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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a negative-image signal processing circuit and,
more particularly, to a processing circuit for a white-balance adjustment.
The invention further relates to a variable-gamma correction circuit and an
image signal processing circuit which uses the gamma correction circuit,
especially an image signal processing circuit for a white-balance
adjustment and gamma correction.
Further, the invention concerns a peak detector circuit for detecting the
maximum or minimum level (peak level) of an input voltage. The peak
detector circuit is well-suited for use in the above-mentioned
negative-image signal processing circuit.
Still further, the invention relates to an electronic image pick-up
apparatus having an electronic shutter function and, more particularly, to
an apparatus well suited for picking up a planar negative image.
The variable-gamma correction circuit according to the present invention is
applicable not only to a color image (signal) but also to monochrome image
(signal). The image signal processing circuit according to a the invention
is applicable not only to a negative image (signal) but also to a positive
image (signal). However, since the image processing circuit is
particularly effective for processing a negative image (signal), its
application in this specification will be described mainly in connection
with a negative image (signal).
2. Description of the Related Art
Negative-image pick-up is necessary in a system of the type in which an
image that appears on a negative film is sensed and either displayed on a
large-size display screen or projected on a screen in the form of a
negative image or upon being converted to a positive image. This is a new
system which has appeared on the scene as a replacement for the optical
overhead projector and finds use in various meetings, lectures, etc. Since
a video signal obtained by picking up a negative image has characteristics
different from those of a video signal obtained by picking up a positive
image, the video signal cannot be handled in the same manner.
FIG. 11 illustrates an example of tone characteristics (alogarithmic
representation) of a negative film. Specifically, FIG. 11 shows the
relationship between incident light quantity when a negative film is
sensitized, and the development density of the negative film after it has
been developed. Density is lowest at a portion A where the film has not
been sensitized at all (a portion where light has been completely shut
out) and highest at a portion B that has been sensitized completely. The
luminance range of the sensed image is not the range from portion A to
portion B but rather the luminace range of the sensed image is the range
from the darkest portion to the brightest portion of the sensed image, as
indicated by the usable range C in FIG. 11. Accordingly, the darkest
portion of the sensed image must be the black level of the video signal,
and the brightest portion of the sensed image must be the white level of
the video signal. This means that it is necessary to detect the upper and
lower limits of the usable range C.
In a case where the negative image is a color image, the color-tone
characteristics of the three primary colors R, G and B which constitute
this color differ from one another, as illustrated in FIG. 12. Moreover,
the used ranges (indicated by the bold lines) of these color-tone
characteristics also differ from one another. This is a significant
problem. When color-tone characteristics differ depending upon the color,
the half-tones of the reproduced image become colored. When the used
ranges differ, color balance cannot be achieved. This problem arises also
with regard to the complementary colors of yellow, magenta and cyan.
FIG. 13 illustrates a negative-image signal processing circuit according to
the prior art. An image pick-up apparatus 70 such as a camera (a video
camera or a still-video camera, etc.) produces color signals G, R and B
representing the three primary colors. The color signals R and B are
applied to variable-gain amplifier circuits 85, 86, respectively, which
perform a white-balance adjustment by a well-known method. In a case where
a negative image is sensed, an adjustment is carried out by the
white-balance adjustment in such a manner that the peak levels of the
negative-image signal (which level is referred to as a "black peak
level"), namely the black peak levels obtained when a reversal is made
from negative to positive, agree for the three primary-color signals G, R
and B.
The color signals G, R and B obtained as a result of the white-balance
adjustment are applied to gamma correction circuits 91, 93, 95,
respectively, and to inverter circuits 71, 73, 75, respectively, where the
signals are inverted from negative-image signals to positive-image
signals. The resulting positive-image signals are applied to respective
blanking mixer circuits 72, 74, 76 which superimpose a blanking signal BLK
on these signals during their blanking intervals. The resulting signals
are applied to gamma correction circuits 92, 94, 96, respectively. The
same gamma correction curve is set in each of the gamma correction
circuits 91, 93, 95 of the positive-image system. Gamma correction curves
are set in the gamma correction circuits 92, 94, 96 of the negative-image
system in dependence upon the tone characteristics of G, R and B. The
arrangement is such that the tone characteristics after the gamma
correction will coincide with the three primary colors of G, R and B.
