Method of enhancing details contained in a color signal and a circuit for implementing the same in a color video apparatus

A method and circuit for reducing distortion of low level signals by efficiently separating noise and signal components using correlativity between respective red (R), green (G) and blue (B) channels and the amplitudes of the detail components. The detail enhancement method includes extracting detail components, determining signal/noise, cancelling noise or enhancing details and outputting a detail-enhanced image signal. Even if a high-pass frequency component of the input signal is smaller than a critical value, the input signal is accurately separated as a signal component or a noise component. The signal is cancelled only when the input signal is classified as a noise component.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to a method for enhancing details contained 
in a color signal and a circuit for implementing the same in a color video 
apparatus. More particularly, the present invention relates to a method 
and circuit for reducing distortion of low level signals by efficiently 
separating noise and signal components using correlativity between the 
respective R, G and B channels. 
2. Description of the Related Art 
Spatial frequency is a measure of how rapidly a parameter changes over 
distance in a prescribed spatial direction and is analogous to temporal 
frequency, which is a measure of how rapidly a parameter changes with the 
passage of time. In television systems using horizontal scanning lines, 
horizontal space is conformably mapped to time by the scanning process, so 
horizontal spatial frequency of the televised image intensity conformably 
maps to temporal frequency in the video signal which is descriptive of the 
televised image. 
In video cameras using a single pickup device, a color pattern filter may 
be used to filter light reaching the pickup device so that color signals 
can be extracted from the electrical signal supplied by the pickup device. 
The color pattern filter customarily contains stripes transmitting light 
of three different colors to the pickup device, which may be a vidicon or 
may be a solid-state imager such as a line-transfer charge-coupled device. 
The direction of the stripes is perpendicular to the direction of the line 
scan in the camera, in which the line scan conventionally proceeds in a 
horizontal direction. The stripes of each color are of uniform width, but 
the stripes of different colors are preferably of different widths to 
simplify the separation of color components from the output signal of the 
pickup device. The respective widths associated with the different colors 
are usually scaled in regard to the contribution of the particular color 
to luminance--that is, to reference white. If the color filter comprises 
red-transmissive, green-transmissive and blue-transmissive stripes, for 
example, the green-transmissive stripes will be the widest and the 
blue-transmissive stripes will be the narrowest. The signals picked up by 
the narrower width stripes have poorer signal-to-noise ratio (S/N), 
particularly in the higher horizontal spatial frequencies containing 
detail. When the video camera is used with a video transmission system 
where the color signals are converted to wideband luminance and narrowband 
color-difference signals, the poorer S/N of the colors contributing less 
to luminance is not of much concern, since detail enhancement or video 
peaking is usually carried out on the shared luminance high frequencies 
rather than on individual color signals. 
However, the video camera can be used with video equipment in which the 
color signals are not combined to form luminance and color-difference 
signals--e.g. certain digital video transmission systems of the so-called 
RGB type where the red (R), green (G) and blue (B) color signals are 
separately digitized and coded. In such equipment detail enhancement or 
video peaking is apt to be performed on the red (R), green (G) and blue 
(B) color signals themselves. Since the human visual system discriminates 
poorly between details of colors as those details become finer, 
enhancement of the color details that have poorer S/N with the color 
details that have better S/N can result in images that have less apparent 
noise in them. Random noise in the green (G) color signal is not 
correlated with random noise in the red (R) and blue (B) color signals, so 
on average the random noise component of the G signal and the random noise 
component of another color signal add as quadrature vectors rather than 
in-phase vectors, which apparently helps the high-frequency S/N when 
enhancing the detail of that other color signal. 
FIG. 1 shows the block diagram of the first embodiment of a conventional 
detail enhancement circuit, which includes, for a red (R) channel, first 
and second 1H delay lines 11 and 12, a vertical high-pass filter (V HPF) 
weight-and-sum circuit 13, a horizontal low-pass filter (H LPF) 14, a 
horizontal high-pass filter (H HPF) 15, first adder 16, a ROM 17 in which 
a noise cancelling and detail enhancement amount determining look-up table 
(LUT) is stored, and a second adder 18. This structure is also applied to 
green (G) and blue (B) channels. 
FIG. 2 is a graph showing input-versus-output characteristics of the noise 
cancelling and detail enhancement amount determining look-up table (LUT) 
stored in ROM 17 of FIG. 1. 
The first embodiment of the conventional detail enhancement circuit will 
now be described with reference to FIGS. 1 and 2. 
