JPH04229788A - Video signal processing circuit - Google Patents

Abstract

PURPOSE:To attain sharper gradation expression by applying gradation correction only to a luminance signal of a video signal so as to improve visual contrast of a picture with comparatively simple circuit CONSTITUTION:An input color signal C1 inputted to an input terminal 52 is outputted from output terminals 54, 55 with a gain in response to a control voltage outputted from a gradation correction circuit 57 to a variable gain amplifier 64. Suppose that a Y signal (luminance signal) is inputted to an input terminal 1, and when an average Y signal is continuously inputted, an APL (average picture level) detected by an APL level detector 8 is nearly 50%. However, when the level of the Y signal is deviated, gradation is corrected by the circuit 57 in response to the detected output APL of the APL level detector 8. In the gradation correction in this case, a white level is expanded when the detection output APL of the APL level detector 8 is high, that is, the picture is very bright, and a black level is expanded when the detection output APL of the APL level detector 8 is low, that is, the picture is very dark.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention uses LCD as a display means.
The present invention relates to a circuit for processing video signals of a monitor using a panel, and particularly relates to a technique for improving the contrast of an LCD.

0002

2. Description of the Related Art The contrast of light in the natural world (natural light) is extremely wide, and the various video equipment that reproduces the contrast generally does not have a dynamic range sufficient to cope with natural light. In particular, in projection monitors that use an LCD panel (liquid crystal display panel) as a light bubble, the dynamic range of gradation expression is extremely limited due to the on/off characteristics of the panel itself or the peak brightness of the light source. He is handicapped. In order to compensate for such handicaps, attempts have been made to improve the contrast by electrically performing non-linear processing such as gradation correction according to the content of the screen.

That is, an active matrix type device using thin film transistors (TFTs) as switching elements.
LCD panels are capable of displaying intermediate gradations and have high image quality, so they are widely used as small liquid crystal TV displays and as light valves for projection TVs. TF
T Active matrix type LCD panels, for example, are normally white driven, that is, when the difference between the counter electrode signal level and a video signal such as a luminance signal is the minimum, the transmittance is maximized, and the brightness and darkness are changed by changing the transmittance. changing the contrast of

However, the transmittance of the LCD panel is not linear with respect to the input video signal, and
It is also different from the γ characteristic of FIG. 12 is a graph showing an example of transmission characteristics in normally white driving, with the vertical axis representing the transmittance and the horizontal axis representing the difference (V) between the video signal voltage and the counter electrode voltage.

[0005] Here, the transmittance is 100% when the LCD panel has the maximum transmittance, and 0% when the transmittance is the minimum. As is clear from FIG. 12, the difference in transmittance is 3 to 5 V.
It decreases rapidly in the front and rear regions, and gradually decreases in other regions. Therefore, correction processing such as extending the black level of the video signal has been conventionally performed in order to obtain linearity between the transmittance and the video signal. FIG. 13 is a graph showing an example of conventional black expansion, with the vertical axis representing the output video signal and the horizontal axis representing the input video signal. As is clear from this graph, by making the slope of the black level larger than in other parts, linearity of the black level is obtained, and gradation of the black level is obtained.

[0006]

[Problem to be solved by the invention] However, LCD
The maximum contrast of the panel was far below that of a CRT, and there was a problem in that it was not possible to express sufficient gradation. this is
This is because the maximum/minimum transmittance ratio of the LCD panel is not sufficient compared to the actual contrast between brightness and darkness, and because LCD requires an external light source, the amount of light is limited by the amount of light from that light source.

[0007] Furthermore, gradation correction must be performed for each of the R, G, and B component signals, which increases the circuit scale and complicates adjustment work depending on the circuit. However, if gradation correction is performed only on the luminance signal (Y signal), although the same effect can be obtained as when gradation correction is performed on each of the R, G, and B component signals, the circuit size is also reduced. It's small,
Therefore, the adjustment work is also simplified. However, when performing gradation correction on only the luminance signal, the larger the amount of correction, the more the correlation with the color signal (C signal) collapses, and for example, the color may be pale despite the contrast. There is a high possibility that the image will be unbalanced.