Changeover switches 101, 102, 103 are provided for respective ones of the
color signals G, R, B. Each changeover switch is adapted to switch between
the gamma-corrected color signal of the positive system and the
gamma-corrected color signal of the negative system. The output color
signals G, R and B from the respective changeover switches 101, 102 103
are applied to a matrix circuit 83, whereby the signals are converted into
a luminance signal Y and color-difference signals R-Y, B-Y. These signals
Y, R-Y and B-Y are converted into an NTSC-format video signal by an
encoder 84, which delivers the video signal as an output signal.
In this circuit according to the prior art, gamma correction curves
conforming to the tone characteristics of the respective colors are set in
the gamma correction circuits 92, 94, 96 of the negative system, and the
tone characteristics of the respective color signals following gamma
correction are in agreement. As a result, the aforementioned problem of
half-tone coloring does not arise.
However, in the white-balance adjustment, the gains of the variable-gain
amplifier circuits 85 and 86 are merely adjusted in such a manner that
only the black peak levels of the color signals G, R and B coincide.
Consequently, in the processing for the reversal from negative to
positive, the black peak levels in terms of a positive image coincide but
the white peak levels for the positive image do not. The problem that
results is an inappropriate white balance.
In the conventional circuitry described above, the three-types of gamma
correction circuits 92, 94, 96 in which the different gamma correction
curves have been set are required in order to bring the tone
characteristics of the three primary colors G, R and B into line. The
result is a complicated circuit arrangement.
In general, a variable-gamma correction circuit is useful in varying tone
characteristics in line with the subject photographed and personal
preference. In order to realize variable gamma characteristics, however, a
circuit arrangement of considerable complexity is required. Providing a
plurality of such variable-gamma correction circuits would result in a
much more complicated circuit arrangement,
A peak detector circuit is useful and widely employed in many electrical
circuits and electrical devices. For example, in fields wherein video
signals are handled, detecting the peak level of a video signal is
important for the purpose of white-balance adjustment and monitoring
signal levels.
FIG. 14 illustrates an example of a peak detector circuit according to the
prior art. As shown in FIG. 14, an input voltage is clamped by a clamping
circuit 113 in such a manner that the reference level thereof attains a
predetermined level, after which the clamped signal is fed into an
operational amplifier 112. The output of the operational amplifier 112
charges a holding capacitor 110 via a protecting resistor 115 and diode
111. The maximum level of the input voltage is thus held by the capacitor
110. A switching circuit 114 for resetting purposes is connected across
the holding capacitor 110.
In the conventional peak detector circuit of this kind, a diode 111 for
preventing reverse current is connected between the operational amplifier
112 and the holding capacitor 110. As a result, the voltage held by the
capacitor 110 attains a value lower than the maximum voltage of the input
voltage by an amount equivalent to the forward voltage V.sub.F (ordinarily
several hundred millivolts) of diode 111. Thus, an error, which
corresponds to the foward voltage V.sub.F is produced. Moreover, since the
forward voltage VF varies by several hundred millivolts depending upon
temperature, highly accurate peak detection cannot be achieved.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a processing
circuit capable of realizing appropriate white balance of negative-image
color signals.
Another object of the present invention is to provide a variable-gamma
correction circuit capable of being constructed with a comparative amount
of simplicity.
Still another object of the present invention is to provide a
variable-gamma correction circuit whereby the tone characteristics of a
plurality of color signals can be made to conform in appropriate fashion
or can be adjusted as desired.
