Automatic brightness contrast lighting circuits and luminance/color difference signal processor and video display apparatus comprising the same

Abstract: The object of the present invention is to easily compensate automatic brightness limiting performance and automatic contrast limiting performance even if a picture is displayed with a different aspect ratio and to prevent deterioration of picture quality such as graduation and sharpness. A picture display apparatus includes (1) an ABL (automatic brightness limiter) circuit including a comparison circuit for varying an EHT compensation starting point, an ABL gain control circuit for varying a slope of EHT regulation characteristic against APL, an ABL comparison voltage generator, a DAC for adjusting an ABL compensation starting point and a DAC for adjusting an ABL gain and (2) an ACL (automatic contrast limiter) circuit including another comparison circuit for varying an EHT compensation starting point, an ACL gain control circuit for varying a slope of EHT regulation characteristic against APL, an ACL comparison voltage generator, a DAC for adjusting an ACL compensation starting point and a DAC for adjusting an ACL gain.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a luminance signal adjusting circuit, especially to an improvement of an automatic brightness limiting (ABL, hereafter) circuit and an automatic contrast limiting (ACL, hereafter) circuit of a luminance signal adjusting circuit of a television receiver. DESCRIPTION OF THE PRIOR ART Recently, a higher quality picture is being desired in a television receiver and an ABL control circuit to automatically adjust a DC level of a luminance signal and an ACL control circuit to automatically adjust an amplitude of the luminance signal are inevitable against the EHT (extra high tension) variation of the receiver. In particular, with the spread of television receivers having display devices with a 16:9 aspect ratio and television receivers for a High Vision system, television receivers which can vary an aspect ratio of the displayed picture are increasing. For an example of an ABL and ACL control circuit, for example Japanese Patent Laid Open Application 2-94882 is proposed. A block diagram of an ABL and ACL control circuit responsive to aspect ratio switching is shown in Fig. 1. Block 101 is an extra high tension (EHT, hereafter) generator including a flyback transformer. Symbols 102, 103 and 104 are resistors for converting an EHT current flowing from EHT generator 101 into an EHT detecting voltage. Block 105 is an aspect ratio detector. Block 106 is a switch switched by an output of the aspect ratio detector 105. Blocks 107 and 108 are buffers inputting an EHT detection voltage detected at resistors 102, 103 and 104. Block 109 is a brightness control voltage generator to generate a brightness control voltage. Block 110 is an adder to add the EHT detection voltage outputted from buffer 107 and the brightness control voltage outputted from brightness control voltage generator 109. Block 111 is a contrast control voltage generator to generate a contrast control voltage. Block 112, is an adder to add the EHT detection voltage outputted from buffer 108 and the contrast control voltage outputted from contrast control voltage generator 111. Block 113 is a luminance and color difference signal processor. Symbol 114 is a brightness control voltage input terminal to input an output voltage from adder 110. Symbol 115 is a contrast control voltage input terminal to input an output voltage from adder 112. Symbol 116 is a primary color signal output terminal to output a primary color signal in which brightness and contrast are adjusted by the voltages inputted to brightness control voltage input terminal 114 and contrast control voltage input terminal 115, respectively. Block 117 is a primary color signal output citcuit to amplify a primary color signal outputted from primary color signal output terminal 116. Block 118 is a cathode ray tube (CRT, hereafter) for displaying a picture (CRT, hereafter). Block 119 is an anode of CRT 118 for supplying an EHT for electron beam accerelation from EHT generator 101. Performance of the ABL control circuit and the ACL control circuit configured above is explained below, referring to FIG. 2. The EHT detecting voltages against the same average picture level (APL, hereafter) value are different between when a horizontally wide picture having an aspect ratio such as 16:9 as shown in FIG. 2(a) is displayed and when a picture having an aspect ratio such as 4:3 as shown in FIG. 2(b) is displayed even if the displayed pictures have the same picture information (content). In FIG. 2(b), the hatched areas at the right and left sides are areas where the electron beam is scanned but the phosphor does not emit light because the signal level is in a black level. Therefore, even if FIG. 2(a) and FIG. 2(b) have the same picture information, the average value of the beam current at the displayed picture (b) of a 4:3 aspect ratio is smaller than that at the displayed picture (a) of a 16:9 aspect ratio and the slope of the EHT characteristic against APL value is gentler. That is, as for the EHT detecting voltage generated when the EHT current flowing from EHT generator 101 to anode 119 is converted to a voltage by resistors 102, 103 and 104, the slope of the EHT regulation characteristic against APL varies according to an aspect ratio as shown in FIG. 2(c). In FIG. 2(c), EHT regulation characteristics against APL (a) and (b) correspond to a picture with a 16:9 aspect ratio shown in FIG. 2(a) and a picture with a 4:3 aspect ratio shown in FIG. 2(b), respectively. In FIGs. 2(c) and 2(d), the abscissa is APL value of a luminance signal and the ordinate is an EHT supplied to CRT, respectively. Therefore, a difference occurs in the EHT current and another difference occurs in the effect of ABL and ACL actions according to the difference between aspect ratios although they contain the same picture information. To prevent these differences, a switch 106 is provided across resistor 103 and switch 106 is switched by a control voltage from aspect ratio detector 105 so that the EHT characteristic (a) against APL at a picture with a 16:9 aspect ratio approaches to the EHT characteristic (b) at a picture with a 4:3 aspect ratio. An EHT characteristic (a') against APL compensated by switching the EHT detection voltage by a control voltage of aspect ratio detector 105 is shown in FIG. 2(d). (Change from characteristic (a) to characteristic (a')). According to the above configuration, the EHT, therefore the EHT detection voltage is nearly same as shown in FIG. 2(d) even if the aspect ratio changes from that shown in FIG. 2(a) to that shown in FIG. 2(b) and vice versa and the effects of ABL and ACL are nearly same and the picture quality such as picture graduation and picture sharpness can be made nearly equal. On the above-mentioned configuration, however, the ABL and ACL characteristics are dominated by the values of resistors 102, 103 and 104. Moreover, switch 106 for detecting the aspect ratio and short circuiting a resistor and buffers 107 and 108 and adders 110 and 112 for ABL and ACL, respectively are necessary. The circuit scale becomes large. SUMMARY OF THE INVENTION In the present invention, optimum ABL and ACL characteristics are obtained even if the aspect ratio of the picture changes, by uniting the ABL and ACL input voltages in one terminal and making the ABL and ACL starting points and the performance characteristics variable and the circuit configuration can be simple by switching with a bus bar data such as IIC (inter IC). The present invention provides a control circuit for automatic brightness limiting and automatic contrast limiting (ACL) circuits of a video display apparatus, said control circuit comprising: ABL compensation starting point determining means operating to compare a detected extra high tension (EHT) value with a first reference voltage and outputting zero when the detected EHT value is larger than the first reference voltage and a difference voltage when the detected EHT value is smaller than the first reference voltage; ABL gain control means receiving the output of said ABL compensation starting point determining means whereby the gain of the ABL circuit is varied according to said output; ACL compensation starting point determining means operating independently of said ABL compensation starting point determining means to compare a detected EHT value with a second reference voltage and outputting zero when the detected EHT value is larger than the second reference voltage and a second difference voltage when the detected EHT value is smaller than the second reference voltage; ACL gain control means receiving the output of said ACL compensation starting point determining means whereby the gain of the ACL circuit is varied according to said output, independently of the variation of the gain of said ABL circuit. The ABL and ACL characteristics can be made optimum by controlling the adjustment of the ABL and ACL characteristics not with a hardware but a software. That is, the ABL and ACL characteristics can be set independently and an optimum performance can be obtained by setting their data separately even if a picture with a different