Gradation correcting apparatus

Abstract: A gradation correcting apparatus which can be effectively incorporated into various video devices in order to display a small sub-display within the domain of main-display (what is called "Picture in Picture"), adjusting each gradation of the display automatically dependent on the characteristics of the respective input signal. By combining the window pulse outputted in synchronization with the main-display and the main- and sub-display switching signal, the gradation corrections of the displays are carried out by giving no gradation correction to the sub-display or by giving a proper correction to the same according to the sub-display signal in order to attain easy-to-see overall displays. By employing this apparatus, since the luminance information related to the sub-display disposed within a main-display and the luminance information related to the main-display excluding the domain of the sub-display are independently inputted into the correction signal output means, the arithmetical processing of this information, including the mutual relationship between these, can be processed. Moreover, since the signal delay caused by the arithmetic processing can be precisely compensated by means of said delay means, the gradation corrections for the main- and sub-displays can be exactly executed according to the respective luminance level information, realizing an easy-to-see sub-display also.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a gradation correcting apparatus, by which the gradation of video signals of such devices as televsion receivers, videotape records, video projectors or others are automatically corrected at the optimum level according to the characteristics of the input image. More specifically, this invention relates to a gradation correcting apparatus which can be effectively incorporated into various video devices in order to display a small sub-display screen area, such as a window, on the main display screen area (what is called "Picture in Picture"). PRIOR ARTS In order to improve the TV image quality, various gradation correcting apparatuses dependent on the characteristics of input images have been developed. A conventional and typical gradation correcting apparatus developed in the past is described below by referring to Figures 6 and 8. A block diagram of a conventional gradation correcting apparatus is shown in Figure 6, wherein timing pulse generator 1 generates window pulses by which the range of the histogram on the image screen is determined according to both the horizontal and the vertical synchronization signals. Histogram memory 2 memorizes the distribution of luminance levels of input video signals which are in a range determined by the pulses generated by said timing pulse generator 1. Lookup table arithmetical processing circuit 3 performs accumulative additions of the histograms stored in the histogram memory 2, and normalizes each data so that the maximum accumulative frequency becomes equal to the maximum value of the output luminance signal. Lookup table memory 4 stores the data normalized by the lookup table arithmetical processing circuit 3, and from which the correction signal according to the luminance level of input video signal is read out. Delay circuit 5 delays the input video signal. Adder 6 adds the correction signals obtained by the lookup table arithmetical circuit 3 to the output of the delay circuit 5. The operations and relationships of the above-explained elements constituting the above-shown gradation correcting apparatus are explained below. Figure 7 illustrates a window pulse obtained by said timing pulse generator 1. When a histogram has to be taken within a domain on the screen shown in Figure 7(a), a window signal such as shown in Figure 7(b) is derived. Figure 7(c) shows a horizontal partial enlargement of the signal. The sampling of the histogram is performed only when this window pulse takes a high-level. Figure 8 graphically shows the operations of the luminance conversion performed by the conventional apparatus. In this operation, a histogram, wherein the input luminance levels are divided into suitable numbers as the one shown in Figure 8(a), is produced. The frequency distributions of the divided luminance levels are stored in the histogram memory 2 while the content of the memory is reset at a predetermined interval in order to reset the previously-stored data. This interval is generally set at an interval which equals a single vertical scanning period or an integer multiple. Then, the lookup table arithmetical processing circuit 3 performs an accumulation of the histogram data, and it derives a normalizing coefficient which makes the accumulated maximum value equal to the desired maximum value of the luminance level in the output picture, by multiplying each data of the accumulative histogram by the derived normalizing coefficient. The results of this are stored in the lookup table 4. Figure 8(b) shows these operations. The content of the lookup table memory 4 is read out according to the luminance level of the input signal, and as shown in Figure 8(c), the difference between this read out content and the input luminance levels is outputted as a correction signal. The correction of the gradation is then performed by adding the correction signal to the input signal which is delayed by means of the delay circuit 5 using the adder 6. However, a below-described problem is inevitable with the gradation correcting apparatus construction shown above when it is incorporated in a TV showing double displays, such as the one shown in Figure 9(a), (what is called "Picture in Picture"), wherein a sub-display signal is added to a main display signal. When the main-display is showing a low luminance scene while the sub-display is showing a high luminance scene, the distribution of the accumulative histogram stored in the lookup table contains a considerably high proportion of the low luminance components, as shown in Figure 9(d). Therefore, the correction signal will take such a form shown in Figure 9(e). Thus, the correction is made to bring the luminance of the sub-display toward a higher level and, therefore, the luminance of the sub-display of which luminance level was originally high will be corrected toward a still higher level, bringing a definitely undesirable TV-viewing condition. SUMMARY OF THE INVENTION The present invention relates to a gradation correcting apparatus capable of performing the optimum gradation correction dependent on the characteristics of the input signal, when the input video signal is a composite video signal consisting of