A further object of the present invention is to provide an image signal
processing circuit whereby the tone characteristics of a plurality of
color signals can be made to conform or can be adjusted as desired and
appropriate white balance can be realized.
A further object of the present invention is to provide a peak detector
circuit capable of realizing highly accurate peak detection.
Yet another object of the present invention is to perform a suitable
exposure adjustment in an image pick-up device which performs gamma
corrections separately by using the aforementioned variable gamma
correction circuits in such a manner that the tone characteristics of
three color signals are eventually brought into conformity, thereby making
it possible to perform the gamma corrections appropriately.
According to the present invention, there is provided a negative-image
signal processing circuit comprising a peak detector circuit for detecting
a maximum level (black peak level) and a minimum level (white peak level)
of each of three types of color signals obtained by picking up a negative
image, and an adjusting circuit for adjusting the amplitudes and the
maximum or minimum levels of the three types of colors signals in such a
manner that the detected maximum levels become equal to one another in the
three types of color signals and the detected minimum levels become equal
to one another in the three types of color signals.
In accordance with this aspect of the present invention, the color signals
are adjusted in such a manner that the maximum levels of the three types
of color signals coincide with one another and the minimum levels of the
three types of color signals coincide with one another. Accordingly, the
black peak levels of the three color signals coincide with one another,
and so do the white peak levels of the three color signals. Since the
white and black peak levels are thus made to conform for the three types
of color signals, a proper white-balance adjustment always can be achieved
even if the color signals are left in the negative state or even if a
negative to positive reversal is made.
By way of example, the adjusting circuit includes variable-gain amplifier
circuits for adjusting the amplitudes of at least two types of the color
signals in such a manner that the differences between detected maximum
levels and minimum levels become equal to one another for the three types
of color signals, and a level adjusting circuit for adjusting the levels
of the three types of color signals so that these attain a level at which
the maximum levels or minimum levels become equal to one another.
In a preferred embodiment, gamma correction circuits are provided for
bringing the tone characteristics of the three types of color signals into
conformity, these gamma correction circuits being equipped with the level
adjusting circuits mentioned above.
In another preferred embodiment, clamping circuits for bringing the levels
of DC components of the three types of color signals into conformity are
provided for respective ones of the three types of color signals and are
arranged in front of the peak detector circuit. As a result, reliable
detection of the maximum and minimum levels becomes possible without
influencing the DC signal components.
The negative-image signal processing circuit according to the invention
further comprises blanking mixer circuits for superimposing signal
components representing the detected maximum level and minimum level on
first- and second-half portions, respectively, of blanking intervals of
the three types of color signals adjusted in such a manner that
differences between the maximum and minimum levels become equal to one
another.
A signal component representing the black peak level and a signal component
representing the white peak level are applied in the blanking interval, in
which a signal component representing an image is not present, at
different positions along the time axis. As a result, the signal component
representing the image is not adversely affected by application of the
aforementioned signal components. Moreover, the black and white peak
levels are preserved within the color signals so that these signals can be
used in subsequent signal processing.
According to the present invention, there is provided a variable-gamma
correction circuit comprising a gamma correction circuit having
input/output characteristics represented by an exponential curve for a
gamma correction up to a fixed input range, and bey a KNEE curve having a
smaller slope in a range beyond the fixed input range, and variable-gain
amplifier circuits connected in front of and in back of the gamma
correction circuit for adjusting the used range of the input/output
characteristic curve in the gamma correction circuit.
By adjusting the gains of the variable-gain amplifier circuits, the used
range of the input/output characteristic curve of the gamma correction
circuit can be changed, thereby making it possible to change the gamma
value (the exponent). Accordingly, any gamma correction curve can be
obtained, thereby making it possible to obtain a desired tone
characteristic. Moreover, the circuit arrangement is made comparatively
simple since it suffices merely to provide the gamma correction circuit
whose input/output characteristics possesses the KNEE curve in the range
beyond the predetermined input range, and the variable-gain amplifier
circuits.
