filter 8 ir~ Fig. 17 is essen-tially the same as the color density signal outputted fron CA 0223~10~ 1998-04-17 the color density detector l in Fig. 13.
(EmbodimerIt 8) Fig. 18 is a diagram showing the configuration of a color density detector in an inIage quality correction circuit according to arlotI-e1- embodiIIlellt (an eighth embodiment) of the present inventiorl. In Fig. 18, the color-difference signal R-Y is inputted to a maximum value detection circuit 97 and also to a multiplier 91. The color-difference signal B-Y also is inputted to the nIaximum value detection circuit 97 and the multiplier 91. The output of the multiplier 91 is fed to a coefficient multiplier 92 actiIlg as an anlplifier circuit, where it is multiplied by a coefficient k to produce a signal 98 expressed as l~(R-Y)(B-Y). This signal is inputted to the maximum value detection circuit 97. The maximum value detection circuit 97 outputs a color densi-ty detection signal 26. The configuration other than the color density detector is the same as tl1at shown in Fig. 13.
II1 the operation of Fig. 18, the incoming color-difference signals R-Y and B-Y are first multiplied together in the multiplier 91, and then multiplied by the coefficient k in the coefficient multiplier 92. The result is fed to the maximum value detection circuit 97 to which the color-difference signals R-Y and B-Y are also inputted directly.
The Jnaximum values among the three are then detected, and CA 0223~10~ 1998-04-17 the resulting output is the color density detection signal 2~.
Referring now to Fig. 19, we will describe why the three signals, R-Y, B-Y, and k(R-Y)(B-Y), are needed. Fig.
19 shows coloI~ vector coordinates wilh the ~hase of the burst signal at 180~ . In the figure, the nullleral 118 indicates the vector direction of the B-Y axis, 119 in-dicates the vector direction joining the 33~ point of the Q
signal to the 213~ point of the -Q signal, 120 indicates the vector direction joining the 55.8~ point; of -(G-Y) to the 235.8~ of G-Y, 121 indicates the vector directiorl joining the 60.7~ point of magenta to the 240.7~ point of green, 123 indicates the vector direction joinitlg the 103.5~ point of red to the 283.5~ of cyan, and 12~1 indicates the vector direction jQining the 123~ pOillt of the +I signal to the 303~ point of the -I signal. Further, the numeral 126 indicates the vector orientation where R-Y in Fig. 18 is at a maximulll, which is close to the orientation of the red vector. The numeral 127 indicates the vector orientation where B-Y is at a maximum, which is close to the orientatio of the blue vector. The numeral 128 indicates the vector orientation where k(R-Y)(B-Y) in Fig. 18 is at a maximum in the first quadrant, which is close to the orientation of the vector of magenta-hued colors, while 129 indicates the vector orientation where k(R-Y)(B-Y) is at a maximum in the CA 0223~10~ 1998-04-17 third quadran~, which is close to ~he orientation of the vector of green-hued colors.
Tlle vector orientations, 126, 127, 128, and 129, in Fig. 19 approximately match the vector directions of the colors having a large drop in the high-frequency areas.
These represents the directions in which the three signals, R-Y, B-Y, and k(R-Y)(B-Y), in Fig. 18 are a~ their respect-ive maximum values. Therefore, when the maxinlulll value of each of these three signals is detected, the detected amount of color density is larger for colors llaving a larger drop in the high-frequerlcy areas.
Fig. 20 shows the results of color density detectioll performed on a color bar signal by using tlle configuration shown in Fig. 18. Fig. ZOA shows ~he waveform of the l~-Y
signal, 20B the waveform of the B-Y signal, and 20C the waveform of k(R-Y)(B-Y) for k=1.5. T~le diagram 20D shows the color density detection signal 26 obtained as a result of detecting the maximulll vàlues from 20A, 20B and 20C. In this figure, the numbers attached alongside the respective waveforms indicate the amplitude ratios when the 100%
amplitude is expressed as 1Ø Numeral 161 indicates a reference voltage for each signal of Figs. 20A, 20B, 20C and 20D. The respective reference voltages for each signal of Figs. 20A, 20B, 20C and 20D have the same value. Observa-tion of the waveform of the color density detection signal CA 0223~10~ 1998-04-17 26 shown in Fig. 20D reveals that the amplitu~e of the color density detectioll signal 26 is relatively small for colors containing a larger luminance component, such as yellow and cyan, while the amplitude is relatively large for darker colors containing a smaller lumirlaIlce component, such as blue and red. It can therefore be seen that the amplitude ratio of the luminance signal is considered in the detection o-f the color density. SLrictly speakirlg, the ratio given is not the optimun- value because, as previously mentioned, division by the luminance signal would be necessary to give the accurate value. In reality, however, there is not much need to give the accurate ratio.
Fig. 21 shows a specific example of the configuration of the maximum value detection circuit 97. Shown in the blocks 13'1, 135, and 136 in Fig. 21 are equivalent circuits.