aspect ratio is displayed. Other preferred features of the invention are described in annexed claims 2 to 5. US-A-4253110 discloses a kinescope beam current limiter which produces a contrast control beam current limiting signal (voltage) and a brightness control beam current limiting signal (voltage). The contrast control voltage is produced when the detected EHT voltage exceeds a certain theshold and the brightness control voltage is produced when the EHT voltage exceeds another threshold. In contrast to the present invention, the control of the brightness depends to some extent on the control of the contrast and the two are not independant. For example, control of brightness alone begins when the contrast control beam current limiting signal has been clamped at a fixed level. According to the present invention contrast and brightness are controlled independently. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a principal part of a television receiver including an ABL and ACL control circuit in accordance with the prior art. FIG. 2(a) shows a picture with a 16:9 aspect ratio displayed on a CRT with a 16:9 aspect ratio. FIG. 2(b) shows a picture with a 4:3 aspect ratio displayed on a CRT with a 16:9 aspect ratio. FIG. 2(c) is an EHT characteristic against APL. FIG. 2(d) is an EHT characteristic against APL of a circuit shown in FIG. 1 when switch 106 is switched, in accordance with the prior art. FIG. 3 is a block diagram of a principal part of a television receiver including an ABL and ACL control circuit in accordance with an exemplary embodiment of the present invention. FIG. 4(a) shows a picture with a 16:9 aspect ratio displayed on a CRT with a 16:9 aspect ratio. FIG. 4(b) shows a picture with a 4:3 aspect ratio displayed on a CRT with a 16:9 aspect ratio. FIG. 4(c) is an EHT characteristic against APL (without compensation). FIG. 4(d) is a compensated EHT characteristic of a circuit shown in FIG. 3. FIG. 4(e) is another compensated EHT characteristic of a circuit shown in FIG. 3. DETAILED DESCRIPTION OF THE INVENTION A block diagram of a principal part of a television receiver including an ABL and ACL control circuit in accordance with an exemplary embodiment of the present invention is shown in FIG. 3. Block 1 is an EHT generator including a flyback transformer. Symbols 2 and 3 are resistors for converting an EHT current flowing from EHT generator 1 into an EHT detecting voltage. Block 4 is a luminance and color difference signal processor. Terminal 5 is an EHT detecting voltage input terminal inputting an EHT detection voltage generated at resistor 3. Block 6 is an ABL comparator for adjusting an ABL compensation starting point by comparing the input signal inputted at EHT detecting voltage input terminal 5 and a first reference voltage. Block 7 is an ABL comparison voltage generator for generating the first reference voltage to be supplied to ABL comparator 6 and to determine an ABL compensation starting point. Block 8 is a digital-to-analog converter (DAC, hereafter) for controlling ABL comparison voltage generator 7. Block 9 is an ABL gain control circuit for controlling an ABL control characteristic inputting the output of ABL comparator 6. Block 10 is an ABL gain control DAC for controlling ABL gain control circuit 9. Block 11 is a brightness control voltage generator for generating a brightness control voltage. Block 12 is an adder for adding the ABL compensation voltage outputted from ABL gain control circuit 9 and the brightness control voltage outputted from brightness control voltage generator 11. Block 13 is a brightness control DAC for controlling brightness control voltage generator 11. Block 14 is a brightness control circuit inputting the output voltage from adder 12. Block 30 is an ABL control circuit block which unifies circuits concerning ABL. Block 15 is an ACL comparator for adjusting an ACL compensation starting point by comparing the input signal inputted at EHT detecting voltage input terminal 5 and a second reference voltage. Block 16 is an ACL comparison voltage generator for generating the second reference voltage to be supplied to ACL comparator 15 and to determine the ACL compensation starting point. Block 17 is a ACL compensation starting point adjust DAC for controlling ACL comparison voltage generator 16. Block 18 is an ACL gain control circuit for controlling an ACL control characteristic inputting the output of ACL comparator 15. Block 19 is an ACL gain control DAC for controlling ACL gain control circuit 18. Block 20 is a contrast control voltage generator for generating a contrast control voltage. Block 21 is an adder for adding the ACL