JP2961719B2 - Image signal processing circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing circuit for color balance adjustment and gamma correction.

The image signal processing circuit according to the present invention is applicable not only to a negative image (signal) but also to a positive image (signal), but is particularly useful for processing a negative image (signal). It mainly refers to negative images (signals).

In this specification, a negative is a negative.
And the negative image represents a negative image or a negative image. The term “positive” is an abbreviation for “positive”, and the term “positive image” refers to a positive image or a positive image.

2. Description of the Related Art Negative image capture is performed, for example, in a system in which an image represented on a negative film is photographed, and the image is displayed on a large display screen or projected on a large display screen as it is or as a negative image. Required. This system is a new system that has appeared in place of an optical so-called overhead projector, and is used in briefings, research presentations, and the like. A video signal obtained by capturing a negative image cannot be handled in the same manner because it has different characteristics from a video signal obtained by capturing a positive image.

FIG. 5 shows an example of the gradation characteristics (logarithmic expression) of a negative film, which shows the relationship between the amount of incident light when the negative film is exposed and the developed density of the negative film after development caused by the exposure. ing. The density is lowest in the part A that is not exposed at all (completely light-shielded part), and is highest in the part B that is completely exposed. The luminance range of the photographed image is not the range from the above-mentioned parts A to B, but is the range from the darkest part to the brightest part of the photographed image as shown as a use range C. Therefore, the darkest part of the captured image must be the black level of the video signal, and the brightest part must be the white level of the video signal. Therefore, it is necessary to detect the upper limit and the lower limit of the use range C.

When the negative image is a color image, the color gradation characteristics of the three primary colors R, G, and B constituting the color are different from each other as shown in FIG. ) Are different from each other.
If the color gradation characteristics are different depending on the color, the halftone of the reproduced image will be colored, and if the usage range is different, the color balance will not be balanced. The above problem also occurs for the complementary colors of yellow, magenta, and cyan.

FIG. 7 shows a conventional negative image signal processing circuit.
Of the three primary color signals R, G, and B output from the imaging device 60 such as a camera (video camera, still video camera), the color signals R and B are supplied to variable gain amplifier circuits 85 and 86, and are known. White balance adjustment is performed by a method. When a negative image is captured, the peak level of the negative image signal (this is called a black peak level) by the white balance adjustment, that is, the black peak level when the image is inverted to positive, is converted into the three primary color signals G, R and B is adjusted to match.

The color signals G, R, and B after the white balance adjustment are supplied to gamma correction circuits 61, 63, and 65, respectively, on the one hand, and inverted by inversion circuits 71, 73, and 75, respectively (the negative image signal is a positive image signal). The blanking signal BLK is superimposed during the blanking period in the blanking / mixing circuits 72, 74 and 76, and further supplied to the gamma correction circuits 62, 64 and 66, respectively. The same gamma correction curve is set in the positive gamma correction circuits 61, 63 and 65. A gamma correction curve is set in the negative gamma correction circuits 62, 64, and 66 in accordance with the G, R, and B gradation characteristics, and the gradation characteristics after the gamma correction are G,
The three primary colors R and B match each other.

The changeover switches 51, 52, and 53 are provided for each of the color signals G, R, and B, and switch between a positive color signal after gamma correction and a negative color signal after gamma correction. The output color signals G, R of these switches 51, 52 and 53
And B are supplied to a matrix circuit 83 and converted into a luminance signal Y and color difference signals RY and BY. Furthermore, these signals Y, RY and BY are
It is converted to a video signal in TSC format and output.

In such a conventional circuit, a gamma correction curve corresponding to the gradation characteristics of each color is set in the negative gamma correction circuits 62, 64, and 66, and the gradation characteristics of each color signal after gamma correction become uniform. The above-described problem of halftone coloring does not occur.

However, in the white balance adjustment, the color signals G, R
And the gains of the variable gain amplifier circuits 85 and 86 are merely adjusted so that only the black peak levels of B and B match, so that the black peak level on the positive matches when this is inverted to positive. But the white peak on the positive
There is a problem that the levels do not match and the white balance is not appropriate.

