Circuit for enhancing a color difference signal by utilizing the change of a luminance signal

An enhancing circuit wherein a color difference signal (b) is enhanced by utilizing the change of a luminance siganal (a). The luminance signal (a) is sent, via a low-pass filter (2), a first delay circuit (3), a first differentiation circuit (4), a detector (5), a slicer (6), and a second differentiation circuit (7), to a comparator set (8) to produce two signals (i and j). The two signals (i and j) are sent to first and second sample-and-hold circuits (11 and 12). Meanwhile, the color difference signal (b) is delayed in a second delay circuit (10). In thses two sample-and-hold circuit (11 and 12), such a delayed color difference signal (c) and color difference signal (b) are respectively held in response to the two signals (i and j). Then, such two held signals (I' and II') and the delayed color difference signal (d) are sent to a switching circuit (13) for selecting one of these three signals (I', II', III') in accordance with the combination of the two signals (i and j) thereby producing an enhanced color difference signal (k). Since the luminance signal (a) has a wide frequency band, color-ringing of an image can be satisfactorily reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to an enhancing circuit which is used for 
the image processings of TVs, VTRs, or the like, and particularly to a 
circuit arrangement for enhancing a color difference signal of a video 
signal by utilizing the change of a luminance signal of the video signal. 
2. Prior Art 
As is well known in the art, there are circuits for enhancing a luminance 
signal. If these circuits are applied to a circuit for enhancing a color 
diffence signal, the ringing of colors occurrs in every color-changing 
portions of images. In the signal processing method of home VTRs, a color 
difference signal is converted into a low-band signal by an AM recording 
method in order to reduce the undesirable effect of jitter. Since the 
frequency-converted color difference signals are of low frequency, i.e. 
several hundreds of kilohertz (for example, 629 Hz for VHS format and 688 
kHz for Betamax format), the degree of azimuth loss is negligibly low 
thereby causing crosstalk between adjacent tracks on a recording medium. 
In addition, the AM recording method has disadvantages such as an 
unsatisfactory SN ratio and a narrow frequency band. Although the SN ratio 
has been improved by a noise reduction circuit using a field memory in 
recent years, there is a problem of color-ringing originated from the 
narrow frequency band. Besides, although there is a home VTR whose 
frequency band of a luminance signal is wider, since the same color 
difference signal processing method as previous VTRs is used in the home 
VTR, the color-ringing problem still remains as a large drawback. 
Hence, the enhancement of the color difference signal is considered. One 
example of the conventional enhancing circuit is disclosed in "Picture 
Signal Improvement in Color. TV Receivers", IEEE, CE31, No. 3, August 
1985. In this enhancing circuit, however, since only information in the 
color difference signal is used for the enhancement, satisfactory enhanced 
color difference signal is hardly produced because the frequency band is 
narrow, i.e. information is not enough. Accordingly, the deterioration of 
SN ratio cannot be satisfactorily improved. 
SUMMARY OF THE INVENTION 
The present invention has been developed in order to remove the 
above-described drawbacks inherent to the conventional circuit for 
enhancing a color difference signal. 
It is, therefore, an object of the present invention to provide a new and 
useful circuit for enhancing a color difference signal by using the change 
of a luminance signal. 
It is another object of the invention to provide an enhancing circuit 
suffering no deterioration of SN ratio at flat portions of a color 
difference signal. 
It is a further object of the invention to provide an enhancing circuit in 
which the color-ringing of an image can be satisfactorily reduced. 
It is a still further object of the invention to provide an enhancing 
circuit which is easily applicable to a digital circuit. 
