JPH0233265A - White level compressor for video signal - Google Patents

Abstract

PURPOSE:To easily obtain a proper white level compression characteristic by transferring a ROM table having the white level compression characteristic in response to an input video signal level to a RAM table. CONSTITUTION:A maximum value and a mean value of an input brightness signal are detected respectively by a maximum value detection circuit 8 and a mean value detection circuit 9 and the output is given to a control signal generating circuit 11 via a mixing circuit 10. The ROM table 12 having a compression characteristic corresponding to all mixture outputs is prepared and the ROM table 12 is transferred to the RAM table 13. The input brightness signal is subject to white level compression by using the RAM table 13.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a video signal white compression device that is installed in a video camera or the like and compresses the white level (high level) of a video signal.

2. Description of the Related Art In general, video cameras and the like compress the white level (high level) within a specified signal amplitude in response to a signal input at a specified level or higher, thereby widening the range in which gradation can be reproduced.

FIG. 5 is a typical characteristic diagram showing the operation of such a conventional white compression device. In Figure 6, the horizontal axis.

The vertical axis indicates the amplitude level of the input video signal and the output video signal, respectively, and each is expressed as a percentage when the amplitude level of a standard signal corresponding to the circuit design of the video camera is set to 100%. In the same figure, from the origin 0 is 2 + K1
The straight lines passing through the line indicate the characteristics when white compression is not performed, and the straight lines B, C, and D indicate the three types of white compression characteristics of the conventional example.

In video cameras, white clipping is usually performed at about 110% of the standard signal in order to match the dynamic range of a video tape recorder or the like. Therefore, when no white compression is performed at all, as in the case of linear video, only the gradations up to 110 pixels of the input video signal can be reproduced, and signals of 110% or more become white at the same gradation, resulting in what is called a white-washed state. It ends up.

Therefore, in order to add some gradation to a signal of 100 pixels or more, white compression is performed at a constant compression rate from a signal near 100% of the input video signal to prevent white collapse. The straight line B has a compression characteristic that connects 1 and Pl. That is 1o
The compression start point (knee point) is 0%, and input video signals of up to 200 inches are white-compressed to 110 inches. With this characteristic, even if it is possible to reproduce gradations up to 200 inches, it is an insufficient characteristic as whites may be crushed in the case of a high contrast subject exceeding 200 inches. Furthermore, the straight line C has a compression characteristic that increases the compression ratio of the characteristic of the straight line B, and connects K1 and P2. In other words, gradation up to 400% can be reproduced, expanding the gradation reproduction range. However, the contrast in the compressed portion is lower than the characteristic of straight line B, resulting in an unnatural screen. On the other hand, the compression characteristic of a straight line has a compression start point of 100
It is a characteristic that connects K2 and P2.

Therefore, the gradation reproduction range is wide, and sufficient contrast can be obtained in the compressed area. However, this hinders accurate gradation reproduction below 100% and deteriorates image quality.

Conventional white compression, as shown in the white compression characteristics of straight lines B, C, and D, has advantages and disadvantages, and it is difficult to widen the gradation reproduction range without deteriorating image quality. Ta. This is due to the fact that the white compression characteristic is a fixed compression characteristic regardless of the level of the input video signal.

As a method for solving the above-mentioned technical problems, there is a method called auto-knee, in which the compression start point (knee point) and compression rate are made variable depending on the level of the input video signal. The conventional example will be explained below with reference to FIGS. 6 to 8.

FIG. 6 is a block diagram showing the configuration of a conventional white compression device. In FIG. 6, 1 is an input terminal, 2 is a white peak detection circuit that detects the white peak level of a video signal applied from a preamplifier (not shown) through input terminal 1, 3 is a variable resistor that provides a reference level, and 4 is a variable resistor that provides a reference level. a differential amplifier circuit that compares the white peak level obtained from the white peak detection circuit 2 and the reference level of the variable resistor 3 and provides a difference output; 6 is a limiting circuit that limits the output level of the differential amplifier circuit 4; Reference numeral 7 indicates a white compression circuit which compresses the white peak of the video signal obtained through the input terminal 1 at a constant degree of compression in accordance with the output signal of the limiting circuit 6. Reference numeral 7 indicates an output terminal.