~irst, the internal configuration of the block 13~ will be described. The input color-clifference signal R-Y is applied to the base of a transistor 131 whose ealitter is coupled to power supply V via a resistor ]32 as well as to the base of a transistor 133. The collector of the transistor 131 is grounded. The emitter of the transistors 133 is coupled to a constant voltage source 27 via a resistor 137, and is also connected to the emitters of colrespollding transistors in the blocks 135 and 136, the result being outputted as tlle voltage of the color density detection signal 26. The . CA 0223~10~ 1998-04-17 Golor-dirferellce signal R-Y inputted to tlle block 134 corresponds to the color-difference signal B-Y in the block 135 and the k(R-Y)(B-Y) signal in the block 136. The cons-tant voltage outputted from the constant voltage source 27 serves as the reference voltage for the color density detection signal 26, and represents the voltage level when no color is present. The constant voltage outputted froln the constant voltage source 27 also serves as the reference voltage for each of the R-Y, B-Y, and k(R-Y)(B-Y) signals, so that the voltage level when no color is present is equal to the reference voltage. The reference voltage for the constant voltage source 27 corresponds to the reference voltage 161 in Fig. 20.
In Fig. 21, the color-difference signal R-Y is applied to the base of the transistor 131, bu-t since the transistor 131 forms arl emitter follower together with the resistor 132, the AC amplitude of the voltage changes very little, while the DC component is increased by the base-emitter voltage of tlle transistor 131 and is outputted from the emitter and applied to the base of tlle transistor 133. The transistor 133 also forms an emitter follower together with the resistor 137, but because the emitters of the output transistors in the blocl~s 135 and 136 are also connected to the common line, formiIlg an emitter follower with the resistor 137, current flows into the resistor 137 from the CA 0223~10~ 1998-04-17 transistor whose base voltage is the highest of the three, and the remaining two transistors are pu~ in the off state.
As a result, the greatest value among R-Y, B-Y, and k(R-Y)(B-Y) is outputted from the COIIIIIIOII emitter as the color density detection signal 26.
Further, since the transistor 133 forllls an emitter follower, the AC amplitude is not amplified, but only -the DC
component decreases relative to the base by the base-emitter voltage and is outputted from the emitter. Therefore, for no-color portions, the input voltage is equal to the output voltage. That is, since the input voltage for no-color portions becomes equal to the reference voltage which is the voltage of the constant voltage source 27, the output voltage for no-color portions also becomes equal to the voltage of the constant voltage source 27.
In this embodiment, only (R-Y)(B-Y) is assigned a weight, but R-Y and B-Y may also be weighted respectively, after which maximunl values among the three si~nals may be detected.
(Embodiment 9) Fig. Z2 shows the configuration of a color density detector in an image quality correction circuit accor~ing to another embodilllellt (a nintll embodimellt) of the present invention. In Fig. 22, the chrolllinarlce signal I is input-ted to a subtractor 99 and a maximum value detection circuit CA 0223~10~ 1998-04-17 103, while the chrominance signal Q is inputted to an inversion circuit 101 as we11 as to the subtractor 99. The output of the subtractor 99 is multi~lied by a coefficient n in a coefficient nlultiplier 100 acting as all aolplirier circuit, and the result is fed in~o the maximum value detection circuit 103. On the other hand, the output of the inversion circuit 101 is mu:Ltiplied by a coefficient m in a coefficient multiplier 102 acting as an attenuation circuit, and the result is fed in~o the maximum value detection circuit 103. The maximulll value de-tection circuit 103 outputs a color density detectioll signal 2G.
Tl~e processing in Fig. 22 will now be described. The chrominance signals I and Q inputted to -the subtractor 99 are subtracted one from the other to produce the result Q -I, which is multiplied by n in tile coefficient multiplier 100 and then fed into the maximum value detection circuit 103 as a first input signal. On the other harld, the chrominance signal Q inverted through -the inversion circuit 101 is multiplied by m in the coefficient multiplier 102 and then fed into the maximum value detectiorl circuit 103 as a second input signal. The chrominance signal I is also inputted directly to the maxilllum value detection circuit 103 as a third input signal. The chrominance signal Q is also inputted directly to the maxilllum value detection circuit 103 as a fourth input signal. The maximum value de-tection CA 0223S10~ 1998-04-17 circuit 103 detects maxilllulll values amorlg the first, second, third and fourth signals, and outputs the result as the color density detection signal 2~. A circuit configuration equivalent to ~hat shown in Fig. 21 may ~e used as a specific example of the nlaximum value detectiorl circuit 103.
Fig. 23 is a diagralll sllowillg the vec~or orientatiorls indicated by the maximunl values of -the first, second, third and fourth inpu-t signals. In Fig. 23, the nunleral 138 indicates the vector orientatiorl when the first input signal is at its maximulll value in the maxinlulll value detection circuil 103, whose orientation coincides with the vector orientatioll of blue. The numeral 139 indicates the vec-tor orientation when the second illpUt signal is a-t its maxilllulll value, whose orientation is close to the vector orientation of green. The numeral 14~ indicates the vector orientation wherl the chromirlallce signal I as the tllird input signal is at its maximum value, whose orientation is close to the vector orientation of red. The numeral 162 indicates the vector orientation when the chrominance signal Q as the four-th input signal is at its maximum value, whose orientation is close to the vector orientation of magenta.
Thus, as in the exam~le of Fig. 19, the color density detected is higher for colors containing a smaller high-frequency component.