In accordance with the present invention there is provided a circuit 
arrangement for enhancing a color difference signal of a video signal, 
comprising: filter means responsive to a luminance signal of the video 
signal for limiting the bandwidth of the luminance signal; first delay 
circuit means for delaying an output signal from the filter means; first 
differentiation circuit means for differentiating an output signal from 
the first delay circuit means; detector means for detecting an output 
signal from the first differentiation circuit means; slicer means 
responsive to an output signal from the detector means for slicing the 
output signal from the detector means at a predetermined slice level 
thereof; second differentiation circuit means for differentiating an 
output signal from the slicer means; comparator set means responsive to an 
output signal from said second differentiation circuit means for producing 
first and second signals by comparing said output signal from said second 
differentiation circuit means with two predetermined signal levels; second 
delay circuit means for delaying the color difference signal; first 
sample-and-hold circuit means for holding the output signal from the 
second delay circuit means in response to the first signal thereby 
outputting a third signal indicative of a held level of the output signal 
from the second delay circuit means; second sample-and-hold circuit means 
for holding the color difference signal in response to the second signal 
thereby outputting a fourth signal indicative of a held level of the color 
difference signal; and switching circuit means responsive to the third 
signal, the output signal from the second delay circuit means, and the 
fourth signal for selecting one of these three signals in accordance with 
combination of the first and second signals thereby outputting a signal 
which is selected therein. 
In accordance with the present invention there is also provided a circuit 
arrangement for enhancing a color difference signal of a video signal, 
comprising: filter means responsive to a luminance signal of the video 
signal for limiting the bandwidth of the luminance signal; differentiation 
circuit means for differentiating an output signal from the filter means; 
detector means for detecting an output signal from the differentiation 
circuit means; comparator means responsive to an output signal from the 
detector means for comparing the output signal from the detector means 
with a predetermined threshold level thereby outputting a first signal; 
delay circuit means for delaying the color difference signal; quadratic 
differentiation circuit means responsive to an output signal from the 
delay circuit means for differentiating the output signal from the delay 
circuit means two times; witching circuit means responsive to the first 
signal and an output signal from the quadratic differentiation circuit 
means for outputting the output signal from the quadratic differentiation 
circuit in response to the first signal; and subtractor means for 
subtracting an output signal of the switching circuit means from the 
output signal of the delay circuit means thereby producing a signal 
indicative of an enhanced color difference signal. 
In accordance with the present invention there is further provided a 
circuit arrangement for enhancing a color difference signal of a video 
signal, comprising: filter means responsive to a luminance signal of the 
video signal for limiting the bandwidth of the luminance signal; 
differentiation circuit means for differentiating an output signal from 
the filter means; detector means for detecting an output signal from the 
differentiation circuit means; comparator means responsive to an output 
signal from the detector means for comparing the output signal from the 
detector means with a predetermined threshold level thereby outputting a 
first signal; delay circuit means for delaying the color difference 
signal; quadratic differentiation circuit means responsive to an output 
signal from the delay circuit means for differentiating the output signal 
from the delay circuit means two times; switching circuit means responsive 
to the first signal and an output signal from the quadratic 
differentiation circuit means for outputting the output signal from the 
quadratic differentiation circuit in response to the first signal; 
amplifier means for amplifying an output signal from the switching 
circuit; and subtractor means for subtracting an output signal of the 
amplifier means from the output signal of the delay circuit means thereby 
producing a signal indicative of an enhanced color difference signal. 
In accordance with the present invention there is still further provided a 
digital circuit arrangement for enhancing color difference signal data 
derived from video signal data, comprising: low-pass filter means 
responsive to luminance signal data of the video signal data for limiting 
the bandwidth of the luminance signal data; first difference circuit means 
responsive to output signal data from the low-pass filter means for 
differentiating the output signal data from the low-pass filter means; ROM 
circuit means responsive to the output data from the first difference 
circuit means for looking up a ROM table in the ROM circuit means thereby 
outputting data in response to the output data from the first difference 
circuit means; second difference circuit means responsive to output signal 
data from the ROM circuit means for differentiating the output signal data 
from the ROM circuit means; first magnitude comparator means for comparing 
output signal data from the second difference circuit means with two 
predetermined threshold level data thereby outputting a first signal; 
second magnitude comparator means for comparing the output signal data 
from the second difference circuit means with two predetermined threshold 
level data thereby outputting a second signal; delay circuit means for 
delaying the color difference signal data; first latch means for latching 
the output signal data from the delay circuit means in response to the 
first signal thereby outputting first signal data indicative of latched 
level of the output signal from the delay circuit means; second latch 
means for latching the color difference signal data in response to the 
second signal thereby outputting second signal data indicative of latched 
level of the color difference signal data; and data selector means 
responsive to the first signal data, the output signal data from the delay 
circuit means, and the second signal data for selecting one of these three 
signal data in accordance with combination of the first and second signals 
thereby outputting signal data which is selected therein. 