The white peak detection circuit 2 is, for example, a peak detection circuit including a diode, etc., detects the white peak level of the video signal from the input terminal 1, and inputs it to the inverting input of the differential amplifier circuit 4. Further, a reference level set by the variable resistor 3 is applied to the non-inverting input. This reference level is set equal to the white peak level 110, so that the output shown in FIG. 7a is obtained from the differential amplifier circuit 4.

That is, as the white peak level becomes larger than 110%, the output becomes negative, and conversely, as the white peak level becomes smaller than 110%, the output becomes positive.

The output signal of the differential amplifier circuit 4 controls the white compression start point of the white compression circuit 6, and a limiting circuit 6 is provided to prevent the white compression start point from becoming too low and deteriorating the image quality. Due to this limiting circuit 5, the relationship between the output signal of the differential amplifier circuit 4 and the white compression start point of the white compression circuit 6 becomes as shown in FIG. 7, for example. Tsumashi differential amplifier circuit 4
The limit value of the white compression starting point that can be lowered by υ due to negative output is 80ch, and for positive output,
It is said to be 110chi.

By controlling the white compression start point of the white compression circuit 6 as described above, output video signals as shown by straight lines E, F, and G in FIG. 8 are obtained from the output terminal 7. In FIG. 8, the horizontal axis indicates the amplitude level of the input video signal, and the vertical axis indicates the amplitude of the output video signal, which are expressed as a percentage when the amplitude level of the standard signal is taken as 100%.

A straight line
%, and after all, it is not subjected to white compression and lies on this straight line E.

Furthermore, the straight line F connecting 3 and P3 is for the case where the input video amplitude level is 200ch. At this time, the output signal of the differential amplifier circuit 4 is negative, so the white compression starting point falls to the 100th factor of 5, and white compression up to 200% is performed. Furthermore, when the input video amplitude level rises and is around 40 degrees, the output signal of the differential amplifier circuit 4 further falls.
The white compression start point falls to 80 inches at Kan4, and the white is compressed as shown by straight line G connecting X4 and P4.

Also, the white peak level of the input video signal is between 110 and 20
When the coefficient is 0 or lower, the compression start point is at a level of 100 or more, and when the white peak level exceeds 200, the compression start point is at a level of less than 100% and 80 or more.

In this way, when the white peak level is 200% or less, the white compression starting point is at a relatively high level of 100% or more, so there is almost no deterioration in image quality, and moreover, the gradation up to the peak level of 200% are all reproduced. Also,
When the white peak level exceeds 200%, the white compression starting point will be at a level of less than 100% ao%, so there will be some deterioration in image quality, but the gradation reproduction range will be widened to 400%, so the overall Overall, the image quality is improved.

In the above, the white peak, that is, the peak of a signal similar to the luminance signal, was used to control the white compression start point, but the white NAM signals (R, (maximum value of G, B signals), each R, G, B
There is also a method that improves image quality by controlling the starting point of signal compression and changing the compression ratio (for example, Mitsuo Mino et al., "High Latitude Camera", August 1986, 2).
7゜Television Society Technical Report).

Problems to be Solved by the Invention However, in the conventional white compression device including the above-mentioned configuration, the response of detecting the white peak level and performing white compression based on that level depends on the resistance provided in the white peak detection circuit, etc. It was determined by a time constant determined by the capacitance of the capacitor. Therefore, once the −time variable constant is determined, it is difficult to change it. Also, depending on how the time constant is selected, if a subject with a high video amplitude level flickers across the screen, the white compression will respond immediately and the screen will feel restless and strange, or conversely, the response may be slow. teeth,
There is a problem that the effect of white compression cannot be obtained and the image quality deteriorates.

If the JG and H signals were to be compressed separately, the compression characteristics would become uneven due to variations in the temperature characteristics of the analog circuits, which could impair image quality.

In view of this, an object of the present invention is to provide a white compression device that can easily change and set the response speed of white compression and can obtain optimal compression characteristics.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a RAM to which a video signal is input and which non-linearly converts the high level part of the video signal.
a ROM table with various conversion characteristics;
A maximum value detection circuit and an average value detection circuit detect the maximum value and average value of the video signal for each field or frame, and the maximum value and average value output from the maximum value detection circuit and the average value detection circuit are By using a mixing circuit that adds the ratio and outputs a mixed value, the value of the current field of the mixed value output from the mixing circuit, and the mixed value of each field up to several fields before the current field, arbitrary values in the ROM table can be calculated. The configuration includes a control signal forming circuit that selects a characteristic table and obtains a control signal for transferring it to the RAM table.