Even though the above-described gamma correction circuit is applicable to
both a color video signal and a monochromatic video signal, the invention
is particularly effective when applied to a color video signal. The
circuit arrangement in such a case will now be described.
Specifically, a variable-gamma correction circuit suited to a color video
signal in accordance with the present invention comprises a gamma
correction circuit to which are inputted three types of color signals
obtained by imaging a subject, and which has an input/output
characteristic represented by an exponential curve for a gamma correction
up to a fixed input range, and by a KNEE curve having a smaller slope in a
range beyond the fixed input range, and variable-gain amplifier circuits
connected in front of and in back of the gamma correction circuit, and
provided for at least two types of the three types of color signals, for
adjusting, for every color signal, the used range of the input/output
characteristic curve in the gamma correction circuit.
In accordance with the invention, merely providing the gamma correction
circuit having the KNEE characteristics and at least two variable-gain
amplifier circuits makes it possible to achieve conformity among the tone
characteristics of the three types of color signals having tone
characteristics that differ from one another. It is possible also to set
these tone characteristics to any desired tone characteristics.
In accordance with the present invention, there is provided a signal
processing circuit comprising a peak detector circuit for detecting a
maximum level and a minimum level of each of three types of color signals
obtained by imaging a subject; first variable-gain amplifier circuits for
adjusting the magnitudes of at least two types of color signals of the
three types of color signals in such a manner that differences between the
detected maximum and minimum levels attain a predetermined ratio for the
three types of color signals; a gamma correction circuit, which is
connected in back of the first variable-gain amplifier circuits and to
which the three types of color signals are inputted, and which has
input/output characteristics represented by an exponential curve for a
gamma correction up to a fixed input range, and by a KNEE curve having a
smaller slope in a range beyond the fixed input range; and second
variable-gain amplifier circuits connected in back of the gamma correction
circuit, and provided for at least two types of the three types of color
signals, for adjusting, for every corresponding color signal, a used range
of the input/output characteristic curve in the gamma correction circuit
in relation to the ratio of the three types of color signals.
In accordance with the image signal processing circuit according to the
present invention, the tone characteristics of a plurality of color
signals can be brought into conformity or adjusted at will. In addition,
suitable white-balance adjustment of each of a plurality of color signals
is possible.
In a preferred embodiment, an adjustment is performed by the second
variable-gain amplifier circuits in such a manner that the differences
between the maximum and minimum levels of the finally outputted three
types of color signals become equal to one another with regard to the
three types of color signals.
Thus, the differences between the maximum levels (the black peak levels in
case of a negative image) and the minimum levels (the white peak levels in
case of a negative image) of the three types of color signals agree with
one another with regard to the three types of color signals. Accordingly,
the white and black peak levels can be made to conform in the three types
of color signals whenever necessary by a clamping processing or the like.
As a result, a suitable white-balance adjustment can always be achieved.
In a preferred embodiment, clamping circuits for bringing the levels of DC
components of the three types of color signals into conformity are
provided for respective ones of the three types of color signals and are
arranged in front of the peak detector circuit. As a result, reliable
detection of the maximum and minimum levels becomes possible without
influencing the DC signal components.
The signal processing circuit further comprises blanking mixer circuits for
superimposing signal components representing the detected maximum level
(black peak signal) and minimum level (white peak level) on first- and
second-half portions, respectively, of blanking intervals of respective
ones of the three types of color signals.
A signal component representing the black peak level and a signal component
representing the white peak level are applied in the blanking interval, in
which a signal component representing an image is not present, at
different positions along the time axis. As a result, the signal component
representing the image is not adversely affected by application of the
aforementioned signal components. Moreover, the black and white peak
levels are preserved within the color signals so that these signals can be
used in subsequent signal processing.