The present invention has been made in view of the above circumstances, and by performing gradation correction on only the luminance signal and compensating for color gain at the same time, it is possible to achieve a relatively simple circuit configuration. The purpose of the present invention is to provide a video signal processing circuit capable of performing gradation correction to obtain clear images.

Furthermore, by expanding the black level and white level of the video signal according to the average brightness, the linearity of the transmittance of the LCD panel can be corrected, the dynamic range can be used effectively, and the visual contrast ratio can be improved. The purpose is to provide a video signal processing circuit that can express clearer gradation.

Furthermore, when the average brightness of the video signal is low, only the black level is determined, and when the average brightness is high, only the white level is determined.
When the average brightness is in between, both levels are expanded individually, and when only the black or white level is expanded, the color gain is increased compared to when both levels are expanded, thereby increasing the dynamic range. The purpose of the present invention is to provide a video signal processing circuit that can be used to improve visual contrast ratio and express clearer gradation.

[0011]

[Means for Solving the Problems] A video signal processing circuit according to the present invention includes a circuit that performs gradation correction only on the luminance signal in a video signal processing circuit that processes a video signal including at least a luminance signal. It is characterized by

Further, the video signal processing circuit according to the present invention is used in a drive circuit of a liquid crystal display panel, and is used in a video signal processing circuit that processes a video signal including at least a luminance signal. circuit, a white expansion circuit for expanding the white level of a video signal, average brightness detection means for detecting average brightness from a brightness signal, and a black expansion circuit and a white expansion circuit according to the average brightness detected by the average brightness detection means. and means for controlling expansion of the black level and white level of the circuit.

Furthermore, the video signal processing circuit according to the present invention includes:
In a video signal processing circuit used in a drive circuit of a liquid crystal display panel and processing a video signal including at least a luminance signal, an average luminance detection means for detecting average luminance from the luminance signal, and an average luminance detection means for detecting a low detection result by the average luminance detection means are provided. In case 1, only the black level of the video signal is expanded, and in the second case, where the detection result by the average brightness detection means is high, only the white level of the video signal is expanded, and the detection result by the average brightness detection means is intermediate. In the third case, the gradation correction means corrects to expand both the black level and the white level of the video signal, and in the first and second cases, compared to the third case, the color gain The invention is characterized in that it includes a gain control means for controlling the gain to a large extent.

[0014]

[Operation] In the present invention, when a video signal including a luminance signal is input, gradation correction is performed only on the luminance signal,
Visual contrast is improved.

Further, in the present invention, when a video signal including a luminance signal is input, the average luminance of the luminance signal is determined by the average luminance calculation means, and the contrast of the white level or black level is corrected according to the determined average luminance. Ru. This allows effective use of the dynamic range of the liquid crystal display panel and improves visual contrast.

Furthermore, in the present invention, when a video signal including a luminance signal is input, the average luminance of the luminance signal is determined by the average luminance calculation means, and according to the determined average luminance, for example, a bright video signal with a high average luminance is determined. In the case of a dark video signal with low average brightness, only the white level is expanded and the contrast of the black level is corrected, and then only the black level is expanded and the contrast of the black level is corrected. When the average brightness is medium, both the black level and white level are expanded to correct the contrast of both levels, and the color gain is controlled to change depending on each case to prevent distortion due to the result of gradation correction. Correlation between the luminance signal and the color signal, which would otherwise occur, is compensated for. This allows effective use of the dynamic range of the liquid crystal display panel and improves visual contrast.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to drawings showing embodiments thereof.

FIG. 1 is a circuit diagram showing the configuration of a video signal processing circuit according to the present invention. In FIG. 1, 1 is an input terminal for a luminance signal Y, and the luminance signal (input luminance signal) YI input to this input terminal 1 is a white expansion circuit 2 and a black expansion circuit 3.
and APL level detector 8 separately.