Fig. 24 shows the results of color density detection CA 0223~10~ 1998-04-17 performed on a color bar signal by USillg the configuration of this embodimerlt. Fig. 24A shows the chrominance signal I, 24B the chrominance signal Q, and 24C the output when the coefficient n in the coefficient multiplier 100 in Fig. 22 is set to 1.0, which produces tlle result 1.0 (Q-I). Fig.
2~D shows the output when the coefficiellt m in the coef-ficient multiplier 102 is set to 0.8, which produces the result -0.8Q, while 24E shows the color density detection signal 26, the output of tlle maximunl value detection circuit 103 that has detected tlle maxinlum values from 24A, C and D.
In this case also, it can be seen tllat the amplitude ratio of the luminance signal is considered in the creation of the color density detection signal 26.
In this embodiment, only Q-I and -Q are assigned weights, but the signal I and Q may also be weighted, after which maximum value amorlg the four signals may be detected.
Also, specified plural signals may be selected from the four kinds of input signals of I, n(Q-I), -mQ and Q, and the maximum value among the selected signals may be detected.
(Embodiment 10) Fig. 25 shows the configuration of a color density detector in an image quali-ty correc-tion circuit according to another embodiment (a 10th embodiment) of the present invention. In Fig. 25, four kinds of signals are inputted to a maximum value detection circuit 110. First, the CA 0223~10~ 1998-04-17 chromirlallce signal Q invertcd througll an inversion circuit 105 is inpu~ted (the rirs~ inl)lJ~ si~llal). Second, the chromillance signal Q is direct,:Ly inputted (the second input sigrlal). Tl-lird, ~he chrolllinall(,e signal l is direct,ly inputted (the third input. signal). Fourth, the chrominance signal Q inputted to a multiplier 107 is multiplied l)y the chrominarlce signal I passed tllrough an inversion circuit 106 and a half-wave rectifier 130, tl-le result of the multipli-cation is further multiplied by a coefficiellt b in a coef-ficient multiplier 108, and the resulting signal 109 is inputted (the fourth input si~nal). The maxilllulll value detection circuit 110 detects maximum values among the four kinds of inputted signals, and outputs the result as the color density detection signal 2G. A circuit configuration equivalent to t,hat shown in Fig. 21 may be considered as a specific example of the maximum value detection circuit 110.
Fig. 26 shows the vector orientations when the first, second, third, and fourth input, signals are at their re-spective maxinlum values. The numeral 141 indicates the vector orientation when the first input signal -Q is at its maximum value, which is a green-llued color. The numeral 142 indicates the vector orientation when the second input signal Q is at its maximum value, which is a magenta-hued color. The numeral 143 indicat;es the vector orientation when the third input signal I is at its maxilllum value, which CA 0223~10~ 1998-04-17 is a red-hued color. The numeral 144 indicates the vector orientation when the fourth input signal 109 (-I Q) is at its maximum value, which orien-tation substantially coincides with the vector orientation of blue. It can be seen that almost all colors llavirlg smaller high-frequency components can be covered by the maximulll values of the four signals.
Fig. 27 sllows the waveforms representillg the results of color density detection performed on a color bar signal by using the configuration of this embodimen~. Fig. 27A shows the third input signal, i.e., -the chromillallce signal I, 27B
the chrolllinance sig~lal I, designed by -I, inverted through the inversion circuit 106, 27C the second input signal, i.e., -the chrominance signal Q, and 27D the fourth input signal 109 when the coefficient b in the coefficient multiplier 108 is set to 10. The diagram 27E shows the color derlsity detection sign~l 26 obtained by detecting the maximum values among the four kinds of input signals, i.e., 27A, C, D and the first input signal -Q (the inverted signal of 27C. As can be seen, the amplitude ratio of the lumirlarlce signal is reflected, except tha~ the magni-tude of the detected amount for cyan and yellow is inverted.
In this embodiment, only -IQ is assigned a weight, but -Q, Q, and I may also be weighted respectively, after which the maximum values may be detected from the four kinds of signals. Furthermore, in this embodiment, the maximum CA 0223~10~ 1998-04-17 values are detected from the rour Icinds of signals, but alterllatively, the maximulll values nlay be detected from the three kinds of signals (-Q, I, and -IQ), i.e. the first, third, and fourth signals, to obtain the color density detection signal.
(Embodiment 11) Fig. 28 shows the configuration of a color density detector in an inlage quality correction circuit according to another embodiment (an 11th embodiment) of the present inven-tion. The only difference from the color density detector 1 of the sixth embodiment (see Fig. 13) is that the inversion circuit 11 is replaced by a binary circuit 111 in the 11th embodinlent. The binary circuit lll outputs a relatively low voltage when the luminallce signal Y is higher than a tl-reshold voltage e, and a relatively high voltage when the lunlillance signal Y is lower than ~he threshold voltage e, thus converting the luminarlce signal Y into a binary form to produce a binary luminance signal 112. The binary lumlnance signal 112 is inputted to the multiplier 9, where it is multiplied by a signal, outputted from the adder 13, corresponding to tlle color density; thus, the output of the adder 13 is corrected to a relatively low level when the luminance signal is at a relatively high level, and to a relatively high level when the luminance signal is at a relatively low level.