In accordance with the present invention, there is provided an image signal
processing circuit characterized by comprising first variable-gain
amplifier circuits provided for at least two types of color signals for
adjusting white balance of three types of color signals obtained by
imaging a subject; a gamma correction circuit, which is connected in back
of the first variable-gain amplifier circuits and to which the three types
of color signals are inputted, and which has an input/output
characteristics represented by exponential curve for a gamma correction up
to a fixed input range, and by a KNEE curve having a smaller slope in a
range beyond the fixed input range; and second variable-gain amplifier
circuits connected in back of the gamma correction circuit, and provided
for at least two types of the three types of color signals, for adjusting,
for every corresponding color signal, a used range of the input/output
characteristic curve in the gamma correction circuit, and for finely
adjusting white balance of the color signals.
In accordance with the present invention, the tone characteristics of a
plurality of color signals can be brought into conformity or adjusted at
will. In addition, suitable white-balance adjustment of each of a
plurality of color signals is possible.
In the present invention, the curves mentioned above are intended to cover
the meaning of polygonal lines as well.
A peak detector circuit in accordance with the present invention comprises
a holding capacitor for holding a peak voltage; a current source for
charging the holding capacitor; and a comparator for comparing an input
voltage and the voltage held by the holding capacitor, and executing
control in dependence upon whether a peak to be detected is a maximum
value or a minimum value in such a manner that if the held voltage has not
attained the input voltage, the current source is activated to charge the
holding capacitor.
According to the present invention, in a case where the voltage held by the
holding capacitor has not attained the input voltage, this is detected by
the comparator and the holding capacitor is charged by the current source
until the held voltage becomes equal to the input voltage. Since the
charging of the holding capacitor is performed by the current supplied
from the current source controlled by the comparator, a diode for
preventing a reverse current need not be provided between an input
terminal and the holding capacitor, as is required in the prior art. As a
result, an error arising from a voltage in the forward direction no longer
is produced and accurate peak detection can be achieved. In addition,
since a comparator generally operates at a high speed, peak detection can
be performed more rapidly.
In order to arrange it so that both maximum and minimum levels of the input
voltage may be detected, two of the above-described peak detector circuits
are provided, one for detecting the maximum level and one for detecting
the minimum level. Further, an input circuit is provided, which applies
the input voltage directly to the peak detector circuit, for maximum-level
detection, and the peak detector circuit for minimum-level detection is
also provided with a voltage obtained by subtracting the input voltage
from a predetermined reference voltage.
By virtue of such an arrangement, the upper and lower peak levels can be
detected at the same time as the white and black peak levels of a video
signal in the same manner.
An electronic image pick-up apparatus having an electronic shutter function
according to the present invention comprises a solid-state electronic
imaging device having an electronic shutter function; a driver circuit for
controlling charge accumulating time of the solid-state electronic imaging
device in dependence upon a given shutter speed, and reading out an
accumulated signal charge at a predetermined period; a gamma correction
circuit, to which are inputted three types of color signals obtained by
processing a signal read out of the solid-state electronic imaging device,
and which has input/output characteristics represented by a power-function
curve for a gamma correction up to a fixed input range, and by a KNEE
curve having a smaller slope in a range which exceeds the fixed input
range (the curve includes polygonal lines); variable-gain amplifier
circuits connected in front of and in back of the gamma correction
circuit, and provided for at least two types of the three types of color
signals, for adjusting, for every corresponding color signal, the used
range of the input/output characteristic curve in the gamma correction
circuit; a peak detector circuit for detecting a maximum level and a
minimum level of each of the three types of color signals on an input side
of the gamma correction. circuit; shutter-speed control means for
deciding, with regard to a predetermined color signal, a shutter speed for
obtaining an exposure at which a difference between the detected maximum
and minimum levels attains a predetermined value (which may be a value
within a predetermined range), and for controlling the driver circuit so
as to operate at this shutter speed; first gain control means for
controlling the gains of the variable-gain amplifier circuits, which are
connected in front of the gamma correction circuit, in such a manner that
with regard to the other two types of color signals of the three types of
color signals, differences between detected maximum and minimum levels
attain respective predetermined values (which may be values which
correspond to the difference between the levels of the aforesaid
predetermined color signal); and second gain control means for controlling
thee gains of the variable-gain amplifier circuits, which are connected in
back of the gamma correction circuit, in such a manner that the
differences between the maximum and minimum levels become equal to one
another in the three types of color signals after they have been
gamma-corrected.