main- and sub-display signals. In other words, the gradation of the sub-display can be corrected to an improved viewing condition by utilizing a main- and sub-display switching signal shown in Figure 9(c) which is outputted in synchronization with the sub-display signal. The gradation correcting apparatus of the present invention in accordance with claim 1 comprises: timing pulse generating means for generating window pulses, said window pulses being derived from a horizontal synchronizing signal and a vertical synchronizing signal of an input video signal and timed relative to said synchronizing signals so as to define a predetermined display domain on a display screen; first switching means for producing a first control signal by using said window pulses and a main and sub-display switching signal, said main and sub-display signal, said main and sub-display switching signal being a second control signal timed relative to said window pulses so as to determine a sub-display domain within said predetermined display domain; a histogram memory for storing luminance levels of said input video signal in order to generate a histogram, wherein said histogram is produced by storing, in response to said control signals, the luminance levels of said input video signal either for said sub-display domain or for said predetermined display domain excluding said sub-display domain; correction signal output means for generating a correction signal to be added to the input video signal for gradation correction, said correction signal being calculated from a predetermined relationship between said histogram and desired output luminance levels of the picture to be displayed; delay means for delaying both said input video signal and said main and sub-display switching signal for a predetermined period; second switching means, responsive to said delayed main- and sub-display switching signal, for outputting said correction signal form said correction signal output means either during the display of said sub-display domain or during the display of said predetermined display domain excluding said sub-display domain; and adding means for adding aid correction signal to said delayed input video signal to produce an output video signal with gradation correction. The signals of the main-display and the sub-display can be accurately separated by using said timing pulse generating means generating the window pulses and the first switching means using the window pulses and the main- and sub-display switching signals. Furthermore, based on the separated signals, a histogram of the luminance levels is produced in the histogram memory. Since the luminance information of the sub-display disposed within the main-display and the luminance information of the main-display excluding the sub-display are independently inputted to the correction signal outputting means, the arithmetical processing of the information including the mutual relationship between these two displays can be performed, and as the result of this, the correction signal can be produced. Moreover, after the signal delay caused by the arithmetic processing is compensated by said delay means, the correction signal is composited with the input video signal by means of the second switching means and the adding means. By means of this produced composite sigtnal, the gradation correction of the input video signal can be preformed according to the respective luminance level information of the main- and sub-displays. In addition to this, since the gradation correction of the same can be performed at the precise timing by means of the delay means, an easy-to-see sub-display can be produced. Moreover, since the histogram memory stores the luminance levels of the input video signal representing either the sub-display domain or the predetermined domain excluding the sub-display domain according to the output of the first switching means in producing the histogram, the gradations of these two domains can be corrected independently, preventing possible interferences between these two domains. Furthermore, the correction signal output means is constituted of said lookup table arithmetical processing means which sequentially accumulates the frequencies of the luminance levels stored in the histogram memory and normalizes the maximum accumulated frequency so that it becomes equal to the desired maximum value of the luminance level in the output picture, and the lookup table memory which memorizes the output of the lookup table arithmetical processing means, simultaneously produces a correction signal corresponding to the difference between said output and the luminance level of the input luminance signal. Therefore, a correction signal precisely matched to the desired luminance level in the output picture can be produced. Furthermore, the construction of the invented gradation correcting apparatus can be highly simplified by employing an interlocking switching circuit capable of switching the first and the second switching means simultaneously. A second embodiment of the invention is claimed in independent claim 6. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of the gradation correcting apparatus showing a first exemplary embodiment of the invention. Figure 2(a) shows a typical display screen produced by applying the first exemplary embodiment, and this shows also a pattern explaining a domain range from which the histograms are taken. Figure 2(b) shows a waveform explaining the operation at the horizontal rate obtained by applying the first exemplary embodiment of the invention. Figure 3(a) shows another example of the video signal display produced by applying the the first exemplary embodiment of the invention, and this also shows a pattern explaining a domain range from which the histograms are taken. Figure 3(b) is to show a pattern explaining the relationship between the main- and sub-displays. Figure 4 is a block diagram showing the construction of the gradation correcting appartus of the second exemplary embodiment of the invention. Figure 5(a) is a waveform showing an example of the main- and sub-display switching signal. Figure 5(b) shows histograms of the frequencies of the input luminance level and the normalized output luminance level of the main display. This shows also a correction signal outputted according to the historgrams. Figure 5(c) shows a histogram of the frequencies of the input luminance levels and the normalized output luminance levels of the sub-display and, at the same time, a correction signal outputted according to the histogram. Figure 6 is a block diagram showing the construction of a conventional gradation