~ CA 0223~10~ 1998-04-17 Fig. 29 shows the waveforms representing the results of color density detection performed on a color bar signal by using the configuration of this embodiment. The diagram 29A
shows the luminance signal Y, and 29B shows the binary luminance signal 112. In the example ShOWIl, tlle threshold voltage 11~ is set at 50 IRE. The l~ullleral 115 indicates a reference voltage for the binary luminance signal 112 to be inpu-tted to the adder 9, and the voltage differential between the binary luminance signal 112 and tlle reference voltage 115 is subjected to the nlultiplication operation.
The diagram 29C shows the signal outputted from the adder 13, corresponding to the color density, and the numeral 116 indicates a reference voltage for the signal corresponding to the color density, to be inputted to the variable gain amplifier 5, the voltage level representing the voltage when no color is present. The diagram 29D shows the color density detection signal 26 obtained by multiplying 29B by 29C, and the rlumeral 117 indicates a reference voltage for it and represents the voltage level when no color is present. The color density detectiorl signal 26 in this case also is corrected to a relatively low level for colors for which the,amplitude of the lulninance signal contained therein is higher than 50 IRE, and to a relatively high level for colors for which the amplitude of the luminance signal'contained therein is lower than 50 IRE. Thus, the CA 0223S10~ 1998-04-17 amplitude ratio of the luminance signal is roughly reflected in this confi~uration.
(Embodiment 12) Fig. 30 shows the confi~uration of a color density detector in an image quality correction circuit according to another embodilllent (a 12th embodiment) of the present invention. The only difference from the color density detector of the seventh embodiment ~see Fig. 17) is that the inversion circuit 11 is replaced by the binary circuit 111.
The signal correspondirlg to the color densi-ty, outputted from the full-wave rectifier 9~1 is almost equivalerlt to the signal corresponding to the color density, outputted from the adder 13 in Fig. 28. Therefore, the operation of the section including tlle binary circuit 111 and multiplier 113 is substantially the same as described ~bove, except that the configuration of Fig. 30 requires tlle provision of a low-pass filter 8 because the output of the multiplier 113 contains harmonics of the chrominance carrier frequency.
It will be ap~reciated that the detection method of the color density detection signal, as described in the 6th to 12th embodiments, can also be applied to the prior art image quality correction circuit (Fig. 1) in which the slice circuit 6 is not provided.
(Embodiment 13) Fig. 31 shows the configuration of an image quality CA 0223~10~ 1998-04-17 correction circuit according to another embodiment (a 13th embodilllerl-t ) of the present inven~ion . In Fig . 31, the same parts as those shown in Fig. 13 are designated by the same reference numerals. Also, blocks 153 and 154 in Fig. 31 may be constructed with the same circuit; configuratiorl of the image quality correction signal generator 14 shown in Fig. 3 or 12. The configuration of Ei'ig. 31 llas the following features: only the positive half-cycles of the color-difference signal R-Y are separated by a half-wave rectifier 147 to produce a color density detection signal 151 associated only with the color-difference signal R-Y, and sinlilarly, only the positive half-cycles of the color-difference signal B-Y are separated by a half-wave rectifier 148 to produce a color density detection signal 152 associ-ated only with tlle color-difference signal B-Y; and the high-frequency signal 79 detected in the luminance signal l~y the high-frequency component detector 78 and fed into a variable gain alllplifier 155 is controlled in accordance with the R-Y color density detection signal 151 so that the gain is increased in higher color-density portiorls of the R-Y
vector and is reduced in lower color-density portions of the R-Y vector, after which only the negative high-frequency components are separated by a slice circuit 156 and fed to an adder 159 for addition to the color-~ifference signal R-Y, not to the original luminance signal as in tlle configu-CA 0223~10~ 1998-04-17 ration of Fig. 13. The block 153 where the negative high-frequency componellts Or the lulnirlance signal are super-imposed on the color-difference signal ~-Y is equivalent in func-tion to the block 154 where the B-Y signal is processed in like manner. Tlle variable gain alnplifiers 157, 155, the slice circuits 156, 158, and the adders 159, 16U are also functionally equivalent to each other.
The same effect as achieved by the con~iguration of Fig. 13 can also be expected froln the configuration of Fig.
31. As a variant form of ~ig. 31, the half-wave rectifier 148, variable gaill amplifier 157, slice circuit 15R, and adder 160, which are resporlsible for -the processing of the color-difference signal B-Y, nlay be eliminated, and pro-cessing may be perforlned only on red that llas a particularly significant effect on visual perceptions. In this case, the slice circuit plays an important role, since, without the slice circuit, a change in hue, not a drop in saturation, would occur in portions where the positive high-rrequency components are amplified.
Alternative configurations of color density detectors 145, 146 of Fig. 31 are shown in Figs. 32 and 33. Fig. 32 shows an example where the configuration shown in Fig. 13 is applied. In the half-wave rectifier 147 ShOWII in Fig. 32, as in the half-wave rectifier 147 of Fig. 31, the positive half-cycles of the color-difference signal R-Y are sepa-CA 0223~10~ 1998-04-17 rated. In Fig. 31, the result is directly outputted as the color density detection signal 151. In Fig. 32, Oll the other hand, in order to reflect the amplitude ratio of the luminance signal contained in the previously mentioned various colors in the color density to be detected, the luminance signal Y is inverted by tlle inversion circuit 11 and fed to a multipliel 149 where tlle output of the half-wave rectifier 147 is multiplied by the inverted luminance signal, so that the color density detection signal 151 is corrected to a relatively low level wheIl the lulninance is high and to a relatively high level when the luminance is low.
Fig. 33 shows an example in which the inversion circuit 11 in Fig. 32 is replaced by a binary circuit 111. This is an application of the configuration shown in Fig. 28. When the amplitude of the luminance signal Y is large, the binary circuit 111 outputs a low voltage so that the color density detection signal is corrected to a low level by the multi-plier 149. Conversely, when the amplitude of the luminance signal Y is slnall, the binary circuit 111 outputs a high voltage so that the color density detection signal is corrected to a high level by the multiplier 149. After that, the result is passed through a delay circuit 150 to obtain the color density detection circuit 151.
In the above embodiment, the output signal of the CA 0223~10~ 1998-04-17 subtractor (t~le high-frequency componellt Or the luminance signal) is first alnplified by the variable gain anlplifier and'then sliced at a prescribed amplitude level by the slice circui-t before being fed to -the adder, but it will be recognized that the order of the variable gain amplifier and the slice circuit may be reversed. Such a configuration will be described below as modiried examples of tlle first and sixth embodinlents.
Fig. 34 shows a nlodified e~ample of the configuration of Fig. 3, in WhiCIl the order ol the variable gaill anlplifier 5 and the slice circuit 6 :is reversed. Fig. 35 shows a modified exanlple of the configuration of Fig. 13, in which the order of the variable gain amplifier 5 and the slice circuit 6 is reversed. In Figs. 34 and 35, the amplitude of the output signal of tlle subtractor 4 (the high-frequency componellt of the lunlinance signal) is first sliced at a prescribed value by the slice circuit 6, and -then the output of the slice circuit 6 is amplified under control by the color density detection signal 26 in such a manner that the gain is increased when the detected color density is high, and is decreased when the detected color density is low.
The result is fed to the adder 7 as the inlage quality correction signal. In other respects, the operation is the same as that of the first and sixth embodinlents, and therefore, description thereof is not repeated here.