In accordance with the present invention there is still further provided a 
digital circuit arrangement for enhancing color difference signal data 
derived from video signal data, comprising: low-pass filter means 
responsive to luminance signal data of the video signal data for limiting 
the bandwidth of the luminance signal data; first difference circuit means 
responsive to output signal data from the low-pass filter means for 
differentiating the output signal data from the low-pass filter means; ROM 
circuit means responsive to the output data from the first difference 
circuit means for looking up a ROM table in the ROM circuit means thereby 
outputting a signal indicative of a switching signal in response to the 
output data from the first difference circuit means; delay circuit means 
for delaying the color difference signal data; latch means responsive to 
output signal data from the delay circuit means for latching the output 
signal data from the delay circuit means; second difference circuit means 
responsive to the output signal data from the delay circuit means for 
differentiating the output signal data from the delay circuit means two 
times thereby outputting first signal data indicative of difference signal 
data; and switching circuit means for outputting the difference signal 
data from the second difference circuit means in response to the switching 
signal outputted from the ROM circuit means; and adder means for 
substantially subtracting output signal data of the switching circuit 
means from output signal data of the latch means thereby outputting signal 
data indicative of enhanced color difference signal data.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a block diagram of an enhancing circuit for the enhancement of 
a color difference signal according to a first embodiment of the present 
invention. In FIG. 1, the enhancing circuit generally comprises a low-pass 
filter 2, a first delay line 3, a first differentiation circuit 4, a 
detecting circuit 5, a slicer 6, a second differentiation circuit 7, a 
comparator set 8 including first and second comparators 8a and 8b, a 
second delay line 10, first and second sample-and-hold circuits 11 and 12, 
and a switching circuit 13. In this structure, the change of a luminance 
signal (a) is utilized for enhancing a color difference signal (b). 
Assuming that the signal level of the luminance signal (a) is Y, this 
luminance signal can be obtained by the following formula: 
EQU Y=0.3R+0.59G+0.11B 
where R, G, and B are the signal levels of three primary colors. 
The color difference signal (b) is presented by R-Y, and B-Y. It is to be 
noted that when the information of R-Y and B-Y changes, the luminance 
signal (a) also changes generally. In other words, there is few cases that 
only the information of R-Y and B-Y changes without the change of the 
luminance signal (a). Accordingly, the use of the information of the 
change of luminance signal (a) causes an effective enhancement of color 
difference signal (b). 
Now, the operation of the enhancing circuit according to the invention will 
be described with reference to FIGS. 1 and 2A-2K. The luminance signal (a) 
shown in FIG. 2A is inputted into a terminal 1, and is sent to the 
low-pass filter 2. In this low-pass filter 2, the bandwidth of the 
luminance signal (a) is limited within several times of the bandwidth of 
the color difference signal (b) shown in FIG. 2B because the luminance 
signal (a) has so large bandwidth in comparison with the color difference 
signal (b), and this band-limited signal is outputted to the first delay 
line 3. In the first delay line 3, the band-limited signal is delayed by a 
delay time which will be described later, and then a delayed signal (c) is 
produced (see FIG. 2C). The delayed signal (c) is differentiated in the 
first differentiation circuit 4, and a differentiated signal (e) is 
outputted to the detector 5. Here, the changing portions of the 
band-limited and delayed luminance signal (c) respectively correspond to 
the changing portions of the output signal (e) from the first 
differentiation circuit 4 (see FIGS. 2C and 2E). Then, detecting the 
differentiated signal (e) in the detector 5, a detected signal (f) is 
produced as shown in FIG. 2F. After this, the detected signal (f) is sent 
to the slicer 6. 