In accordance with the present invention, a proper exposure is obtained
using an electronic shutter. By setting the exposure properly, it is
possible to realize, in correct fashion, a function through which
conformity is obtained among the tone characteristics of the three types
of color signals using the gamma correction circuit having the
aforementioned KNEE characteristics. As a result, a sensed image having a
correct color balance can be obtained even if the image is a negative
image such as found on a negative film. Since it is so arranged that the
proper exposure is obtained using an electronic shutter, it is not
necessary to provide an expensive auto iris mechanism. The image pick-up
apparatus of the invention is capable of operation sufficiently without
being provided with an iris at all. In a case where a planar subject is to
be photographed and the distance to the subject has been decided, proper
image pick-up is possible even if the iris is open (which includes the
case in which there is no iris), i.e., even if the depth of field is
small. It goes without saying that the present invention is applicable
also to an electronic image pick-up apparatus equipped with an iris. The
reason is that when a negative film or the like is imaged, the operator is
instructed to fully open the iris and allow the exposure to be adjusted by
the electronic shutter.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed
description given hereinbelow and the accompanying drawings which are
given by way of illustration only, and thus are not limitative of the
present invention, and wherein:
FIG. 1 is a block diagram illustrating an embodiment of a negative-image
signal processing circuit according to an embodiment of the present
invention;
FIG. 2 is waveform diagrams showing the input and output signals for each
block in the circuit of FIG. 1;
FIG. 3 is a circuit diagram showing the principle of a peak detector
circuit according to an embodiment of the present invention;
FIG. 4 is a circuit diagram showing a circuit for detecting white peak
levels and this and black peak level, circuit illustrates an embodiment of
a peak detector circuit according to the present invention;
FIG. 5 is a block diagram illustrating an embodiment of a variable-gamma
correction circuit;
FIG. 6 is graphs showing input/output characteristics for describing the
function of the variable-gamma correction circuit;
FIG. 7 is a block diagram showing an embodiment of an image signal
processing circuit;
FIG. 8 is waveform diagrams showing the input and output signals of each
block in the circuit shown in FIG. 7;
FIG. 9 illustrates the manner in which the tone characteristics of color
signals G, R and B are made to coincide by using a variable-gamma
correction circuit;
FIG. 10 is a block diagram illustrating the electrical construction of an
embodiment of an electronic image pick-up apparatus having an electronic
shutter function according to the present invention;
FIG. 11 is a graph showing the tone characteristic of an image which
appears on a negative film;
FIG. 12 is a graph showing how tone characteristics differ depending upon
color;
FIG. 13 is a block diagram showing a negative-image signal processing
circuit according to the prior art; and
FIG. 14 is a circuit diagram showing a peak detector circuit according to
the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates an embodiment of a circuit for processing a
negative-image signal according to the present invention.
An image pick-up apparatus 10 such as a video camera or a still-video
camera produces the three primary-color signals G, R and B representing
the image of a subject. In a case where the image is at rest, such as the
image on a negative film, the color signals representing the image on the
negative film are repeatedly outputted at a fixed period (e.g., 1/60 sec).
The color signals G, R, B outputted by the image pick-up apparatus 10 are
as shown in FIG. 2a. As illustrated, the color signals G, R, B reflect the
differences in the tone characteristics of the colors, as described
earlier. Also, levels of the DC signal components generally differ from
one another.