7~

CA 0223~l0~ l998-04-l7 (EnlbOdiment 14 ) Fig. 36 is a block diagram showing the configuration of an image quality correction circuit according to another embodiment (a 14th embodimellt) of the present invention. In Fig. 36, tlle color density detector 1 is supplied with the chronlillarlce signal or with both the cllrolninal-lce and lumi-nance signals, and outputs a color density detection signal 176. The color density detection signal 176 corresponds to the color density detection signal outputted from the color density detector 1 in Elnbodiments 1 - 13. A boundary re-duction circuit 177 accepts the color density detection signal 176 and outputs a corrected color density detection signal 26. The corrected color density detection signal 26 is applied to the control terminal of tlle variable gain amplifier 5 (at which, in Embodiments 1 - 13, the color density detection signal 26 is applied). This color density detection signal 26 corresponds to the control voltage for the variable gain amplifier 5 in Embodiments 1 - 13.
The color density detector 1 detects the color density from the inputted chrominance and luminance signals. The boundary reduction circuit 177 produces the color density detection signal 26, with its amplitude reduced at the edge portions, which signal is used to control the high-frequency signal gain of the variable gain amplifier 5. Since the amplitude of the color densi-ty detection signal 26 is CA 0223~10~ 1998-04-17 reduced at the boundaries of colors, i.e., at the edge portions of the color density detection signal 176, the gain of the variable gain alnplifier 5 is reduced at the bound-aries of colors. The net effect of this is to suppress the enhancement of unnatural contours at the color boundaries.
Fig. 37 shows the circuit configuration of Embodiment 14 in more detail. In Fig. 37, a delay circuit 178 delays the input color density detection signal 176 by a predeter-mined time and supplies the output to a minimum value de-tection circuit 179. Tlle millilllulll value detection circuit 179 is also supplied with the color density detection signal 176 not passed through the delay circuit 178, and outputs the corrected color density detection signal 26. The configuration of the minilnum value detection circuit 179, where minimum values at the same instant of time between the color density detection signal 176 delayed through the delay circuit 178 and the color density detection signal 176 not passed through the delay circuit 178 are obtained to reduce the amplitude of the edge portions, can be readily imple-mented by applying the maximum value detection circuit configuration shown in Fig. 21.
~ ig. 38 shows the waveforms taken at points a, b, and c in Fig. 37, along with a color density-controlled high-fre~uency signal 180 that the variable gain amplifier 5 would output when the circuit of Fig. 37 is connec-ted to the 7~

CA 0223~10~ 1998-04-17 image quality correction circuit of Fig. 13, and a corrected luminance signal 181 (Y') in that case. In Fig. 38, i-t is sllown that the signal c obtained by taking the minimum values between tlle signals a and b has its aolplitude reduced at the edges of the color densi-ty detection signal a. It can also be seen that in the corrected lulninance signal 181 the amount of high-frequency correction is reduced in areas near the boundaries of colors. However, the edges indicated at E and H in the corrected luminance signal 181 correspond to the boundaries between non-color and color areas, and no correction is made in -these portions, while on ~he other hand, the efrect of reduction is reduced at the boundaries between different colors or between different tonal den-sities, as indicated by F and G. Notwithstanding this shortcoming, the 14th embodiment provides an effective method because of its extremely simple construction.
The 14th embodiment is also effective in application to an image quality correction circuit that does no-t use a slice circuit; the embodiment is effective not only in the configuration where image quali~y correction is applied to the luminance signal, but also in the configuration of Fig.
31 where image quality correction is applied to the chromi-nance signal, the configuration of Fig. 31 in the case where the slice circuit is not provided, and ~he configuration where image quality correction is applied to the primary CA 0223~10~ 1998-04-17 color signal.
(Embodiment 15) Fig. 39 is a block diagram showing ~he configuration of an image quality correction circuit according to another embodiment (a l5th embodiment) of the present invention. In Fig. 39, a delay circuit 182 delays the input color density de-tection signal 176 by a predeternlirled tinle, arld supplies the output to a delay circuit 183 and a nlultiplier 189. The delay circuit 183 delays the input signal by a predetermined time, and supplies the output to subtractors 184 and 185.
The color density detection signal 176 is also inputted directly to the subtractors 184 and 185. The subtractor 184 subtracts the density detection signal 176 frolll the output signal of t}le delay circuit 183, and outputs the result as a first edge signal which is supplied to a maximum value detection circuit 186. The subtractor 185 subtracts the output signal of the delay circuit 183 from the color density detection signal 176, and outputs the result as a second edge signal which is supplied to the maximum value detection circuit 186. The maxilllulll value detection circuit 186 detects the maximum values at the same instant of time between the first and second edge signals, and thereby detects the absolute value signal of the first edge signal or second edge signal, the absolute value signal then being supplied to a comparator 187. The comparator 187 compares CA 0223~10~ 1998-04-17 the absolute value signal with a predetermirled voltage 188, and supplies a binary signal to the multiplier 189 in accordance with the result of the comparisoll. The output signal of the delay circuit 182 is also suyplied to the multiplier 189. The multiplier 189 multiplies the two input signals together, and outputs a corrected color density detection signal 26. The section 213 enclosed by a dotted line can be considered as a circuit for obtaining the absolute value of the first edge signal.
The first edge signal, which is produced by subtracting the color densi~y de~eotion sigrlal 17~ directly fed to the subtractor 184 from the color density detection signal 176 delayed through the delay circuits 182 and 183 in series, and the second edge signal, an inverted version of the first edge signal, outputted from the subtractor 185, are supplied to the maxim,un- value detection circuit 186 where the maximum values between the first and second edge signals are ob-tained. That is, the output of the maxilllulll value detec-tion circuit 186 is the absolute value signal of the first edge signal. When -the amplitude of the absolute value signal exceeds a certain level, the comparator 187 outputs the result of the comparison between the absolute value signal and the predetermined threshold value voltage 188, as a binary signal, i.e., an edge correction signal for edge correction. The edge correction signal goes high for edge CA 0223~10~ 1998-04-17 port,ions und goes low Cor othcr portiolls. rl'llis edge correc-tion signal is multiplied in the multi~lier 189 by the output signal of the delay circuit 182, thus producing the corrected color density detection signal 2G. The color density detection signal 26 drops to the same voltage level as the no-color level for portions where -the edge absolute value signal exceeds the threshold value voltage 188 in amplitude.
Fig. 40 shows the waveforms taken at pOilltS a, d, e, f, g, h, i, and j in ~ig. 39, along with a color density-con-trolled high-frequency signal 193 that the variable gain amplifier 5 would output when the circuit of Fig. 39 is connected to the image quality correction circuit of Fig.
13, and a corrected luminance signal 194 (Y') in that case.
The numeral 190 indicates the level of the thresllold value voltage 188. The numeral 191 indicates the reference signal for the edge correction signal i, the reference signal being calculated as O in the multiplication operation performed by the multiplier 189. It can be seen that the edge correction signal i drops down to the reference voltage 191 in areas near the boundaries of colors. The numeral 192 indicates the reference voltage for the corrected color density de-tection signal j, the reference voltage being the voltage for no-color portions. The corrected color density detec-tion signal j drops down to the reference voltage 192 in CA 0223~10~ 1998-04-17 areas near the boundaries of colors. It can be seen that the high-frequerlcy signal 193 controlled by the variable gain amplifier 5 in accordance with the color density detection signal j is suppressed for portions corresponding to the color boundaries. In the luminance signal 194 after image quality correction, no image correction is applied in areas near the color boundaries. This eliminates the possibility of overcorrection or contour reversion at the boundaries of colors as was the case with the prior art previously described.
The difference between the configurations of Figs. 37 and 39 is that the configuration of Fig. 39 achieves a much greater improvenlerlt ~lOt only at tlle boundaries hetween non-color and color areas but also at the bourldaries between different colors.
The 15th embodiment is also effective in application to an image quality correction circuit that does not use a slice circuit; the embodiment is effective not only ir-l the configuration where image quality correction is applied to the luminance signal, but also in the configuration of Fig.
31 where image quality correction is applied to the chrolni-nance signal, the configuration of Fig. 31 in the case where the slice circuit is not provided, and the configuration where inlage quality correction is applied to the primary color signal.