The white expansion circuit 2 and the black expansion circuit 3 expand the white level and black level of the input luminance signal YI, and correct the linearity of the transmittance of the LCD panel at these levels. Further, the APL level detector 8 detects the average luminance level of the input luminance signal YI and outputs a detection output APL corresponding to the average luminance level. The output of the white expansion circuit 2 is applied to a buffer 7 via an analog switch 5 and a resistor 4 for synthesis, and similarly the output of the black expansion circuit 3 is applied to a buffer 7 via an analog switch 6 and a resistor 4. The output from the buffer 7 is output from the output terminal 13 as a corrected luminance signal YO. The buffer 7 is for impedance conversion and reduces the impedance seen from the output terminal 13.

Detection output APL of APL level detector 8
is applied to the − input terminal of comparator 9 and the + input terminal of comparator 10. A reference voltage V1 is applied to the +input terminal of the comparator 9, and a reference voltage V2 (V2 < V1) is applied to the -input terminal of the comparator 10.

Comparators 9 and 10 are of open collector type, and voltage +B is applied to their output terminals via pull-up resistors 14 and 14, and the voltage level when outputting H level is set to +BV.

FIG. 2 is a diagram showing the output levels of the comparators 9 and 10, FIG. 2(a) shows the relationship between the average luminance level and the detection output APL, and FIGS. 2(b) and (c) show the output levels of the comparators 9 and 10, respectively. 9 and 10 are shown.

The comparator 9 outputs an H level when the detection output APL is lower than the reference voltage V1 (APL<V1), and the comparator 10 outputs an H level when the detection output APL is higher than the reference voltage V2 (APL>V2). Output. The output of the comparator 9 is given to the analog switch 6, and the analog switch 6 is connected to the comparator 9.
Closed when the output is H level. Also, comparator 10
The output of is given to the analog switch 5, which is similarly closed when the output of the comparator 10 is at H level.

Next, the contents of signal processing of the video signal processing circuit of the present invention configured as described above will be explained.

FIG. 3 is a graph showing the relationship between the input brightness signal YI and the corrected brightness signal YO, with the vertical axis representing the corrected brightness signal YO and the horizontal axis representing the input brightness signal YI. Also, in Fig. 3(a), APL >V1,
In other words, when the output of the comparator 9 is at L level and the output of comparator 10 is at H level, FIG. 3(b) shows the V
2 <APL <V1, that is, the comparator 9 and the comparator 10 are both at H level, and FIG. It shows.

As shown in FIG. 3(a), APL>V1
In this case, that is, when the brightness is relatively high, in other words, when the image is very bright, only the comparator 10 becomes the H level, only the analog switch 5 is closed, and only white expansion is performed. Also, as shown in Figure 3(b), V2 < APL
<V1, that is, when the brightness is intermediate, in other words, when the brightness of the image is normal, the comparators 9 and 10
both become H level, analog switches 5 and 6 are both closed, and white expansion and black expansion are performed. Furthermore, Figure 3(c
), when APL < V2, that is, when the brightness is relatively low, in other words, when the image is very dark, only the comparator 9 goes to the H level, only the analog switch 6 closes, and only black extension is performed. It will be done.

By performing white expansion and black expansion according to the average brightness level in this way, even when a high brightness or low brightness video signal with little change in the transmittance of the LCD panel is input, it is possible to Even when a luminance video signal is input, the white level and/or black level are expanded, and visual contrast can be improved.

Next, a second embodiment of the present invention will be described. FIG. 4 is a circuit diagram showing the configuration of a video signal processing circuit according to a second embodiment of the present invention.

Unlike the first embodiment described above, the second embodiment does not use an analog switch and directly operates the comparators 9 and 9.
10 controls the expansion operations of the white expansion circuit 2 and the black expansion circuit 3 separately. This is to prevent the screen from flickering when the analog switch is turned on and off. Hereinafter, the explanation will focus on the differences from the first embodiment.