A slice level is predetermined in the slicer 6 whereby an output signal of 
the slicer 6 is produced only when the detected signal (f) is larger than 
the predetermined slice level. The purpose of the use of the slicer 6 is 
to enhance the color difference signal (b) only when the degree of 
amplitude change of the luminance signal (a) is larger than a 
predetermined level. This is because the color-ringing is hardly caused 
when the degree of amplitude change of the luminance signal (a) is small. 
In this embodiment, such an output signal (g) is presented as shown in 
FIG. 2G. 
The output signal (g) is differentiated in the second differentiation 
circuit 7 thereby producing a signal (h) shown in FIG. 2H. Here, the 
positive portion of the signal (h) indicates a first half time period from 
the begining of the change to a maximum according to the luminance signal 
(a), and the negative portion of the signal (h) presents a latter half 
time period from the maximum to the end of the change. Then the signal (h) 
is inputted into the comparator set 8. 
Since the signal (h) includes noise generally, first and second threshold 
levels are predetermined for using in the respective first and second 
comparators 8a and 8b. More specifically, when the level of the signal (h) 
is higher than the first threshold level, a first signal (i) indicative of 
the first half time period is outputted from the first comparator 8a, and 
when the level of the signal (h) is lower than the second threshold level, 
a second signal (j) indicative of the latter half time period is outputted 
from the second comparator 8b. Thus, the first and second signals are 
produced in accordance with two predetermined threshold levels. The first 
signal (i) is sent to the first sample-and-hold circuit 11 and the 
switching circuit 13, and the second signal (j) is sent to the second 
sample-and-hold circuit 12 and the switching circuit 13. 
Meanwhile, the color difference signal (b) is inputted into a terminal 9, 
and sent to the second delay circuit 10 and the second sample-and-hold 
circuit 12. In the second delay circuit 10, the color difference signal 
(b) is delayed by a delay time which is preferably selected between 200 ns 
and 500 ns, as shown in FIG. 2D. Here, the first signal (i) from the first 
comparator 8a is produced during the above-mentioned first half time 
period, and the level of the delayed color difference signal (d) starts 
changing. This delayed signal (d) is also inputted into the first 
sample-and-hold circuit 11. In the first sample-and-hold circuit 11, the 
first signal (i) is used as a first hold signal, i.e. the voltage of the 
delayed signal (d) is sampled at the leading edge of the first hold signal 
whereby this sampled voltage is continuously outputted when the first hold 
signal is valid. Therefore, the voltage value at the begining of the 
change of the delayed signal (d) is held in the first sample-and-hold 
circuit 11. Then the first sample-and-hold circuit 11 outputs a signal 
indicative of a first held level of the delayed signal (d). 
Similarly, the second signal (j) from the second comparator 8b is produced 
during the above-mentioned second half time period. At this time, the 
level change of the color difference signal (b) is terminated. 
Furthermore, the color difference signal (b) is also inputted into the 
second sample-and-hold circuit 12. In the second sample-and-hold circuit 
12, the second signal (j) is used as a second hold signal, i.e. the color 
difference signal (b) is held therein in response to the second hold 
signal. Therefore, the voltage value at the end of the change of the color 
difference signal (d) is held in the second sample-and-hold circuit 12. 
Then the second sample-and-hold circuit 12 outputs a signal indicative of 
a second held level of the color difference signal (b). 
The operation of the switching circuit 13 will be described hereinbelow. 