In addition, in the above-described conventional circuit, three types of gamma correction circuits 62, 64 and 6 in which different gamma correction curves are set in order to equalize the gradation characteristics of the three primary color signals G, R and B, respectively.
6 is required, and the circuit configuration becomes complicated.

Generally, the variable gamma correction circuit is useful for changing the gradation characteristics according to the subject and the taste, but the circuit configuration becomes considerably complicated to realize the variable gamma characteristics. If a plurality of such variable gamma correction circuits are provided, the circuit configuration becomes more complicated.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image signal processing circuit that can perform appropriate color balance adjustment and match the tone characteristics of color signals.

An image signal processing circuit according to the present invention is a first color balance adjustment circuit provided for at least two types of color signals for adjusting the color balance of three types of color signals obtained by imaging a subject. The first color balance adjustment circuit is connected to the subsequent stage, and the three color signals subjected to the color balance adjustment in the first color balance adjustment circuit are input, and a power function for gamma correction is used in the first half of the used range. A gamma correction circuit having input / output characteristics represented by a KNEE curve in the latter half of the curve, and at least one of three types of color signals connected to the subsequent stage of the gamma correction circuit and gamma corrected by the gamma correction circuit A second color balance adjustment circuit provided for two types is provided.

By adjusting the first color balance adjustment circuit, the use range of the input / output characteristic curve of the gamma correction circuit can be changed, thereby making it possible to change the gamma value (that is, the power). Therefore, an arbitrary gamma correction curve can be obtained, and a desired gradation characteristic can be obtained. The gradation characteristics of three types of color signals having mutually different gradation characteristics can be made uniform. Further, appropriate color balance adjustment of a plurality of color signals can be performed. The first color balance adjustment circuit and the second color balance adjustment circuit can be manually adjusted.

A peak detection circuit that detects the maximum level and the minimum level of each of the three types of color signals obtained by imaging the subject, and a difference between the maximum level and the minimum level detected by the peak detection circuit is determined for the three types of color signals. A first gain adjustment means for adjusting the gain of the first color balance adjustment circuit so as to achieve the ratio of May be provided with a second gain adjusting means for adjusting the gain of the second color balance adjusting circuit so that the values of .gamma.

In this way, the difference between the maximum level (black peak level in the case of a negative) and the minimum level (white peak level in the case of a negative) of the three types of color signals matches each other in the three types of color signals. . Therefore, the white and black peak levels can be made uniform for the three types of color signals as required by a clamp process or the like, so that appropriate color balance adjustment is always achieved. In addition, automation of color balance adjustment can be achieved.

Blanking for superimposing signal components representing the detected maximum level (black peak level) and minimum level (white peak level) in the first half and the second half of the blanking period of each of the three types of color signals. It is preferable to further include a mix circuit. Since the signal component representing the black peak level and the signal component representing the white peak level are added at different positions on the time axis during the blanking period when no signal component representing the image exists, the signal component representing the image is added. Is not adversely affected by this, and since the black and white peak levels are stored in the color signal, this can be used in the subsequent signal processing circuit.

Preferably, a clamp circuit for adjusting the level of the DC component of each of the three types of color signals to a constant level is provided before the peak detection circuit for each of the three types of color signals. As a result, it is possible to reliably detect the maximum level and the minimum level without being affected by the DC signal component.

In the present invention, each of the curves is a concept including a broken line.

FIG. 1 shows an embodiment of a variable gamma correction circuit.
In this figure, input and output signal waveforms before and after each block are schematically shown.

The variable gamma correction circuit includes a first variable gain amplifier circuit 42 at the preceding stage, a gamma correction circuit 41 having KNEE characteristics, and a variable gain amplifier circuit 43 at the subsequent stage.