compensation voltage outputted from ACL gain control circuit 18 and the contrast control voltage outputted from contrast control voltage generator 20. Block 22 is a contrast control DAC for controlling contrast control voltage generator 20. Block 23 is a contrast control circuit inputting the output voltage from adder 21. Block 31 is an ACL control circuit block which unifies circuits concerning ACL. Terminal 24 is a primary color signal output terminal for outputting primary color signals in which the brightness and the contrast are adjusted by the voltages inputted at brightness control circuit 14 and contrast control circuit 23. Block 25 is a primary color signal output circuit for amplifying the primary color signals outputted from primary color signal output terminal 24. Block 26 is a CRT having an aspect ratio, for example 16:9. Terminal 27 is an anode through which an EHT for CRT 26 is supplied from EHT generator 1. Block 28 is a micro processor for supplying data to DACs 8, 10, 13, 17, 19 and 22. Terminal 29 is a data input terminal for supplying data from micro processor 28 to luminance and color difference signal processor 4. The performance of a television receiver in accordance with an exemplary embodiment of the present invention is explained below. The value of the EHT current flowing in resistor 3 differs between the case in which a picture having a 16:9 aspect ratio is displayed on a screen having a 16:9 aspect ratio as shown in FIG. 4(a) and the case in which a picture having a 4:3 aspect ratio is displayed on the same screen having a 16:9 aspect ratio as shown in FIG. 4(b). That is, the EHT detecting voltage converted from the EHT current flowing in resistors 2 and 3 varies according to the aspect ratio of the picture to be displayed even if the same picture information is displayed and the slopes of the EHT characteristics against APL are different as shown in FIG. 4(c). In FIG. 4(c), the characteristic (a) is an EHT regulation against APL when a picture having a 16:9 aspect ratio is displayed on a screen having a 16:9 aspect ratio as shown in FIG. 4(a) and the characteristic (b) is an EHT regulation against APL when a picture having a 4:3 aspect ratio is displayed on the same screen having a 16:9 aspect ratio as shown in FIG. 4(b). Therefore, the EHT current differs according to the difference between the aspect ratios and a difference occurs in the ABL and ACL performance even if the displayed pictures have the same information (content). To solve this problem, ABL control circuit block 30 and ACL control circuit block 31 are used and the ABL or ACL compensation starting point and the slope of the EHT characteristic against APL are set separately according to the aspect ratio and to what degree the ABL or ACL characteristic is controlled is made almost independent of the aspect ratio. When a bright signal having a high APL value is inputted, the beam current is large and the EHT decreases. The decreasing of the EHT is detected at resistor 3. The detected EHT value Vin is compared with a reference voltage Vref generated at ABL comparison voltage generator 7 at ABL comparator 6. When Vin is smaller than Vref, the difference voltage, dV = Vref - Vin is outputted from ABL comparator 6. The input voltage dV is gain adjusted at ABL gain control circuit 9. The gain controlled dV is added to a brightness control voltage outputted from brightness control voltage generator 11 at adder 12. Brightness control circuit 14 lowers the picture brightness according to the output of adder 12. Then, the beam current of CRT 26 decreases and EHT generator 1 works to raise the EHT and to prevent EHT decreasing. This performance corresponds to a right side region of the ABL compensation starting point (m) shown in FIG. 4 (d). When a dark signal having a low APL value is inputted, the EHT decreases not so much as the case of a bright signal and because the EHT holds a compratively high voltage, Vin is larger than Vref, there is no output at ABL comparator 6 and the EHT is not compensated. That is, the EHT regulation characteristic against APL is the same as the characteristic (a). This performance corresponds to a left side region of the ABL compensation starting point (m) shown in FIG. 2(d). The ACL performance is quite similar to that of the ABL performance and its explanation is omitted. FIG. 4(d) and 4(e) show EHT characteristics against APL for ABL or ACL in the above-mentioned exemplary embodiment. FIG. 4(d) shows an example in which the EHT characteristic for ABL or ACL is varied so that the EHT characteristic (a) approaches the EHT characteristic (b) and results in a compensated EHT