The white expansion circuit 2 relatively expands the white by compressing the portion other than the area to be expanded, and its output is given to the variable gain amplifier 18 via the combining resistor 4. Similarly, the black expansion circuit 3 relatively expands the black, and its output is given to the variable gain amplifier 18 via the combining resistor 4.

When white and black are expanded by the operation described above, the entire dynamic range, that is, the peak to
Since the peak value (P, P value) becomes small, it is necessary to amplify it with the variable gain amplifier 18. In addition, the detection output APL of the APL level detector 8 is determined by the comparators 9 and 10.
is applied to each − input terminal of . Comparator 9
, 10 are supplied with reference voltages V1 and V2, respectively. The output of comparator 9 is black expansion circuit 3
and one end of the AND gate 17. The black expansion circuit 3 performs an expansion operation when this output is at H level.

The output of the comparator 10 is applied to the white expansion circuit 2 and also to the other end of the AND gate 17 via the inverter 16. The white expansion circuit 2 performs an expansion operation when this output is at L level. The output of the AND gate 17 is given to the variable gain amplifier 18, and the output of the ``
The gain can be switched to two levels, large and small, depending on ``H'' and ``L''.

FIG. 5 is a diagram showing changes in the detection output APL, the outputs of the comparators 9 and 10, and the output of the AND gate 17 when the average luminance level changes.

Comparator 9 is APL level detector 8
The comparator 10 outputs an H level when the detection output APL is lower than the reference voltage V1, and the comparator 10 outputs an H level when the detection output APL is lower than the reference voltage V2. Further, the AND gate 17 outputs an H level when the output of the comparator 9 is an H level and the output of the comparator 10 is an L level.

Next, the operation of the video signal processing circuit of the second embodiment will be explained.

FIG. 6 is a graph showing the relationship between the input luminance signal YI and the corrected luminance signals YO', YO, with the corrected luminance signals YO', YO on the vertical axis and the input luminance signal YI on the horizontal axis. ing. Also, the corrected brightness signal YO' before amplification is shown on the left, and the corrected brightness signal Y after amplified is shown on the right.
O is shown respectively. Also, Figure 6(a) shows the APL
>V1, the same (b) is V2 ≦APL ≦
(c) shows the case of APL <V2.

As shown in FIG. 6(a), APL>V1
In this case, that is, when the brightness is high and the screen is bright, the outputs of both comparators 9 and 10 are both at the L level, and only the white expansion circuit 2 operates, and only white expansion is performed. In this case, the output of the AND gate 17 applied to the variable gain amplifier 18 is at L level, and its gain becomes low level and is amplified as shown on the right side of FIG. 6(a).

As shown in FIG. 6(b), V2≦APL
When ≦V1, the output of comparator 9 becomes H level, the output of comparator 10 becomes L level, white expansion circuit 2 and black expansion circuit 3 operate together, and white and black expansion are performed. In this case, the dynamic range is further reduced than when only white extension or black extension is performed, and the P and P values become smaller. A high level of gain is required to compensate for this. In this case, the AND gate 17 becomes H level, so the variable gain amplifier 18 amplifies the signal with a high level gain.

As shown in FIG. 6(c), APL <V2
In this case, that is, when the brightness is low and the screen is dark, the outputs of both comparators 9 and 10 are both at H level, and only the black expansion circuit 3 operates, and only black expansion is performed. In this case, the output of the AND gate 17 given to the variable gain amplifier 18 is at L level, and its gain is at a low level.
The signal is amplified as shown on the right side of FIG. 6(c).

As described above, when the APL is medium, the dynamic range of the output characteristics is made constant by increasing the gain of the variable gain amplifier 18. However, in FIG. 6, the DC levels are aligned for comparison, but in reality, the DC levels of the amplification results are slightly shifted upwards, so it is necessary to align the DC levels by clamp processing at a subsequent stage of the circuit of the present invention.