The switching circuit 13 is supplied with output signal (I') indicative of 
the first held level from the first sample-and-hold circuits 11, delayed 
color difference signal (II'), and output signal (III') indicative of the 
second held level from the second sample-and-hold circuit 12. One of these 
three signals (I'), (II'), and (III') is selected in accordance with the 
combination of the first and second signals (i) and (j) outputted from the 
comparator set 8. In the switching circuit 13, when the first signal (i) 
is "H", the signal (I') is selected and outputted to a terminal 14. When 
the second signal (j) is "H", the signal (III') is selected and outputted 
to the terminal 14. When both the first and second signals (i) and (j) are 
"L", the signal (II') is selected and outputted to the terminal 14. As a 
result, an enhanced color difference signal (k) from the switching circuit 
13 is produced as shown in FIG. 2K. In FIG. 2K, reference numerals (I), 
(II), and (III) respectively denote a time period in which the signal (I) 
is selected, a time period in which the signal (II) is selected, and a 
time period in which the signal (III) is selected. 
It is to be noted that the delay time of the delay line 3 is predetermined 
so as to satisfy the condition that inflection points N1 and N2 of the 
waveform of the band-limited and delayed luminance signal (c) respectively 
substantially coincide with middles M1 and M2 of the rising and falling 
portions of the waveform of the color difference signal (d) delayed by the 
delay line 10. 
FIG. 3 is a block diagram showing a digital enhancing circuit to which the 
enhancing circuit of the first embodiment is applied. The digital 
enhancing circuit generally comprises a low-pass filter 22, first and 
second difference circuits 24 and 27, a ROM circuit 25, first and second 
magnitude comparators 28 and 29, first and second latch circuits 31 and 
32, a delay line 30, and a data selector 33. In this structure, luminance 
signal data (a') is inputted into a terminal 21, and sent to the low-pass 
filter 22 to limit the bandwidth of the luminance signal data (a'). An FIR 
(Finite Impulse Response) filter can be applied to the low-pass filter 22 
in this case, and is shown in FIG. 4. In addition, the FIR filter can be 
also applied to the first embodiment low-pass filter 2. Reference numerals 
k0-kn represent filter coefficients. 
Then the band-limited signal data is inputted into the first difference 
circuit 24. The first difference circuit 24 includes a latch 24a and a 
subtractor 24b. In the latch 24a, the band-limited signal data is delayed 
by n clock pulses, and this delayed signal data is sent to the subtractor 
24b. The band-limited signal data is also inputted into the subtractor 
24b. In the subtractor 24b, one of the input signal data of the subtractor 
24b is subtracted from the other of the input signal data thereof to 
produce output signal data (e'). This signal data (e') is sent to the ROM 
circuit 25 as read address. The ROM circuit 25 corresponds to the detector 
5 and the slicer 6 of FIG. 1. FIG. 5 is a graph showing address-data 
characteristics which are programmed in a ROM of the ROM circuit 25. In 
the ROM circuit 25, this ROM table is looked up whereby signal data (g') 
is produced in response to the outputted signal data (e'). 
The second difference circuit 27 generally comprises a latch 27a and a 
subtractor 27b. Since the structure and the operation of the second 
difference circuit 27 are the same as the first difference circuit 25, the 
detail description thereof will be omitted here. Output signal data (h') 
from the second difference circuit 27 is sent to the magnitude comparators 
28 and 29. By comparing the output signal data (h') with predetermined two 
threshold level data for each of these comparators 28 and 29, signals (i') 
and (j') are produced. More specifically, since these comparators 28 and 
29 operate as the comparator set 8 of FIG. 1, two threshold level are 
respectively required to produce the signals (i') and (j'). 
Similar to the enhancing circuit of the first embodiment, the signal (i') 
is sent to the first latch 31 and the data selector 33, and the signal 
(j') is applied to the second latch 32 and the data selector 33. The first 
latch 31 and the second latch 32 respectively operate as the first and 
second sample-and-hold circuit 11 and 12 of FIG. 1, and the data selector 
33 operates as the switching circuit 13 of FIG. 1. Meanwhile, color 
difference signal data (b') which is inputted into a terminal 26 is 
delayed in the delay line 30 thereby producing signal data (d'). The 
output signal data (d') of the delay line 30 is sent to the latch 31 and 
the data selector 33, and the data selector 33 is supplied with three 
signal data (Ia'), (IIa'), and (IIIa'). In the data selector 33, one of 
these signal data (Ia'), (IIa'), and (IIIa') is selected in accordance 
with the combination of the signals (i') and (j') outputted from the first 
and second magnitude comparators 28 and 29. Thus, enhanced color 
difference signal data (k') is obtained from a terminal 34 by the similar 
structure and operation to the enhancing circuit of the first embodiment. 