An example of the input / output characteristics of the gamma correction circuit 41 is shown in FIG. 2a. In this figure, the solid line represents the input / output characteristics of the circuit 41. For simplicity, the variable gain amplifier circuit 42 and
Assume that each of the 43 gains is 1. Input is 0 to 100%
The curve shown by the solid line in the range of (point a) is a power function curve. When the input is 100%, the output is also 100%. Generally CR
Since the T display device has a light emission characteristic of 2.2 power, the exponent (gamma) is often selected to be 0.45, but it is needless to say that the value is not limited to this value. In particular, in the case of imaging a negative film, another appropriate value is selected. 100% of input
In the range exceeding, the dashed line shows the above power function curve, and the solid line shows the KNEE
It is a curve. The slope of the KNEE curve is set smaller than that of the power function curve. This can be realized, for example, by sequentially turning on the diodes connected to different biases and connected in parallel as the input voltage increases, and gradually decreasing the ratio of the output increase to the input increase.

If the gain of the variable gain amplifying circuit 42 at the preceding stage is made larger than 1, the input of the gamma correction circuit 41 becomes apparently large, so that the change range of the input signal exceeds 100% and enters the area of the KNEE curve. By making the gain of the variable gain amplifier circuit 43 at the subsequent stage smaller than 1, the output of this amplifier circuit 43 becomes 100
% Range. This corresponds to a relative reduction in the horizontal axis and the vertical axis in the graph of FIG. 2a. The input / output characteristics when the gain of the variable gain amplifying circuit 42 in the preceding stage is set to be larger than 1 and the gain of the variable gain amplifying circuit 43 in the subsequent stage is set to be smaller than 1 are shown by solid lines in FIG. 2b. this is,
This is equivalent to apparently changing the gamma correction curve from a broken line (input / output characteristics shown in FIG. 2a) to a solid line. Thus, gamma can be changed by adjusting the gains of the variable gain amplifier circuits 42 and 43.

FIG. 3 shows an image signal processing circuit according to the present invention, particularly a portion relating to white balance adjustment and gamma correction.

An imaging device 10 such as a still video camera or a video camera outputs three primary color signals G, R, and B representing a captured subject image. If the subject image is stationary like a negative film, color signals representing the image of the negative film are repeatedly output at a constant period (for example, 1/60 second). The color signals G, R and B output from the imaging device 10 are shown in FIG. 4a. As described above, the color signals G, R, and B reflect the above-described difference in gradation characteristics for each color, and the level of the DC signal component is also generally different.

Of these color signals, the color signal G is directly supplied to the clamp circuit.
The other color signals R and B are amplified by variable gain amplifier circuits 15 and 16 with an appropriate gain described later, and then applied to clamp circuits 22 and 23, respectively. The same clamp level is set in the clamp circuits 21, 22, and 23, and the DC signals of the color signals G, R, and B are uniformly arranged by the clamp circuits 21, 22, and 23.

The color signals G, R output from the clamp circuits 21, 22, and 23
And B on the other hand are blanking mix circuits 31, 32
And 33, respectively, and on the other hand, to a changeover switch (multiplexer) 24. The changeover switch 24 is switched by the microcomputer 11 at regular intervals, and the color signals G, R and B are sequentially supplied to the peak detection circuit 12.

The peak detection circuit 12 detects the maximum level and the minimum level of the input signal. When the color signals G, R and B represent a negative image such as a negative film, the maximum level corresponds to the black peak level and the minimum level corresponds to the white peak level. Use When the color signal G is input to the peak detection circuit 12 by the changeover switch 24, the black and white peak levels of the color signal G are detected. The time during which the changeover switch 24 selects the color signal G may be the time during which an appropriate area within one screen is scanned. This is equivalent to setting a window on the screen and detecting black and white peaks in the window. The same applies to the other color signals R and B.