Operation The present invention uses the above-described configuration to arithmetic process the maximum value and average value of several fields of the video signal, and uses those values to transfer the optimal characteristic table in the ROM table to the RAM table to perform white compression. Therefore, the compression response speed can be easily changed and selected, and therefore smooth response and optimal white compression can be performed for various subjects.

Embodiment FIG. 1 is a block diagram showing the structure of a video signal white compression apparatus according to an embodiment of the present invention.

In FIG. 1, 1 is an input terminal into which a video signal such as a luminance signal is input, 8 is a maximum value detection circuit for detecting the maximum value for each field of the input luminance signal, and 9 is a maximum value detection circuit for detecting the maximum value for each field of the input luminance signal. Average value detection circuit that detects average value, 1
o is a mixing circuit that adds the maximum value and average value output from the maximum value detection circuit 8 and the average value detection circuit 9 in an appropriate ratio and outputs a mixed value; 12 is an RO with various conversion characteristics;
M table 13 is a RAM table for non-linearly converting the high level part of the input luminance signal; 11 is a RAM table for calculating the mixed values of several fields output from the mixing circuit 1o, and storing them in the ROM
A control signal forming circuit 7 forms a control signal for selecting an arbitrary characteristic table of the table 12 and transferring it to the RAM table 13, and 7 is an output terminal, and each circuit is composed of a digital circuit.

The operation of the white compression device of this embodiment configured as described above will be explained with reference to FIGS. 2 to 4.

FIG. 2 is a diagram showing the compression characteristics of the white compression device of this embodiment. In Figure 2, the horizontal and vertical axes represent the input video signal, respectively.
The amplitude level of the output video signal is shown, and as in the conventional example, each is expressed as a percentage when the amplitude level of the standard signal of the video camera is 1oo'4. MO+ M+ in the figure
e...Ml...MnMn+1 indicates the output mixing value of the mixing circuit 1o in FIG. 1 for the input video signal, and when the input video signal is 110ch, MO, 3oon
When it is 600, the value is Ml, and when it is 600, it is Mn. At the input signal level at which this mixed value is input, the output video signal becomes 110chi. This mixed value is obtained by adding the maximum value and average value of each field obtained from the maximum value detection circuit 8 and the average value detection circuit 9 in FIG. 1 at an appropriate ratio. ＋ ＋ K2 ＋・・・・・・K6
...Kn is '1 + '2 ...
In the white compression device of this embodiment, a straight line "j+T2 +...T6,...
. . . It has compression characteristics similar to Tn. Also, when the input video signal is 110 inches or less, the straight line To connecting the origin 0 and Mo is set, and the compression start point is not set at Kn, that is, 80% or less, but compression also starts when the mixed value (Mn++) exceeds Mn. The point is Kn, and the straight line is Tn +1.

As described above, in this embodiment, since the compression start point is controlled by the sum of the maximum value and the average value in an appropriate ratio thereof, the same effect as the conventional example can be obtained. In this embodiment, such compression characteristics are achieved as shown in FIG.

In other words, the RO of the compression characteristic corresponding to all mixed value outputs is
Prepare M table 12 and write those characteristic tables as
By transferring the signal to the RAM table 13, white compression of the video signal input to the RAM table 13 is performed. By appropriately selecting the characteristics of this ROM table 13, various characteristics can be easily obtained. Here, the control signal forming circuit 11 controls the transfer rate.

The operation will be explained below using FIGS. 4a to 4f.

FIGS. 4a and 4d show examples of mixed values output from the mixing circuit 1o, in which mixed values (values corresponding to input amplitude levels (%)) in each field are shown. Also in the same figure,
c, e, and f indicate output examples of the control signal forming circuit 11 corresponding to a and d in the figure, respectively.

Figure 4 & indicates that the output of the mixed value is 0 after field F3.
This shows the case where the value corresponds to the input video amplitude level of 600%. In this case, if the white compression responds immediately with the output of this mixed value, the characteristics of To will immediately change to the characteristics of Tn shown in FIG. 2, and an unnatural feeling will be felt on the screen. Therefore, the control signal forming circuit 11
The output of the mixed value is converted into the control signal value shown in c in the figure, and the ROM table 12 is converted into the RAM table 13 using this control signal value as shown in FIG.
By transmitting the image to the image, the white compression is made to respond in a certain number of steps. In the case shown in the figure, the control signal is obtained using the following equation.