FIG. 6 is a block diagram showing an enhancing circuit of a second 
embodiment according to the present invention. In this embodiment, the 
enhancing circuit comprises a low-pass filter 102, a differentiation 
circuit 103, a detector 104, a comparator 105, a delay circuit 107, a 
quadratic differentiation circuit 108, a switching circuit 110, and a 
subtractor 109. In this structure, the low-pas filter 102, the 
differentiation circuit 103, the detector 104, and the delay circuit 107 
respectively have the same function as those of the first embodiment of 
FIG. 1, and the low-pass filter 102 is made of the FIR filter of FIG. 4. 
The operation of this enhancing circuit will be described with reference to 
FIGS. 7A-7J. A luminance signal 1a is inputted into a terminal 101, and 
the bandwidth thereof is limited in the low-pass filter 102. Such a 
limited signal 1c is differentiated in the differentiation circuit 103, 
and this produced differentiated signal 1d is detected in the detector 
104. Then, this detected signal 1e is inputted into the comparator 105. I 
the comparator 105, when the amplitude level of the detected signal 1e is 
larger than a predetermined threshold level, a signal is outputted to the 
switching circuit 110. This is because the degree of the color-ringing is 
negligibly low when the change of colors is small. 
Meanwhile, a color differnce signal 1b is inputted into a terminal 106, and 
is delayed in the delay circuit 107. An output signal 1g from the delay 
circuit 107 is sent to the quadratic differentiation circuit 108 and the 
subtractor 109. If an output signal 1h from the quadratic differentiation 
circuit 108 is always subtracted from the delayed signal 1g in the 
subtractor 109, the enhancement of the color difference signal 1b is the 
same as that of the luminance signal 1a thereby causing the deterioration 
of SN ratio in flat portions of the waveform of the color difference 
signal 1b. In order to improve this problem, the second embodiment 
enhancing circuit has the switching circuit 110 for producing a signal 1i 
in accordance with the output signal 1f from the comparator 105. In this 
embodiment, the switching circuit 110 is in "ON" state only when the 
signal 1f is "H". Thus, the signal 1i is subtracted from the signal 1g 
during the change of the luminance signal 1a in the subtractor 109 
whereby an enhanced signal 1j is outputted to a terminal 111 as shown in 
FIG. 7J. 
In the case that the enhancement of the color difference signal 1b is not 
enough, the signal 1h may be subtracted from the signal 1g with the signal 
1h being amplified. To this end, there is provided another enhancing 
circuit which is shown in FIG. 8. FIG. 8 is a block diagram showing an 
enhancing circuit of a third embodiment according to the present 
invention. Since the structure is the same as that of the enhancing 
circuit of the second embodiment except that an amplifier 112 is provided 
between the switching circuit 110 and the subtractor 109, a further 
discription thereof will be omitted here. According to the third 
embodiment, the color difference signal 1b can be satisfactorily enhanced 
whereby an enhanced signal 2j can be obtained from the terminal 111. 
FIG. 9 is a block diagram showing a digital enhancing circuit to which the 
enhancing circuit of the second embodiment is applied. This digital 
enhancing circuit comprises a low-pass filter 102', a first difference 
circuit 103' including a latch 103a' and a subtractor 103b, a ROM circuit 
104', a delay line 107', a second difference circuit 108', a latch 113', a 
switching circuit 110', and an adder 109'. The second difference circuit 
108' includes two latchs 108a' and 108b', two adders 108c' and 108d', and 
two multipliers 108e' and 108f'. In this structure, the first and second 
defference circuits 103' and 108' respectively correspond to the 
differentiation circuit 103 and the quadratic differentiation circuit 108 
of FIG. 6, and the ROM circuit corresponds to the detector 104 and the 
comparator 105 of FIG. 6. Furthermore, the low-pass filter 102', the delay 
line 107' the switching circuit 110', and the adder 109' respectively 
correspond to the low-pass filter 102, the delay circuit 107, the 
switching circuit 110, and the subtractor 109. 