In this way, the black and white peak levels of each of the detected color signals G, R and B are given to the microcomputer 11. Microcomputer 11
Is the difference between the black peak level and the white peak level of the color signal G and the black peak level and the white peak level of the color signal R using the data representing the input black and white peak levels. The gains of the variable gain amplifying circuits 15 and 16 are controlled so that the difference between the difference and the black peak level and the white peak level of the color signal B becomes a constant ratio, and provisional white balance adjustment is performed. FIG. 4b shows the color signals G, R and B after the provisional white balance adjustment and the DC component clamping are performed in this way. Three color signals G, R and B
Have the same DC component level. Color signals G, R and B
The difference between the black peak or white peak and the gradation characteristic has not been adjusted yet. In FIG. 4b, the white peaks in the color signals G, R and B are drawn so as to coincide with each other, but this is for the sake of simplicity.

The color signals G, R and B temporarily adjusted in white balance are applied to the blanking / mixing circuits 31, 32 and 33, respectively, as described above. These blanking mix circuits 31, 32 and 33 have two types of blanking circuits.
Timing signals BLK1 and BLK2 are input. The blanking timing signal BLK1 is a signal that goes high in the first half of the blanking period, and the blanking timing signal BLK2 is a signal that goes high in the second half of the blanking period. The blanking mix circuits 31, 32 and 33 also have respective black and white peak levels adjusted to a fixed ratio of the color signals G, R and B.
Signals representing the levels are provided from the microcomputer 11 respectively. Blanking / mixing circuit 3
1, 32 and 33 superimpose a pulse signal representing the white peak level on the color signals G, R and B while the blanking timing signal BLK1 is at the H level, and the blanking timing signal BLK2 has the H level. During this time, a pulse signal representing the corresponding black peak level is superimposed on the color signals G, R and B.

Thus, the white peak level and the black peak level
FIG. 4c shows the color signals G, R and B to which the pulse signals representing the levels are applied during the blanking period, together with the blanking timing signals BLK1 and BLK2. Color signal G, R
Since the white peak level and the black peak level are stored during the blanking periods of B and B, these white and black peak levels are stored.
The black peak level can be used in subsequent circuits. For example, after the black peak level is inverted to positive,
Used as a black reference level of the video signal. The white peak level is used in a clamp process for adjusting these peak levels to a constant level.

Output signals of the blanking / mixing circuits 31, 32 and 33 are applied to a gamma correction circuit 40 having KNEE characteristics. The gamma correction circuit 40 is the gamma correction circuit 41 described above (FIG. 1).
(With the same input / output characteristics)
R and B respectively. The color signals G, R, and B input to the gamma correction circuit 40 are gamma-corrected by an input / output characteristic curve partially having a KNEE curve in accordance with the magnitude of the input (the above ratio) and output.
The height (amplitude) of the pulse signal representing the black peak level applied during the blanking period is also gamma corrected. The output signal of the gamma correction circuit 40 is shown in FIG. 4d.

The output signals G, R and B of the gamma correction circuit 40 are positively inverted on the one hand by inverting circuits 57, 58 and 59, respectively (the inverted signal is shown in FIG. 4e), and on the other hand the changeover switches 51, It is provided to positive terminals P of 52 and 53, respectively.

Of the color signals inverted by the inverting circuits 57, 58 and 59, the signals R and B are supplied to variable gain amplifying circuits 55 and 56, respectively. As shown in FIG. 4f, the gains of the variable gain amplifier circuits 55 and 56 are such that the gradation characteristics of the output signals R and B of the amplifier circuits 55 and 56 and the output signal G of the inverting circuit 57 match, and The difference between the level and the white peak level is adjusted so that the color signals G, R, and B match (complete white balance adjustment). In other words, the ratio of the difference between the black peak level and the white peak level in the provisional white balance adjustment described above is determined by the gamma correction circuit 40 having the KNEE characteristic and the variable gain amplifier circuits 55 and 56 by the three primary color signals G. , R, and B are appropriately set so that their gradation characteristics match and the difference between the black peak level and the white peak level matches. Color signal G, R
And B, black peak level or white peak level
The levels are adjusted by a clamp process or the like as necessary in the encoder 14 or the like described later. In this way,
Signals G, R, and B whose tone characteristics and the difference between the black peak level and the white peak level (that is, the white balance is completely adjusted) are applied to the negative terminals N of the changeover switches 51, 52, and 53, respectively. Given.