Next, a third embodiment of the present invention will be described.

In the two embodiments described above, the luminance signal Y was subjected to black and white expansion. In this case, if the γ characteristics of the LCD panels differ individually, especially if they differ for each of R, G, and B, the stretching operation cannot be adjusted according to those characteristics. In this third embodiment, each color signal of R, G, and B is subjected to black and white expansion, so that correction can be performed according to the characteristics of the LCD panel.

FIG. 7 is a circuit diagram showing the configuration of the video signal processing circuit of the third embodiment, in which each color signal input terminal 1R, 1G,
The color signals RI, GI, BI input to 1B are sent to the R expansion circuit 15R, the G expansion circuit 15G, and the B expansion circuit 15B.
Each is given separately.

The R extension circuit 15R includes a white extension circuit 2R, a black extension circuit 3R, resistors 4R, 4R, and an analog switch 5.
The configuration is the same as that of the first embodiment shown in FIG. 1. Correction color signal RO of expansion result
is output from the output terminal 13R.

The G and B expansion circuits 15G and 15B have a similar configuration, and their corrected color signals GO and BO are outputted from output terminals 13G and 13B.

In addition, the input luminance signal YI is given to the APL level detector 8 as in the first embodiment, and the comparators 9 and 10 output H and L based on the detection output APL corresponding to the average luminance level detected there. . The outputs of comparators 9 and 10 are sent to each expansion circuit 15R, 15G, 15
B and simultaneously controls the expansion operation for each color.

In this case, by adjusting the amount of expansion of the white expansion circuit 2 and the black expansion circuit 3 for each color, it is possible to perform an expansion operation according to the γ characteristics of the LCD panel.

Next, a fourth embodiment of the video signal processing circuit of the present invention will be described. FIG. 8 is a block diagram showing the circuit configuration of the fourth embodiment.

In FIG. 8, reference numeral 1 is an input terminal for the luminance signal Y, and the input luminance signal YI is A
The signal is applied to the PL level detector 8, the gradation correction circuit 57, and the synchronization separation circuit.

The APL level detector 8 receives the input luminance signal YI.
A detection output APL that detects the average image level of is given to the gradation correction circuit 57.

The gradation correction circuit 57 operates as already explained in each of the above embodiments, that is, it expands the black level, the white level, or both levels according to the detected output APL of the input luminance signal YI by the APL level detector 8. This circuit performs the following to correct the gradation of the Y signal. The corrected luminance signal YO whose gradation has been corrected by the gradation correction circuit 57 is outputted from the output terminal 13. Also, this gradation correction circuit 57
As will be described later, a signal for controlling color gain, that is, a control voltage for the variable gain amplifier 64 is generated and applied to the variable gain amplifier 64.

Reference numeral 52 denotes an input terminal for a color signal C, and the input color signal CI is applied to a gate 60 and a variable gain amplifier 64.

The synchronous separation circuit 58 synchronously separates the input luminance signal YI and supplies the resulting synchronous signal to the burst gate 59 . The burst gate 59 controls the opening and closing of the gate 60 according to the synchronization signal. Therefore, the input color signal CI is sent from the gate 60 to an auto phase controller (APC) in synchronization with the synchronization signal.
r) given in 62.

61 is a low pass filter (LPF: Low
63 is a voltage controlled oscillator (VCO).
It processes the input color signal CI given to the APC 62 to generate a color demodulation subcarrier, and sends it directly to the phase detector 66 and to the phase detector 67 via the π/2 phase shifter 65. I'm giving it to you. Note that 69 is an output terminal of this color demodulation subcarrier.

The variable gain amplifier 64 inputs the input color signal CI input to the input terminal 52 to the gradation correction circuit 57.
The signal is amplified with a gain corresponding to the control voltage given from the control voltage and output to the phase detectors 66 and 67. Double phase detector 66
, 67 output a color difference signal I and a color difference signal Q from output terminals 54 and 55, respectively, in accordance with the phase of the color demodulation subcarrier given from the VCO 63.