The first delay line 11 delays an image signal of the R channel by a 1H 
period and the second delay line 12 delays the 1H-delayed image signal 
output from first delay line 11 by another 1H period. Here, the image 
signal output from the first and second delay lines 11 and 12 is a 
gamma-corrected signal. 
The V HPF weight-and-sum circuit 13 performs a high-pass filtering 
operation with respect to the original signal, 1H-delayed signal and 
2H-delayed signal output from first and second delay lines 11 and 12, 
respectively, to extract vertical detail components present in a 
predetermined high-frequency band. 
The H LPF 14 performs a horizontal low-pass filtering operation with 
respect to vertical detail components output from the V HPF weight-and-sum 
circuit 13 to eliminate diagonal detail components contained in the 
vertical detail components output from V HPF weight-and-sum circuit 13, 
thereby preventing double enhancement of the diagonal detail components. 
The H HPF 15 performs a horizontal high-pass filtering operation with 
respect to the 1H-delayed image signal output from the first delay line 11 
to extract horizontal detail components present in a predetermined 
high-frequency band. 
The first adder 16 sums the vertical detail components output from the H 
LPF 14 with the horizontal detail components output from the H HPF 15. The 
summed output is applied to ROM 17 in which the noise cancelling and 
detail enhancement amount determining look-up table (LUT) is stored. 
The input-versus-output characteristics of the noise cancelling and detail 
enhancement amount determining look-up table (LUT) are the same as those 
in FIG. 2. At this time, input signals (the abscissa) are divided into a 
section A, a section B and a section C. Here, a critical point (CP) 
represents a boundary value between the sections A and B, that is, a 
reference value for noise identification. 
If the signal level applied to ROM 17 is greater than +CP or less than -CP 
(i.e., is within the A section), the LUT stored within ROM 17 determines 
and outputs the corresponding amount of detail enhancement, ranging from 
zero to the respective maximum detail enhancement amounts (DE.sub.max and 
-DE.sub.max) with respect to the magnitude of the input horizontal and 
vertical detail components, as shown in FIG. 2. 
On the other hand, if the signal level applied to ROM 17 is less than +CP 
or greater than -CP (i.e., is within the B and C sections), the LUT stored 
within ROM 17 functions for cancelling the high-spatial-frequency noise. 
That is to say, if an input signal is present between -CL and +CL (i.e., is 
in the C section), the signal output from ROM 17 is a signal corresponding 
to inverted high-frequency noise. When the input signal to ROM 17 is 
primarily high-frequency noise from the R signal, which is applied as one 
summand to the second adder 18, the inverted high-frequency noise output 
from ROM 17, which is applied as another summand to the second adder 18, 
cancels the high-frequency noise from the R' signal supplied from second 
adder 18. That is to say, if the input signal level to ROM 17 is between 
-CL and +CL (i.e., is in the C region), the corresponding output signal 
level read from ROM 17 is the same amplitude as the input signal, but is 
of opposite polarity, as shown in FIG. 2. 
If an input signal is present between -CP and -CL, or between +CL and +CP 
(i.e., is in the B section), the output signal level is expressed in the 
form of an exponentially increasing function as the input signal level 
ranges upward from a point +a to the point on the abscissa corresponding 
to +CP, or ranges downward from the point -a to the point on the abscissa 
corresponding to -CP, as shown in FIG. 2. 
In other words, the horizontal and vertical detail components input to ROM 
17 determine the types of output signals depending on the respective 
magnitudes thereof. If the magnitude of the input signal is less than the 
absolute value of CP, a noise-cancelled horizontal and vertical detail 
component is output. If the magnitude of the input signal is greater than 
the absolute value of CP, a horizontal and vertical detail component whose 
corresponding detail enhancement amount is determined is output. 
The second adder 18 functions to add or subtract a high-frequency component 
output form the LUT stored in ROM 17 with respect to the 1H-delayed signal 
output from first delay line 11. At this time, the second adder 18 
functions as a noise canceler unit during subtraction and functions as a 
detail enhancer during addition. 
FIG. 3 shows a block diagram of a second embodiment of the conventional 
detail enhancement circuit, which includes, for a red (R) channel, first 
and second delay lines 31 and 32, a vertical low-pass filter (V LPF) 
weight-and-sum circuit 33, a horizontal low-pass filter (H LPF) 34, a 
subtracter 35, a ROM 36 in which a noise cancelling and detail enhancement 
amount determining look-up table (LUT) is stored, and an adder 37. This 
structure is also applied to the green (G) and blue (B) channels. 