The color signal G from among these color signals is applied directly to a
clamping circuit 21, and the other color signals R and B are applied to
respective clamping circuits 22, 23 upon first being amplified at an
appropriate gain (less than one in some cases), described below, by
variable-gain amplifier circuits 15 and 16, respectively. The same
clamping level is set in the clamping circuits 21, 22 and 23. The DC
signal components of the color signals G, R and B are made to conform by
the clamping circuits 21, 22 and 23.
The color signals G, R and B outputted by the respective clamping circuits
21, 22 and 23 are applied to respective blanking circuits 31, 32 and 33
and also to a changeover switch (multiplexer) 24. The changeover switch 24
is changed over at a fixed time interval by a microcomputer 11 so that the
color signals G, R and B are successively applied to a peak detector
circuit 12.
As will be described in detail later, the peak detector circuit 12 detects
the maximum level and minimum level of the input signal applied thereto.
In a case where the color signals G, R and B represent a negative image
such as that on a negative film, the maximum level corresponds to the
black peak level and the minimum level corresponds to the white peak
level, and therefore the terms "black peak level" and "white peak level"
will be used hereinafter. When the color signal G is being applied to the
peak detector circuit 12 by the changeover switch 24, the peak detector
detects the black and white peak levels of the color signal G. The time
during which the changeover switch 24 is selecting the color signal G
preferably is an interval during which an appropriate region of one frame
is scanned. This is equivalent to setting a window on the frame and
performing white and black peak detection within the window. The
description for the color signal G applies to the other color signals R
and B as well. The microcomputer 11 applies a reset pulse to the peak
detector circuit 12. This reset pulse is outputted every vertical blanking
interval, by way of example.
The detected black and white peak levels of each of the color signals G, R
and B are applied to the microcomputer 11. The latter uses the data
representing the inputted black and white peak levels to perform a
white-balance adjustment by controlling the gains of the variable-gain
amplifier circuits 15, 16 so as to establish equality among the difference
between the black peak level and white peak level of the color signal G;
the difference between the black peak level and white peak level of the
color signal R; and the difference between the black peak level and white
peak level of the color signal B (these differences are indicated by D in,
FIG. 2b). The color signals G, R and B after thus being white-balance
adjusted and having their DC components clamped are as shown in FIG. 2b.
The black peak levels of the three color signals G, R and B still do not
coincide, and neither do the white peak levels of these signals. In
addition, the disparity among the tone characteristics of the color
signals G, R and B has not yet been adjusted.
The white-balance adjusted color signals G, R and B are respectively
applied to blanking mixer circuits 31, 32 and 33, as described earlier.
Two types of blanking-timing signals BLK1, BLK2 are inputted to each of
the blanking mixer circuits 31, 32 and 33. The blanking-timing signal BLK1
is a signal that attains the H level in the first half of the blanking
interval, and the blanking-timing signal BLK2 is a signal that attains the
H level in the second half of the blanking interval. The microcomputer 11
supplies the blanking mixer circuit 31 with signals representing the
detected black peak level and white peak level of the color signal G; the
blanking mixer circuit 32 with signals representing the detected black
peak level and white peak level of the color signal R; and the blanking
mixer circuit 33 with signals representing the detected black peak level
and white peak level of the color signal B. The blanking mixer circuits
31, 32 and 33 superimpose pulse signals representing the corresponding
white peak levels on the color signals G, R and B during the time that the
blanking-timing signal BLK1 is at the H level, and superimpose pulse
signals representing the corresponding black peak levels on the color
signals G, R and B during the time that the blanking-timing signal BLK2 is
at the H level.
The color signals G, R and B obtained by applying the pulse signals
representing the white and black peak levels in the blanking intervals are
illustrated, along with the blanking-timing signals BLK1, BLK2, in FIG.