In the above configuration, the input color signal CI input to the input terminal 52 is output from the output terminals 54 and 55 with a gain corresponding to the control voltage output from the gradation correction circuit 57 to the variable gain amplifier 64. will be done.

For example, suppose that a Y signal (luminance signal) having a waveform as shown in FIG. 9(a) is input to input terminal 1. When such an average Y signal is input continuously, the APL level detector 8 detects the
PL (average image level) is approximately 50%. However, if the level of the Y signal is uneven, the gradation is corrected by the gradation correction circuit 57 according to the detection output APL of the APL level detector 8. The gradation correction at this time is performed by the APL level detector 8 as already explained in each of the above embodiments.
When the detection output APL of the APL level detector 8 is high, that is, the image is very bright, the white level is expanded as shown in FIG. 9(b), and the detection output APL of the APL level detector 8 is low.
That is, if the image is very dark, the black level is expanded as shown in FIG. 9(c).

However, since gradation correction only by expanding the Y signal as described above causes a relative decrease in color gain, in the fourth embodiment, the color signal is corrected by using the circuit configuration as shown in FIG. Performs gain change control.

The C signal (color signal) is input from the input terminal 52 and sent to the gate 60, APC 61, L
Color demodulation subcarriers are obtained via PF 61 and VCO 63. This color demodulated subcarrier is directly applied to a phase detector 66 and also applied to a phase detector 67 via a π/2 phase shifter 65. Double phase detector 66
, 67 are supplied with the C signal via the variable gain amplifier 64, and the color difference signals I and Q are respectively supplied to the output terminal 5.
4, output to 55.

Here, the color gain can be achieved by controlling the gain of the variable gain amplifier 64. Therefore, by changing the color gain in response to the tone correction of the input luminance signal YI by the tone correction circuit 57, it becomes possible to compensate for the relative decrease in color gain that accompanies the tone correction.

FIG. 10 is a graph showing the color gain/APL characteristics in the fourth embodiment, which is carried out for the above-mentioned purpose. Specifically, in Figure 10 where APL is high (
W) If the area is shown in (when the white level is expanded) and the APL is low (B) in Figure 10
In the case of the region shown in (when the black level is expanded), the control voltage which is the output from the gradation correction circuit 57 to the variable gain amplifier 64 is made relatively high,
When the APL is in the other region shown by (M) in FIG. 10 (when both levels are expanded), the control voltage that is the output from the gradation correction circuit 57 to the variable gain amplifier 64 is Keep it relatively low.

FIG. 11 is a circuit diagram showing a specific circuit configuration of the tone correction circuit 57 of the fourth embodiment as described above.

In FIG. 11, reference numeral 1 indicates a clamp.
(DC regeneration) Y signal input terminal, negative polarity (synchronous positive).

Reference numeral 72 is an input terminal to which the detection output APL of the APL level detector 8 is input, and the detection output APL is input as a voltage (Vc). Note that the lower the detection output APL of the APL level detector 8, the higher the voltage input to the input terminal 72, and the higher the detection output APL, the lower the voltage input.

The Y signal input from input terminal 1 is sent to two transistors Tr. 1, Tr. 2, the transistors Tr. 15, Tr. 16, Tr. 17 is applied. Furthermore, the transistor Tr. 4, Tr. By appropriately setting the base bias of transistor Tr. The black expanded Y signal is sent from the emitter of transistor Tr. White-expanded Y signals are output from each of the 17 emitters.

The input luminance signal YI is finally converted into a potential waveform generated by the current flowing through the load resistor R1 to the transistor Tr. 6 and output to the output terminal 13. Therefore, this load current is transferred to the transistor Tr. 7 to Tr. 1
In the switching circuit configured by transistor Tr. 15, Tr. 16, Tr. 17, the input/output characteristics from the input terminal 1 to the output terminal 13 change corresponding to each state from black level expansion to white level expansion.