FIG. 4 is a graph showing input-versus-output characteristics of the noise 
cancelling and detail enhancement amount determining look-up table (LUT) 
stored in ROM 36 of FIG. 3. 
The second embodiment for the conventional detail enhancement circuit will 
now be described with reference to FIGS. 3 and 4. Here, descriptions of 
parts that are the same as those in the first embodiment will be omitted. 
The V LPF weight-and-sum circuit 33 performs a low-pass filtering operation 
with respect to the original signal, 1H-delayed signal and 2H-delayed 
signal output from first and second delays 31 and 32, respectively, to 
extract a vertical low-frequency band. 
The H LPF 34 performs a horizontal low-pass filtering operation with 
respect to the vertical low-frequency band output from V LPF 
weight-and-sum circuit 33 to extract a diagonal low-frequency band 
contained in the vertical low-frequency band output from V LPF 
weight-and-sum circuit 33. 
The subtracter 35 subtracts the diagonal low-frequency band contained in 
the vertical low-frequency band output from H LPF 34 from the 1H-delayed 
image signal output from first delay line 31 to output a high frequency 
component around current data, that is, the horizontal and vertical detail 
components. 
Operation of ROM 36 and adder 37 is the same as in the first embodiment. 
However, the aforementioned conventional detail enhancement circuits each 
functions as a noise canceler by inverting an input signal by 180.degree. 
inphase if the level of the signal input to the LUT stored in the ROM is 
less than a critical level (CL). As the result, the following drawbacks 
are caused. 
First, in the case of a low level input signal whose high-frequency 
component is less than a critical level (CL), the input signal is always 
regarded as noise, which is, in turn, cancelled from the original signal. 
Second, in order to solve the first drawback, if the critical level (CL) 
is decreased, the advantage of the detail enhancement circuit as a noise 
canceler is reduced. 
Third, if the amount of variation of the color signal is large and the 
color signal level input to a channel is less than the critical level, the 
color signal input to the corresponding channel is cancelled, thereby 
changing the color signal. 
SUMMARY OF THE INVENTION 
To solve the above-described problems, it is, therefore, an object of the 
present invention to provide a detail enhancement method for improving a 
signal-to-noise (S/N) ratio by performing a detail enhancing function or a 
noise cancelling function after the signal and noise are accurately 
discriminated using the correlativity between red (R), green (G) and blue 
(B) channels and the level of a signal input to the respective channels. 
It is another object of the present invention to provide a detail 
enhancement circuit suitable for implementing the detail enhancement 
method. 
To accomplish the first object, there is provided a detail enhancement 
method comprising the steps of: 
extracting horizontal and vertical detail components contained in an input 
image signal supplied from at least one channel among red (R), green (G) 
and blue (B) channels; 
determining whether the horizontal and vertical detail components 
correspond a signal or noise by checking correlativity between said R, G 
and B channels and amplitude of the horizontal and vertical detail 
components; 
cancelling noise by coring the horizontal and vertical detail components 
output for the respective channels if the horizontal and vertical detail 
components is determined as the noise in the signal/noise determining 
step, and determining and outputting the detail enhancement amount 
corresponding to the amplitude of detail components if the horizontal and 
vertical detail components is determined as the signal; and 
outputting a detail-enhanced image signal for the respective channels 
obtained by summing the 1H-delayed signal of the image signal supplied 
from at least one channel among the R, G and B channels, with the 
horizontal/vertical detail components whose detail enhancement amount is 
determined, or whose noise is cancelled. 
To accomplish the second object of the present invention, there is provided 
a detail enhancement circuit comprising: 
means for extracting horizontal and vertical detail components contained in 
an input image signal supplied from at least one channel among R, G and B 
channels; 
a controller for determining whether the horizontal and vertical detail 
components correspond a signal or noise by checking correlativity between 
the R, G and B channels and amplitude of the horizontal and vertical 
detail components and outputting a selection control signal for selecting 
a Is corresponding look-up table; 
a ROM for storing a first look-up table for determining and outputting a 
detail enhancement amount corresponding to the amplitude of detail 
components and a second look-up table for cancelling the noise of the 
input image signal are stored, and supplying output from the look-up table 
selected according to the selection control signal; and 
means for outputting a detail-enhanced image signal for the respective 
channels obtained by summing the 1H-delayed signal of said image signal 
supplied from at least one channel among the R, G and B channels, with the 
horizontal/vertical detail components whose detail enhancement amount is 
determined, or whose noise is cancelled, in the ROM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 5, the detail enhancement circuit according to a first embodiment 
of the present invention includes a controller 58, and a ROM 59 in which 
first and second look-up tables LUT1 and LUT2 are stored, for use with a 
circuit 50, excluding a part corresponding to ROM 17 of the conventional 
detail enhancement circuit shown in FIG. 1. FIGS. 6A and 6B are graphs 
showing one form of the input-versus-output characteristics of first and 
second look-up tables LUT1 and LUT2 stored in ROM 59 shown in FIG. 5, 
respectively. 