CA 0223~l0~ l998-04-l7 ( Embodiment 16 ) Fig. 41 is a block diagram showing the conLiguratiorl of ,an image quality correction circuit accor~ing to another embodiment (a 16th embodiment) of tlle present invention. In Fig. 41, the color density detector 1 corresponds to the color density detector in Embodiments 1 - 13. The chromi-nance and luminance signals are inputted to the color density detector 1 which then outputs a color density detection signal 176 to a small-amplitude removal circuit 195. The small-amplitude removal circuit 195 outputs a corrected color density detection sign~l 26 which is applied to the control terminal of the variable gain amplifier 5.
More specifically, the color density detection signal 176 synthesized from the chrominance and luminance signals is supplied to the small-amplitude removal circuit 195 where small-ampli-tude por-tions of the color density detection signal are removed in order to reduce the effect of image quality correctiorl for light color areas, and ~he resulting output, i.e., the corrected color density detection signal 26, is applied to the control terminal of tlle variable gain amplifier 5 in Embodiments 1 - 15. When applying the configuration of Fig. 41 to Embodiments 14 and 15, the small-amplitude removal circuit 195 is connected after the boundary reduction circuit 177 in Fig. 36.
Fig. ~2 shows the configuration of the small-amplitude CA 0223~10~ 1998-04-17 removal circuit 195 in more detail. The numeral 196 and 197 designate trarlsis~ors ~laving similar cllaracteris~ics; the color density detection signal 176 is applied to the base of the transistor 196, while a reference voltage 200 is applied to the base of the transistor 197, the reference voltage 200 also serving as the reference voltage for the corrected color density detection signal 26. The collectors of the transistors 196 and 197 are conrlected together and coupled to a power source 198. The emitters of the transistors 196 and 197 are connected together and grounded via a resistor 199. The emitter of each of the transistors 196, 197 is connected to the base of a transistor 201 whose collector is grounded and whose emitter is conrlected to ~he power source 198 via a resis~or 202. The voltage outpu~ted from the emitter of the transistor 201 is the corrected color density detection signal 26. The transistor 201 forms an emitter follower buffer so that the voltage drop by the base-emitter voltage of the transistors 196, 197 is cancelled as it is raised by the base-emitter voltage of the transistor 201.
The circuit operation of Fig. 42 will be described. It is assumed that the reference voltage 200 for the corrected color density detection signal 26 is chosen to be higher than the input color density detection signal 17~. When the color density detection signal 176 applied to the base of the transistor 196 is higher t~lan the reference voltage ,200 CA 0223~10~ l99X-04-17 applied to tlle base of the trallsistor 197, currerlt flows fronl the transistor 196 Lo groun~ via the resistor 199, so that the transistor 197 is off. Therefore, in this case, the color density detection signal 176 is ou-tputted as the corrected color density detection signal 2G ~hrougll the transis~ors 196 ~nd 201. Conversely, wllen the colo~ density detection signal 176 applied to tlle base of the transistor 196 is lower than the reference voltage 200 applied to -the base of the transistor 197, the transistor 196 is off, and current flows from the emitter of the transistor 197 to ground via the resistor 199. Therefore, in this case, the reference voltage is outputted from the emit-ter of the transistor 201.
Fig. 43 explains the circuit operation of Fig. 42 by using specific examples of signal wavefornls. The numeral 203 designates an example of the color density detection signal 176 applied to the base of the transis-tor 196, and a dotted line 204 indicates the reference voltage for tlle color density detection signal 176, the reference voltage being equal to the no-color voltage level. The numeral 205 shows an example of the voltage value of tl~e reference voltage 200, after correction, applied to the base of the transistor 197. The numeral 206 indicates the color density detection signal 26 after correction, and 205 designates the reference voltage applied to the base of the transistor 197.