This transistor Tr. 7 to Tr. 14
The operation of the switching circuit configured by the transistor Tr. 8, Tr. The base potential (V2) of transistor Tr. 12, Tr. 13 base potential (
V1) and the detection output APL of the APL level detector 8
It is determined by the relationship with the voltage (Vc).

In FIG. 11, reference symbols R2 and R3 are resistors for detecting the operating state of the tone correction circuit 57, and the detection result is output to the color gain control terminal 68. The output to this color gain control terminal 68 is as described above.
Since the signal is also supplied to the variable gain amplifier 64, the color gain is controlled to change.

The specific operation of the gradation correction circuit 57 shown in FIG. 11 will be described below with respect to each state according to the three levels of detection output APL of the APL level detector 8.

(i) When Vc>V1>V2, Vc is V1
and V2, that is, when the detection output APL of the APL level detector 8 is low. In this case, the transistor Tr. 7 to Tr. 1
4, current flows through the transistor Tr. 7,
Tr. 10, Tr. 11, Tr. 14, and the remaining transistors are in the off state and no current flows. Therefore, in this case, the transistor Tr. 15 emitter current of transistor Tr. 10, Tr. 11 to the load resistor R1, and as a result, the corrected luminance signal YO, which has undergone black level expansion processing, is output to the output terminal 13.

At the same time, the transistor Tr. 14 also drops due to the current flowing through the load resistor R2. Only the DC component is taken out from this current by an LPF composed of a resistor R4 and a capacitor C1, and the DC component is extracted from the transistor Tr. 18, Tr. The signal is inverted and amplified by the operational amplifier 19 and output to the color gain control terminal 68, that is, the variable gain amplifier 64.

That is, along with the black level expansion operation, the output voltage to the color gain control terminal 68, in other words, the control voltage applied to the variable gain amplifier 64 increases. This state corresponds to the operation in the region (B) of FIG.

(ii) When V1>Vc>V2, Vc is V
1 and V2, that is, the detection output APL of the APL level detector 8 is medium. In this case, the transistor Tr. 7 to T
r. 14, the one through which current flows is the transistor Tr.
7, Tr. 10, Tr. 12, Tr. 13, and the remaining transistors are off and no current flows. Therefore, in this case, the transistor Tr. 16
The emitter current of transistor Tr. 10, Tr. 1
3 to the load resistor R1, the corrected luminance signal YO is outputted to the output terminal 13 without being expanded.

At the same time, the transistor Tr. T for 9
r. 14, no voltage drop occurs due to the load resistors R2 and R3, and therefore the inverted output of the color gain control terminal 68, that is, the variable gain amplifier 6
The output voltage to 4 will be relatively low. This state is shown in Figure 10.
This corresponds to the operation in the region (M) of (M).

(iii) When V1>V2>Vc, Vc
is lower than V1 and V2, that is, the detection output APL of the APL level detector 8 is high. in this case,
A transistor Tr constituting a switching circuit. 7 to Tr. 14, the one through which current flows is the transistor Tr.
．． 8, Tr. 9, Tr. 12, Tr. 13, and the remaining transistors are in the off state and no current flows. Therefore, in this case, the transistor Tr. The emitter current of transistor Tr. 8 to the load resistor R1, and as a result, the corrected luminance signal YO, which has been subjected to white level expansion processing, is output to the output terminal 13.

At the same time, the transistor Tr. The collector potential of No. 9 also drops due to the current flowing through the load resistor R3. From this current, an LP composed of resistor R4 and capacitor C1
Only the DC component is taken out by F, and the transistor Tr
．． 18, Tr. The signal is inverted and amplified by the operational amplifier 19 and output to the color gain control terminal 68 .

That is, as the white level is expanded, the output voltage to the color gain control terminal 68, in other words, the control voltage applied to the variable gain amplifier 64 increases. This state corresponds to the operation in the region (W) in FIG.