FIG. 7 is a detailed block diagram of the controller 58 and ROM 59 shown in 
FIG. 5. The controller 58 includes an RGB channel correlativity determiner 
70, an RGB signal level determiner 74 and an OR gate 79 as a control 
signal output unit. ROM 59 includes the first look-up table 80a, the 
second look-up table 80b and a selector 80c. 
The RGB channel correlativity determiner 70 includes an AND gate 71, a NOR 
gate 72 and an OR gate 73. The RGB signal level determiner 74 includes 
three inverters 75, 76 and 77 and an OR gate 78. 
FIG. 8 shows image signals displayed on a screen when black, gray and green 
colors taken along lines aa', bb' and cc', respectively, are vertically 
formed on a white background part during signal photographing operation. 
FIGS. 9A and 9B show signals of a line when the black color signal and the 
gray color signal shown in FIG. 8 are taken along the horizontal lines aa' 
and bb', respectively. 
FIG. 10 shows signals obtained by performing horizontal and vertical 
filtering operations with respect to the signal shown in FIG. 9A and 
summing the result, for R, G and B channels. FIG. 11 shows signals 
obtained by performing horizontal and vertical filtering operations with 
respect to the signal shown in FIG. 9B and summing the result, for R, G 
and B channels. FIG. 12 shows signals obtained by performing horizontal 
and vertical filtering operations with respect to the green color signal 
for the line cc' in FIG. 8 and summing the result, for R, G and B 
channels. FIG. 13 shows a signal discriminated as the noise for R, G and B 
channels. 
FIG. 14 is a block diagram of a detail enhancement circuit according to a 
second embodiment of the present invention, which includes a controller 
147 and a ROM 148 in which first and second look-up tables LUT1 and LUT2 
are stored, for use with a circuit 140, excluding a part corresponding to 
ROM 17 of the conventional detail enhancement circuit shown in FIG. 3. 
FIG. 15 is a block diagram of a detail enhancement circuit according to a 
third embodiment of the present invention, which includes a controller 158 
and a ROM 159 in which first and second look-up tables LUT1 and LUT2 are 
stored, for use with a circuit 150, excluding a part corresponding to ROM 
17 of the conventional detail enhancement circuit shown in FIG. 3. 
FIG. 16 is a graph showing another form of the input-versus- output 
characteristics of the first LUT (LUT1) stored in the ROM shown in FIGS. 
5, 14 and 15. 
FIG. 17 is a graph showing another form of the input-versus- output 
characteristics of the second LUT (LUT2) stored in the ROM shown in FIGS. 
5, 14 and 15. 
Now, the operation of the present invention will be described in detail and 
descriptions of the pans which are the same as those in the conventional 
detail enhancement circuit will be omitted herein. 
In the controller 58, the RGB channel correlativity determiner 70 detects 
the sign of the signal Ra supplied from the first adder 55 with respect to 
the R channel, as well as the signals Ga and Ba with respect to the G and 
B channels (not shown) and discriminates whether the signs of the Ra, Ga 
and Ba signals are identical, thereby determining the sign correlativity 
among the Ra, Ga and Ba signals. The following Table 1 is an 
input-versus-output table of the RGB channel correlativity determiner 70. 
In Table 1, if the sign of the R, G or B signal is negative, it is 
designated by `1`, and if the sign is positive, it is I0 designated by 
`0`. 
TABLE 1 
______________________________________ 
Ra Ga Ba Output 
______________________________________ 
0 0 0 1 
0 0 1 0 
0 1 0 0 
0 1 1 0 
1 0 0 0 
1 0 1 0 
1 1 0 0 
1 1 1 1 
______________________________________ 
That is, the RGB channel correlativity determiner 70 outputs a `1` if 
correlativity between R, G and B channels exists, and it outputs a `0` 
otherwise. 
The RGB signal level determiner 74 compares the magnitude of the signal Ra 
supplied from the first adder 55 with respect to the R channel as well as 
the signals Ga and Ba with respect to the G and B channels (not shown). 