On=(Mn+Mn-1+Mn-z)/s ・-・
...(1) However, Cn: Control signal value of the current field Mn: Mixed value of the current field Mn-+: Mixed value M of the field before the current field
n-2: Mixed value of two fields before the current field. Also, in the case of C in the same figure, On= (Mn+On-+ + 0n-z)/3
--(2) However, Cn-1: Control signal value one field before the current field Cn-2: Calculated using the formula for the control signal value two fields before the current field.

Using the control signal obtained from the above equation, the white compression characteristic changes from the To characteristic shown in FIG. 2 to the Tn characteristic in two steps, and also changes from the To characteristic shown in FIG. For example, Tn-8+ Tn-6r...Tn-2+"
It changes to n-1 and approaches the characteristics of Tn.

Next, the case shown in FIG. 4d shows a case where the output of the mixed value changes greatly every two fields, and this corresponds to, for example, a case where a high-brightness subject appears flickeringly on the screen.

The control signal values obtained from equations (1) and (2) for this mixed value are as shown in θ and f in the figure.

As can be seen from the figure, the change in the control signal value is suppressed to a certain extent in response to a large change in the mixture value, and therefore the white compression characteristics do not change drastically and do not give any sense of discomfort.

Although the equation for forming the control signal is calculated for three fields, it goes without saying that an appropriate number of fields can be easily obtained by selecting the white compression characteristics. In addition, by appropriately selecting the mixing ratio between the maximum value and the average value in the mixing circuit, if there is a very small high-brightness area in an image with a relatively low average value, the high-brightness area will be whitened. It goes without saying that compression can be performed without deteriorating image quality. Also maximum value detection, average value detection, and mixing.

Addition, division, etc. are strengths of digital circuits, and it goes without saying that each circuit of this embodiment can be easily realized.

Furthermore, in this embodiment, the maximum value detection circuit 8° and the average value detection circuit 9. Mixing circuit 10. Control signal forming circuit 1
It goes without saying that the same effect can be obtained even if 1 is configured with a microcomputer in combination with a digital circuit.

As described in detail, according to the present invention, appropriate white compression characteristics can be obtained by transferring a ROM table prepared in advance and having white compression characteristics according to the input video signal level to a RAM table. It is easy to obtain, the characteristics can be easily changed, and the white compression response can be made to respond smoothly by using a control signal calculated based on an arbitrary ratio mixture value of the maximum value and average value for several fields. White compression without any discomfort can be obtained, and its practical effects are great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a video signal white compression device according to an embodiment of the present invention, FIG. 2 is a compression characteristic diagram of the white compression device of FIG. 1, and FIG. 3 is a control signal forming circuit,
Relationship diagram of ROM table and RAM table, Figure 4a
-r3 is an output characteristic diagram of the mixing circuit and control signal forming circuit, FIG. 6 is a typical characteristic diagram of a conventional white compression device, FIG. 6 is a block diagram of a conventional white compression device, and FIG. 7 a, b
is a relationship diagram between the output of the differential amplifier circuit in Figure 6 and the limiting circuit,
FIG. 8 is a compression characteristic diagram of the white compression device of FIG. 6. 1...Input terminal, 7...Output terminal, 8
...Maximum value detection circuit, 9...Average value detection circuit, 1o...Mixing circuit, 11...
Control signal forming circuit, 12...ROM table, 13...RAM table. Name of agent: Patent attorney Shigetaka Awano and one other person

Claims (1)

[Claims] A RAM table into which a video signal is input and which non-linearly converts the high level part of the video signal, and a RAM table having various conversion characteristics.
OM table, maximum value detection circuit and average value detection circuit that detect the maximum value and average value of the video signal for each field or frame, and the maximum value and average output from the maximum value detection circuit and the average value detection circuit. A mixing circuit that adds values at an appropriate ratio and outputs a mixed value, and a calculation of the current field value of the mixed value output from the mixing circuit and the mixed value of each field several fields before the current field. . A control signal forming circuit that selects a table of arbitrary characteristics from the ROM table and obtains a control signal for transferring it to the RAM table.