Note that the values of the load resistors R2 and R3 and the transistor Tr. 18, Tr. How the color gain corresponds to the detection output APL can be arbitrarily set within a considerable range depending on the gain of the operational amplifier constituted by 19, etc. In the manner described above, it becomes possible to obtain the color gain control voltage corresponding to the detection output APL of the APL level detector 8.

[0079]

As explained above, in the present invention, only the luminance signal of the video signal is gradation corrected, so the visual contrast of the image can be improved with a relatively simple circuit configuration.
Allows for clearer gradation expression.

Furthermore, in the present invention, the video signal is expanded into white and black according to the average brightness, so the linearity of the transmittance of the LCD panel is corrected, the dynamic range is effectively utilized, and the visual contrast is improved. , allows for clearer gradation expression.

Furthermore, in the present invention, when the average luminance of the video signal is high, that is, when the image is bright, the white level is expanded,
When the average brightness is low, that is, the image is dark, the black level is expanded, and when the brightness is intermediate, both levels are expanded, and at the same time, the color gain is adjusted to avoid the correlation with the color signal collapsing. Since the linearity of the transmittance of the LCD panel can be corrected with a relatively simple circuit configuration,
It makes effective use of the dynamic range, improves visual contrast, enables clearer gradation expression, reduces costs, reduces the number of adjustment points on products, and simplifies adjustment work.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of a first embodiment of a video signal processing circuit according to the present invention.

FIG. 2 is a diagram showing the relationship between the average brightness level and the output of a comparator in the first embodiment of the present invention.

FIG. 3 is a graph showing the input/output characteristics of the first embodiment of the circuit of the present invention.

FIG. 4 is a circuit diagram showing the configuration of a video signal processing circuit according to a second embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the average brightness level and the comparator output in the second embodiment of the present invention.

FIG. 6 is a graph showing input/output characteristics of a second embodiment of the present invention.

FIG. 7 is a circuit diagram showing the configuration of a video signal processing circuit according to a third embodiment of the present invention.

FIG. 8 is a block diagram showing the configuration of a fourth embodiment of the video signal processing circuit according to the present invention.

FIG. 9 is a graph showing input/output characteristics of a luminance signal according to a fourth embodiment of the present invention.

FIG. 10 Color gain/APL of the fourth embodiment of the present invention
It is a graph showing characteristics.

FIG. 11 is a circuit diagram showing a specific configuration of a fourth embodiment of the video signal processing circuit according to the present invention.

FIG. 12 is a graph showing transmittance characteristics of an LCD panel.

FIG. 13 is a graph showing input/output characteristics of a conventional video signal processing circuit.

[Explanation of symbols]

2 White extension circuit 3. Black extension circuit 5, 6 Analog switch 8 APL level detector 9, 10 Comparator 57 Gradation correction circuit 64 Variable gain amplifier

Claims (3)

[Claims] 1. A video signal processing circuit that processes a video signal including at least a luminance signal, the circuit comprising: a gradation correction circuit that performs gradation correction only on the luminance signal. 2. A video signal processing circuit that is used in a drive circuit of a liquid crystal display panel and processes a video signal including at least a luminance signal, comprising: a black expansion circuit that expands the black level of the video signal; a white expansion circuit that expands the level; an average brightness detection means that detects average brightness from the brightness signal; and a black level and a A video signal processing circuit comprising means for controlling expansion of a white level. 3. A video signal processing circuit used in a drive circuit of a liquid crystal display panel and processing a video signal including at least a brightness signal, comprising: average brightness detection means for detecting average brightness from the brightness signal; In a first case where the detection result by the means is low, only the black level of the video signal is expanded; in a second case where the detection result by the average brightness detection means is high, only the white level of the video signal is expanded; gradation correction means for correcting to extend both the black level and white level of the video signal in a third case in which the detection result by the average luminance detection means is intermediate; and in the first and second cases; A video signal processing circuit comprising: gain control means for controlling color gain to a greater extent than in the third case.