Luminance signal data 1a' which is inputted into a terminal 101' is sent to 
the ROM circuit 104' as read address via the low-pass filter 102' and the 
first difference circuit 103'. Since the operation of the low-pass filter 
102' and the first difference circuit 103' are the same as those of FIG. 
3, the discription will be omitted here. FIG. 10 is a graph showing a ROM 
table, i.e. address-data characteristics which are programmed in a ROM of 
the ROM circuit 104'. In the ROM circuit 104', the ROM table is looked up, 
and signal data 1f' is produced in accordance with the outputted signal 
data 1e' from the subtractor 103'. Meanwhile, color difference signal data 
1b' which is inputted into a terminal 106' is sent, via the delay line 
107', to the second difference circuit 108' and the latch 113'. 
In the second difference circuit 108', the output signal data from the 
delay line 107' is inputted into the latch 108a' and the adder 108c', and 
the output signal data from the latch 108a' is sent to another latch 108b' 
and another adder 108d'. In the adder 108c', the output signal data from 
the delay line 107' is added to the output signal data from the latch 
108b', and then this added signal data is sent to the multiplier 108e'. In 
the multiplier 108e', this added signal data is multiplied by -1/2, and 
the product signal data is sent to the adder 108d'. In the adder 108d', 
the output signal data from the latch 108a' is added to the output signal 
data from the multiplier 108e'. This added signal data is multiplied by a 
factor in the multiplier 108f' and then sent to the switching circuit 110' 
The multiplying factor of the multiplier 108f' is selected in the order of 
from 1/4 to unity so that a false color is not produced at a border 
between mutually different colors. From the second difference circuit 108' 
, data resulted from the quadratic differentiation of the input signal 
data thereto is produced so that plus/minus sign thereof is reversed. In 
the switching circuit 110', the output signal from the second difference 
signal is selected only when the signal 1f' is outputted from the ROM 
circuit 104', and then outputted to the adder 109'. In the adder 109', the 
output signal from latch 113' is added to the output signal from the 
switching circuit 110', and this added signal is outputted to a terminal 
111'. It should be noted that such quadratic differentiated data is 
substantially subtracted from the output signal of the latch 113' because 
the plus/minus sign is reversed in the second difference circuit 108'. 
Thus, digital enhanced color difference signal data can be produced. 
In the case of the digital enhancing circuit shown in FIG. 9 is used for a 
reproduced video signal in a home-use video tape recorder in which the 
reproduced video signal is sampled by sampling frequency of 14.318 MHz 
(4f.sub.sc), the parameters are preferably selected from the following 
ranges: n of the latches 108a', 108b' and 113' is between 8 and 16; the 
cut-off frequency of the low-pass filter 102' is in the order of 1-3 MHz; 
and n of the latch 103a' is between 4 and 8. In addition, inserting a 
coring circuit (not shown) in the path from the adder 108d' and the adder 
109', the quality of the reproduced picture can be further improved. 
As described hereinabove, the present invention provides a circuit for 
enhancing a color difference signal by using the change of a luminance 
signal, and the circuit can be easily applied to a digital enhancing 
circuit. In addition, although the FIR filter is applied to the low-pass 
filters of the present embodiments, it is not limited to the FIR filter. 
From the figure, it will be apparent that the present enhancing circuit 
has no deterioration of SN ratio at flat portions of the color difference 
signal. Therefore, the color-ringing of an image can be satisfactorily 
reduced. 
The above-described embodiments are just examples of the present invention, 
and therefore, it will be apparent for those skilled in the art that many 
modifications and variations may be made without departing from the scope 
of the present invention.