characteristic (a'). FIG. 4(e) shows another example in which the EHT characteristic for ABL or ACL is varied so that the EHT characteristic (b) approaches the EHT characteristic (a) and results in a compensated EHT characteristic (b'). Thus, a television receiver in accordance with the present invention can have nearly same EHT characteristics both for a display having a 16:9 aspect ratio shown in FIG. 4(a) and for a display having a 4:3 aspect ratio shown in FIG. 4(b) by varying the compensation starting point and the slope of the EHT characteristic for ABL or ACL. Therefore, nearly same picture quality such as picture graduaion and sharpness can be obtained for the displays having different aspect ratios. The compensation starting point and the slope of the EHT characteristic for ABL or ACL are freely variable by inputting data in micro processor 28. Thus, the present invention provides a comparison circuit which varies the compensation starting point and a gain control circuit which varies the slope of the EHT characteristic against APL and an optimum ABL and ACL performance is obtained even for different aspect ratios only by setting each data and without adjusting a hardware for ABL and ACL voltages even if the aspect ratio of the signal varies. Of course, the circuit configuration becomes simpler. Both ABL control circuit 30 and ACL control circuit 31 are not always necessary and either one may be used.

Claims: A control circuit (30,31) for automatic brightness limiting (ABL) and automatic contrast limiting (ACL) circuits of a video display apparatus, said control circuit comprising: ABL compensation starting point determining means (6) operating to compare a detected extra high tension (EHT) value with a first reference voltage (7) and outputting zero when the detected EHT value is larger than the first reference voltage and a difference voltage when the detected EHT value is smaller than the first reference voltage (7). ABL gain control means (9) receiving the output of said ABL compensation starting point determining means (6) whereby the gain of the ABL circuit is varied according to said output; ACL compensation starting point determining means (15) operating independently of said ABL compensation starting point determining means (6) to compare a detected EHT value with a second reference voltage (16) and outputting zero when the detected EHT value is larger than the second reference voltage and a second difference voltage when the detected EHT value is smaller than the second reference voltage (16); ACL gain control means (18) receiving the output of said ACL compensation starting point determining means (15) whereby the gain of the ACL circuit is varied according to said output, independently of the variation of the gain of said ABL circuit. A control circuit as claimed in claim 1 the control circuit being arranged such that the gain of the ABL circuit is varied such that the slope of the EHT versus average picture level characteristic is varied for detected EHT values smaller than the first reference voltage as compared to the slope for detected EHT values above the first reference voltage. A control circuit as claimed in claim 1 or 2 the control circuit being arranged such that the gain of the ACL circuit is varied such that the slope of the EHT versus average picture level characteristic is varied for detected EHT values smaller than the second reference voltage as compared to the slope for detected EHT values above the first reference voltage. A luminance/colour difference signal processor (4) comprising a control circuit as claimed in any preceding claim and further comprising: brightness control means (14) receiving the output of said ABL gain control means (9) and a brightness control voltage (13) and controlling the DC level of a luminance signal; and contrast control means (23) receiving the output of the ACL gain control means (18) and a contrast control voltage (20) and controlling the amplitude of a luminance signal. A video display apparatus comprising a luminance/colour difference signal processor (4) as claimed in claim 4 and further comprising: primary colour signal output means (25) producing primary colour signals from luminance and colour difference signals output from said luminance/colour difference signal processor; and a display device (26) driven by the output signals from said luminance/colour difference signal processor to display a picture.

Inventors: HIROKI KINUGAWA, HIROSHI ANDO, MASAHIKO SASADA, MINORU MIYATA, HIROKI, KINUGAWA, HIROSHI, ANDO, MASAHIKO, SASADA, MINORU, MIYATA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H04N5
IPC codes: H04N5