The changeover switches 51, 52, and 53 are provided for each of the color signals G, R, and B, and switch between a positive color signal after gamma correction and a negative color signal after gamma correction. Of course, these changeover switches 51, 52 and 53
Are preferably linked to each other. Output color signals G, R and B of these switches 51, 52 and 53 are applied to a matrix circuit 13 and converted into a luminance signal Y and color difference signals RY and BY. Further, these signals Y, RY
And BY are converted into an NTSC format video signal by the encoder 14 and output.

The encoder 14 has a blanking timing signal BLK3
Is given. As shown in FIG. 4g, the timing signal BLK3 is a signal representing a blanking period (the L level during this period), and is a pulse-like signal having a width slightly wider than the combined width of the pulse widths of the timing signals BLK1 and BLK2. It is. During the L level period of the timing signal BLK3, the signals Y, RY, and BY are blanked so as to coincide with the respective levels (that is, black levels) during the H level period of the timing signal BLK2. , NTSC format, a signal component representing a blanking period is added to the signals Y, RY and BY. The final NTSC output is shown in FIG. 4h.

The three primary color signals G,
In R and B, the difference between the white peak level and the black peak level may be made to coincide with each other to perform perfect white balance adjustment. In this case, variable gain amplifier circuits 55 and 56 perform only gamma correction. The white balance already adjusted may be slightly distorted by this gamma correction, but in many cases there is no problem.

Further, the gains of the variable gain amplifier circuits 55 and 56 may be manually adjusted. In this case, manual fine adjustment of at least one of white balance and gamma correction will be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a variable gamma correction circuit. FIGS. 2a and 2b are graphs showing input / output characteristics for explaining the function of the variable gamma correction circuit. FIG. 3 is a block diagram showing an embodiment of the image signal processing circuit. 4a to 4h are waveform diagrams showing input / output signals of each block of the circuit shown in FIG. FIG. 5 is a graph showing the gradation characteristics of an image represented on a negative film, and FIG. 6 is a graph showing how the gradation characteristics differ depending on the color. FIG. 7 is a block diagram showing a conventional example. 10 ... Imaging device, 11 ... Microcomputer, 12 ... Peak detection circuit, 15,16,42,43,55,56 ... Variable gain amplifier circuit, 21,22,23 ... Clamp circuit, 31, 32,33: Blanking / mixing circuit, 40,41: Gamma correction circuit.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 9/69 H04N 9/73

Claims (4)

(57) [Claims] A first color balance adjustment circuit provided for at least two types of color signals for adjusting the color balance of three types of color signals obtained by imaging a subject; The three types of color signals, which are connected at the subsequent stage of the adjustment circuit and have been subjected to color balance adjustment in the first color balance adjustment circuit, are input, respectively. In the part, a gamma correction circuit having input / output characteristics represented by a KNEE curve, and a gamma correction circuit connected to a subsequent stage of the gamma correction circuit for at least two of the three color signals gamma corrected by the gamma correction circuit An image signal processing circuit, comprising: a second color balance adjustment circuit provided. 2. A peak detection circuit for detecting a maximum level and a minimum level of each of three types of color signals obtained by imaging a subject, wherein a difference between the maximum level and the minimum level detected by the peak detection circuit is 3 First gain adjusting means for adjusting the gain of the first color balance adjusting circuit so that the ratio of the color signals becomes a predetermined ratio; and 2. The image signal processing circuit according to claim 1, further comprising: a second gain adjustment unit that adjusts a gain of the second color balance adjustment circuit so that a difference between the level and the minimum level is equal. 3. A blanking / mixing circuit for superimposing signal components representing the detected maximum level and minimum level on the first half and the second half of the blanking period of each of the three types of color signals. Item 3. The image signal processing circuit according to Item 2. 4. The apparatus according to claim 2, wherein a clamp circuit for adjusting the level of the DC component of each of the three types of color signals to a constant level is provided at a stage preceding the peak detection circuit for each of the three types of color signals. Image signal processing circuit.