2c. Since the white and black peak levels are preserved in the blanking
intervals of the color signals G, R and B, these white and black peak
levels can be utilized in later circuitry. For example, after the color
signals are inverted from negative to positive, the black peak levels may
be used as the black reference level of a video signal. The white peak
levels are used for the sake of clamp processing in a gamma correction, as
will be described below.
The outputs or the blanking mixer circuits 31, 32 and 33 are applied to
respective negative-image gamma correction circuits 41, 43 and 45 and also
to respective positive-image gamma correction circuits 42, 44 and 46.
Gamma correction curves conforming to the tone characteristics of the
color signals G, R and B are set in the negative-image gamma correction
circuits 41, 43 and 45, respectively. In the negative-image gamma
correction circuits 41, 43 and 45, the respective input signals (the color
signals G, R and B) are clamped in such a manner that their white peak
levels will coincide by attaining a prescribed level, and the input
signals are respectively gamma-corrected in accordance with the gamma
correction curves in such a manner that the tone characteristics after the
gamma correction will coincide for the three primary colors G, R and B.
The output color signals G, R and B of the negative-image gamma correction
circuits 41, 43 and 45 are as shown in FIG. 2d. As will be appreciated
from FIG. 2d, the color signals G, R and B are such that their black peak
levels agree with one another, their white peak levels agree with one
another and their tone characteristics are in conformity with one another.
The output signals from the negative-image gamma correction circuits 41, 43
and 45 are inverted from negative to positive by inverter circuits 57, 58
and 59, respectively. The signals after inversion are as shown in FIG. 2e.
The same gamma correction curve is set in each of the positive-image gamma
correction circuits 42, 44 and 46. Of course, different gamma correction
curves may be set in these positive-image gamma correction circuits 42, 44
and 46 if desired.
The outputs of the gamma correction circuits 41 and 42 are inputted to a
changeover switch 51, the outputs of the gamma correction circuits 43 and
44 to a changeover switch 52, and the outputs of the gamma correction
circuits 45 and 46 to a changeover switch 53.
The changeover switches 51, 52 and 53 are provided for respective ones of
the color signals G, R and B and each changeover switch is for switching
between the gamma-corrected color signal of the positive system and the
gamma-corrected color signal of the negative system. It is preferable, of
course, that the changeover switches 51, 52 and 53 be operatively
associated with one another. The output color signals G, R and B of these
changeover switches 51, 52 and 53 are applied to a matrix circuit 17,
which converts them into a luminance signal Y and color-difference signals
R-Y and B-Y. These signals Y, R-Y and B-Y are converted into an
NTSC-format video signal by an encoder 18, which delivers the video signal
as an output signal.
A blanking-timing signal BLK3 is applied to the encoder 18. As shown in
FIG. 2f, the blanking-timing signal BLK3 is a pulsed signal which
represents the blanking intervals(during which interval the signal is at
the L level) and has a pulse width slightly larger than the combined pulse
widths of the timing signals BLK1 and BLK2. Blanking is performed in the
L-level interval of the timing signal BLK3 in such a manner that the
signals Y, R-Y and B-Y coincide with the respective signal levels (i. e.,
the black levels) in the H-level interval of the timing signal BLK2. As a
result, a signal component representing the blanking interval of the NTSC
format is applied to the signals Y, R-Y and B-Y. The NTSC signal finally
obtained is as illustrated in FIG. 2g.
An embodiment of the peak detector circuit according to the present
invention will now be described in detail.
FIG. 3 is for describing the principle of the peak detector circuit of the
present embodiment.
A holding capacitor 1 is charged by a current supplied by a current source
3. The voltage held by the holding capacitor 1 is outputted, and applied
to the negative input terminal of a comparator 2, via a buffer circuit 4.
An input voltage is clamped by a clamping circuit 6 in such a manner that
the reference voltage thereof attains a predetermined level, after which
the voltage is applied to the positive input terminal of the comparator 2.
When the input voltage is greater tha | | |