CA 0223~10~ 1998-04-17 In Fig. 43, it can be seen that when the color density detection signal 203 is higher than the corrected reference voltage 205, the color density detection signal 203 is outputted as the corrected col.or density detection signal 206; conversely, when the reference voltage 205 is higher than the color density detection signal 203, a voltage equal to the corrected reference voltage 205 is outputted as the corrected color density detection signal 206. As a result, the amount of image quality co.rrection in light color areas is held down.
The l~th embodimerlt is al.so effective in application to an image quality correction circuit tha~ does not use a slice circuit; -the embodiment is effective not only in the configuration where image qual.ity correction is applied to the luminance signal, but also in the configura~ion of Fig.
31 where image quality correction is ap~lied to the chromi-nance signal, the configuration of Fig. 31 in the case where -the slice circuit is not provi.ded, and the configuration where image quality correction is applied to the primary color signal. The boundary reduction circuit of Embodiments 14, 15 may be incorporated into tlle above configuration.
(Embodiment 17) Fig. 44 is a block diagram showing the configuration of an image quality correction circuit according to another embodiment (a 17th embodiment) of the present invention. In CA 0223~10~ 1998-04-17 Fig. 44, the color density detector 1 corresponds to the color density detector in Embodilllents 1 - 13. The chromi-nance and lumirlance signals are inputted to the color density detector 1 which then outputs a color density detection signal 17G to an offset additiorl circuit 208. Tlle offset acldition circui~ 208 outputs a corrected color density detection signal 26. More specifically, the color density detection signal 176 synthesized in the color density detector 1 from the chrominance and luminance signals is inputted to the offset additiorl circuit 208 where a DC component is added to the color density detection signal 176 so that a difference voltage will remain with respect to the reference voltage even in no-color portions, thus allowing a certain degree of image quality correction in no-color portions.
As a specific example, tlle offset addition circuit 208 can be implemented using the same configuration as -the small-amplitude removal circuit shown in Fig. 42. However, in the small-amplitude removal circuit, the reference voltage 200 after correction is made higher than the no-color level of the color density detection signal 176, while on the other hand, in the case of the offset addition cir-cuit 208, tlle reference voltage 200 after correction must be made lower than the no-color level of tlle color density detection signal 176. Further, instead of using the circuit CA 0223~10~ 1998-04-17 such as shown in Fig. ~2, the reference voltage may sinlply be made lower than the no-color level of the color density detection signal 176. Fig. 45 shows specific examples of waveforms. The numeral 209 designates the voltage waveform of the color density detection signal 17~; 210 is the vol-tage level when no color is present; 211 is the color density cletection signal after correction; and 212 is the reference voltage after correction. It can be seen that the signal 211 has a DC voltage to enable a certain degree of correction even in no-color portions. This also provides the effect of aperture correction. Furthermore, if pro-visions are made so that the corrected reference voltage can be variecl using a microcomputer or the lil~e, an operation equivalent to image quality control by aperture correction can be ac,complished.
The l7tll embodimerlt is also effective in application to an image quality correction circuit that does not use a slice circuit; the embodiment is ef-fective not only in the configuratiorl where image quality correction is applied to the lumillance signal, but also in the configuration of Fig.
31 where image quality correction is applied to the chro-minance signal, the configuration o~ Fig. 31 in the case where the slice circuit is not provided, and tlle configu-ration wllere image quality correction is applied to the primary c,olor signal. The boundary reduction circuit of CA 0223~10~ 1998-04-17 Embodiments 14, 15 may be incc)rporated into the above configuration. Furthermore, the small-amplitude removal circuit of Embodiment 16 may be incorporated as well.
Any of Embodilllents 1 to 17 can be applied not only to analog systems but also to digital systellls.
As this invention nlay be embodied in several forms without departing from the s~irit of essential character-istics thereof, the present embodilllerlt is therefore illustrative and not restrictive, since the scope of the invention is defined by the al~ended clainls rather than by the description preceding thelll, arlcl all changes that fall within metes and bounds of the claims, or equivalence of such metes and bound thereof are therefore intelldecl to be embraced by the claims.

Claims (21)

CLAIMS:

1. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes rectifiers for full-wave rectifying two chrominance signals, an adder for summing the two signals after rectification, and a multiplier for multiplying an output signal of said adder by an inverted version of the luminance signal, wherein the color density is detected from the output of said multiplier.

2. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes a rectifier for rectifying a carrier chrominance signal, a multiplier for multiplying an inverted luminance signal and an output signal of said rectifier, and a low-pass filter for extracting a low-frequency component from an output signal of said multiplier, wherein the color density is detected from the output of said low-pass filter.

3. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes a multiplier for multiplying two color-difference signals, a weighting circuit for assigning a weight to an output signal of said multiplier, and a maximum value detection circuit for detecting a maximum value among an output signal of said weighting circuit, one of said two color-difference signal, and the other of said two color-difference signal, wherein the color density is detected from the output of said maximum value detection circuit.

4. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes a subtractor for subtracting a second chrominance signal from a first chrominance signal, and a maximum value detection circuit for detecting a maximum value among an output signal of said subtractor, an inverted signal of the first chrominance signal, and the second chrominance signal, wherein the color density is detected from the output of said maximum value detection circuit.

5. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes a half-wave rectifier for half-wave rectifying an inverted signal of a first chrominance signal, a multiplier for multiplying chrominance signal; and a maximum value detection circuit for detecting a maximum value among three kinds of signals having an output signal of said multiplier, said first chrominance signal, and an inverted signal of the second chrominance signal, wherein the color density is detected from the output of said maximum value detection circuit.

6. An image correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component filtering means for filtering out a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the filtered luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means includes an adder for summing two kinds of chrominance signals after full-wave rectification, and a multiplier for multiplying an output signal of said adder by a binary version of the luminance signal, wherein the color density is detected from the output of said multiplier.

7. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
a variable gain amplifier for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low; and means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal, wherein said color density detecting means comprises a rectifier for rectifying a carrier chrominance signal, a multiplier for multiplying an output signal of said rectifier by a binary version of the luminance signal, and a low-pass filter for extracting a low-frequency component from an output signal of said multiplier, and the color density is detected from the output of said low-pass filter.

8. An image quality correction circuit comprising:
color density detecting means for detecting color density from a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of a luminance signal;
a variable gain amplifier for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal; and a boundary reduction circuit for accepting a color density signal representing the color density detected by said color density detecting means, and for reducing the amplitude of the color density signal at portions near the boundary between no-color and color areas or near the boundary between different colors; wherein an output signal from said boundary reduction circuit is taken as a new color density signal for image quality correction.

9. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective color-difference signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated chrominance signal to be corrected and thereby outputting a corrected chrominance signal; and a boundary reduction circuit for accepting a color density signal representing the color density detected by said color density detecting means, and for reducing the amplitude of the color density signal at portions near the boundary between no-color and color areas or near the boundary between different colors; wherein an output signal from said boundary reduction circuit is taken as a new color density signal for image quality correction.