The following Table 2 is an input-versus-output table of the RGB signal 
level determiner 74, in which `1` designates that the R, G or B signal is 
greater than CP, and `0` designates that the R, G or B signal is less than 
CP. 
TABLE 2 
______________________________________ 
R G B Output 
______________________________________ 
0 0 0 0 
0 0 1 1 
0 1 0 1 
0 1 1 1 
1 0 0 1 
1 0 1 1 
1 1 0 1 
1 1 1 1 
______________________________________ 
The OR gate 79 (control signal output unit) performs an OR operation with 
respect to the outputs of the RGB channel correlativity determiner 70 and 
RGB signal level determiner 74 to output a control signal to selector 80c 
for selecting a look-up table. 
The following Table 3 is an input-versus-output table of the OR gate 79. In 
Table 3, `1` designates the case when the first look-up table LUT1 80a is 
selected, and `0` designates the case when the second look-up table LUT2 
80b is selected. 
TABLE 3 
______________________________________ 
CP &gt; Signal level 
CP &lt; Signal level 
______________________________________ 
Presence of correlativity 
0 1 
Absence of correlativity 
1 1 
______________________________________ 
In the case of using a triple-panel charge coupled device (CCD), data types 
of the respective R, G and B channels obtained from a black-and-white 
object to be photographed are identical. Therefore, the phases of the 
signals output from a spatial filter are the same when the spatial filter 
in the same band is used. At this time, an example of the waveforms are 
shown in FIGS. 10 and 11. In other words, as shown in FIG. 10, if a large 
changing black-and-white signal is input, a DC offset is eliminated in the 
output of the spatial filter so that a signal swinging around zero is 
output. In such a signal as shown in FIG. 11, the phases indicated as a 
positive (+) or negative (-) sign signal are identical. That is, as above, 
when the sign portion of the signal is the same and the signal level is 
greater than a critical point (CP), ROM 59 performs a detail enhancer 
operation by using the first look-up table LUT1 80a. 
On the other hand, as shown in FIG. 11, if a small changing black-and-white 
signal is input, the sign portion of the signal is the same and the signal 
level is less than a critical point (CP). Generally, a signal has 
correlativity among R, G and B channels, whereas noise, being irregular, 
has no correlativity among R, G and B channels. Therefore, if the signs of 
the signals of the respective channels are identical with one another, the 
input image signal is likely to be considered as signal components. For 
the foregoing reasons, when the sign portion of the signal is the same and 
the signal level is less than a critical point (CP), ROM 59 performs a 
detail enhancer operation by using the first look-up table LUT1 80a. 
If a large changing color signal is input, the channel output corresponding 
to the color signal level becomes larger. At this time, an example of the 
waveforms for the respective channels are shown in FIG. 12. In other 
words, the signal shown in FIG. 12 is for the case of a comparatively 
large signal present in the G channel when green vertical lines are 
present. In such a case, although signs of the respective channels are not 
the same with one another, since a large changing color signal, exceeding 
a general noise level, is present in any channel among the R, G and B 
channels, the signal is considered to be a signal component. Thus, for the 
foregoing reason, when the sign portion of the signal is not the same and 
the signal level is greater than a critical point (CP), ROM 59 performs a 
detail enhancement operation by using the first look-up table LUT1 80a. 
On the other hand, as shown in FIG. 13, in the case of a signal having 
different sign portions of the R, G or B signals and a small color change, 
the respective signal levels of the R, G and B channels are at the general 
noise level. Also, since there is no correlativity among the respective R, 
G and B channels, the input image signal is considered as noise. Thus, for 
the foregoing reason, when the sign portion of the signal is not the same 
and the signal level is less than a critical point (CP), ROM 59 performs a 
noise cancellation operation by using the second look-up table LUT2 80b. 
FIGS. 6A and 16 show graphs showing examples of the input-versus-output 
characteristics of LUT1 80a and FIGS. 6B and 17 show graphs showing 
examples of the input-versus-output characteristics of LUT2 80b. 
As described above, in the detail enhancement method and circuit according 
to the present invention, a separability of signal and noise components is 
improved by checking the correlativity among R, G and B channels and the 
levels of the signals input to the respective channels. Therefore, even if 
a high-pass frequency component of the input signal is smaller than a 
critical value, the input signal is accurately separated as a signal or a 
noise component. Then, the signal is cancelled only when the input signal 
is classified as the noise component. 
Also, even if the level of the signal of a channel, except for the channel 
experiencing a large color change, is smaller than the critical value, the 
signal of the channel experiencing a small color change is not cancelled. 
Therefore, there is no change in a color signal.