10. An image quality correction circuit comprising:
color density detecting means for detecting color density from a primary color signal and a luminance signal;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective primary color signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated primary color signal to be corrected and thereby outputting a corrected primary color signal; and a boundary reduction circuit for accepting a color density signal representing the color density detected by said color density detecting means, and for reducing the amplitude of the color density signal at portions near the boundary between no-color and color areas or near the boundary between different colors; wherein an output signal from said boundary reduction circuit is taken as a new color density signal for image quality correction.

11. An image quality correction circuit according to claim 8, wherein said boundary reduction circuit comprises a delay circuit for delaying the color density signal inputted thereto and a minimum value detection circuit for detecting a minimum value between an output signal from said delay circuit and the color density signal not passed through said delay circuit, and an output of said minimum value detection circuit is taken as a new color density signal for image quality correction.

12. An image quality correction circuit according to claim 8, wherein said boundary reduction circuit comprises:
a first delay circuit for delaying the color density signal inputted thereto;
a second delay circuit for delaying an output signal of said first delay circuit;
a subtractor for subtracting the input color density signal from an output signal of said second delay circuit;
an absolute value detection circuit for detecting the absolute value of an output of said subtractor;
a comparator for comparing an output signal of said absolute value detection circuit with a specific voltage and for outputting the result as a binary signal;
and a multiplier for multiplying an output signal of said comparator by the output signal of said first delay circuit, and an output signal of said multiplier is taken as a new color density signal for image quality correction.

13. An image quality correction circuit comprising:
color density detecting means for detecting color density from a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of a luminance signal;
a variable gain amplifier for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal; and a small-amplitude removal circuit for removing small amplitude portions of a color density signal representing the color density detected by said color density detecting means; wherein an output signal from said small-amplitude removal circuit is taken as a new color density signal for image quality correction so that no image correction may be applied to low color-density areas.

14. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective color-difference signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated chrominance signal to be corrected and thereby outputting a corrected chrominance signal; and a small-amplitude removal circuit for removing small amplitude portions of a color density signal representing the color density detected by said color density detecting means, wherein an output signal from said small-amplitude removal circuit is taken as a new color density signal for image quality correction so that no image correction may be applied to low color-density areas.

15. An image quality correction circuit comprising:
color density detecting means for detecting color density from a primary color signal and a luminance signal;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective primary color signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated primary color signal to be corrected and thereby outputting a corrected primary color signal; and a small-amplitude removal circuit for removing small amplitude portions of a color density signal representing the color density detected by said color density detecting means, wherein an output signal from said small-amplitude removal circuit is taken as a new color density signal for image quality correction so that no image correction may be applied to low color-density areas.

16. An image quality correction circuit comprising:
color density detecting means for detecting color density from a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of a luminance signal;
a variable gain amplifier for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal; and offset addition means for adding a DC component to a color density signal representing the color density detected by said color density detecting means; wherein an output signal from said offset addition means is taken as a new color density signal for image quality correction so that a certain degree of image quality correction effect can be obtained for no-color areas.

17. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;

high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective color-difference signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated chrominance signal to be corrected and thereby outputting a corrected chrominance signal; and offset addition means for adding a DC component to a color density signal representing the color density detected by said color density detecting means; wherein an output signal from said offset addition means is taken as a new color density signal for image quality correction so that a certain degree of image quality correction effect can be obtained for no-color areas.

18. An image quality correction circuit comprising:
color density detecting means for detecting color density from a primary color signal and a luminance signal;

high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
variable gain amplifiers, corresponding to respective primary color signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated variable gain amplifier with the associated primary color signal to be corrected and thereby outputting a corrected primary color signal; and offset addition means for adding a DC component to a color density signal representing the color density detected by said color density detecting means; wherein an output signal from said offset addition means is taken as a new color density signal for image quality correction so that a certain degree of image quality correction effect can be obtained for non-color areas.

19. An image quality correction circuit comprising:
color density detecting means for detecting color density from a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of a luminance signal;

a first variable gain amplifier for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said first variable gain amplifier with the luminance signal to be corrected and thereby outputting a corrected luminance signal; and a second variable gain amplifier whose gain to amplify the amplitude of a signal representing the color density to be detected is controlled in accordance with a voltage value specified by a microcomputer or set by a variable resistor; wherein the gain to amplify the high-frequency component of the luminance signal is controlled in relation to an output signal of said second variable gain amplifier.

20. An image quality correction circuit comprising:
color density detecting means for detecting color density from a luminance signal and a signal relating to color;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
first variable gain amplifiers, corresponding to respective color-difference signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated first variable gain amplifier with the associated chrominance signal to be corrected and thereby outputting a corrected chrominance signal; and a second variable gain amplifier whose gain to amplify the amplitude of a signal representing the color density to be detected is controlled in accordance with a voltage value specified by a microcomputer or set by a variable resistor; wherein the gain to amplify the high-frequency component of the luminance signal is controlled in relation to an output signal of said second variable gain amplifier.

21. An image quality correction circuit comprising:
color density detecting means for detecting color density from a primary color signal and a luminance signal;
high-frequency component extracting means for extracting a high-frequency component of the luminance signal;
first variable gain amplifiers, corresponding to respective primary color signals, for amplifying the extracted high-frequency component of the luminance signal by performing control in such a manner that the gain thereof is increased when the detected color density is high, and is reduced when the detected color density is low;
means for combining an image quality correction signal outputted from said associated first variable gain amplifier with the associated primary color signal to be corrected and thereby outputting a corrected primary color signal; and a second variable gain amplifier whose gain to amplify the amplitude of a signal representing the color density to be detected is controlled in accordance with a voltage value specified by a microcomputer or set by a variable resistor; wherein the gain to amplify the high-frequency component of the luminance signal is controlled in relation to an output signal of said second variable gain amplifier.