correcting apparatus. Figure 7(a) shows an example of the display produced by applying the conventional gradation correcting method. Figure 7(b) shows a waveform to explain the operation of the gradation correction at a vertical rate obtained by applying the conventional gradation correction method. Figure 7(c) shows a waveform to explain the operation at a horizontal rate obtained by applying the conventional gradation correction method. Figure 8(a) shows a histogram of the luminance levels of the input signal on the main-display. Figure 8(b) shows a histogram of the output luminance levels normalized for each input luminance level on the main-display. Figure 8(c) shows a graph of the correction signals outputted according to the histograms. Figure 9(a) shows a typical display pattern on the double display TV of the conventional construction. Figure 9(b) shows a waveform of video signal at the horizontal rate obtained by a conventional gradation correcting apparatus. Figure 9(c) shows a waveform of the main- and sub-display switching signal at the horizontal rate obtained by the conventional correcting apparatus. Figure 9(d) shows a histogram of the output luminance level which is normalized for every input luminance level on the double-display TV. Figure 9(e) shows a waveform of the correction signal outputted according to the histograms. DETAILED DESCRIPTION OF THE INVENTION An exemplary embodiment of the invention is now explained by referring to the attached drawings. Figure 1 is a block diagram showing the construction of the gradation correcting apparatus of the first exemplary embodiment. In Figure 1, timing pulse generator 11 produces window pulses from both the horizontal and vertical synchronization signals, first AND circuit 17, as a first switching means, derives the AND between the window pulse and the main- and sub-display switching signal. Histogram memory 12 derives the histogram of gradation by storing the luminance levels of the input video signal according to the output of the first AND circuit 17, and lookup table arithmetical processing circuit 13 normalizes the maximum accumulated frequency in order to make this coincide with the maximum value of the output luminance signal. Lookup table memory 14 stores the normalized data and outputs the correction signals. Therefore, the correction signal output means is constituted of the lookup table arithmetical processing circuit 13 and the lookup table memory 14. Delay circuit 15, as a delay means, delays both the input video signal and the main- and sub-display switching signal. Second AND circuit 18, as the second switching means, derives the AND between the delayed main- and sub-display switching signal and the correction signal outputted at the lookup table memory 14. Adder 16, as the adding means, adds the output of the second AND circuit 18 to the input signal delayed by the delay circuit 15. The relationship between these constituting elements of the gradation correcting apparatus and the operation of these are now explained by referring to Figures 2 and 3. As shown in Figure 2(b), a window pulse is produced by the timing pulse generator 11, and an AND of the window pulse and the main- and sub-display switching signal is produced by the first AND circuit 17. This means that the sampling of histogram is performed when both the window pulse and the main- and sub-display switching signals are in a high-level. And by this, as shown in Figure 2(a), the histogram of the display domain (shown by the hatched lines) excluding the sub-display domain can be derived. That is, the luminance levels of the input video signal are stored in the histogram memory 12 according to the output of the first AND circuit 17 in order to derive the histogram of the gradation. The histograms are then stored in the lookup table arithmetical processing circuit 13, and these values are normalized so that the maximum accumulated frequency becomes equal to the maximum value of the output luminance signal. The normalized data is stored in the lookup table memory 14, and the output of the memory 14 becomes available as the correction signal. Then, the input signal and the main- and sub-display switching signal are delayed for a certain period in order to conduct the arithmetical processing by using the delay circuit 15. The correction signal at the sub-display domain is set at zero by deriving the AND of the correction signal and the delayed main- and sub-display switching signal by means of the second AND circuit 18. The relationship for the horizontal rate is shown in Figure 2(b), wherein the output of the second AND circuit 18 is set at zero during the period of Hd of the main- and sub-display switching signal (or the period of the sub-display). Thus, an appropriate correction according to the histogram of the main display can be given to the main display by adding the output of the second AND circuit 18 to the delayed input video signal by means of the adder 16, and the output signal maintaining the sub-display at its uncorrected state can be obtained. As explained above, according to the embodiment of the invention, the sub-display, held as it is, can be outputted because it is made independent of the gradation correction given to the main-display. This is obtained by combining the window pulse and the main- and sub-display switching signal. Therefore, as a result, the gradation correction makes the sub-display easier to view. A case of the input video signal by which the display shown in Figure 3(b) is produced is now explained below. As shown in Figure 3(b), this is a case where a sub-display-B is additionally provided within the domain of the sub-display-A, which is within the domain of main display. As shown in Figure 3(a), the histogram of the domain (the hatched area) excluding the domain of sub-display-B can be obtained by setting the sub-display-A within the window pulse period Hb. As shown here, since the sub-display-B as it is can be obtained by setting the window pulse at a state to obtain a histogram of the surrounding domain of the sub-display-B, the gradation correction which makes the sub-display easier to see can be obtained as the result of this. When the domains of plural sub-displays are overlapped as shown in this case, the same advantage obtained in the above can be obtained by setting the predetermined domain at the peripheral of the sub-display. The second exemplary embodiment of the gradation correcting apparatus is now explained. Figure 4 is a block diagram showing the construction of a second exemplary gradation correcting apparatus. In Figure 4, timing pulse generator 11 generates window pulses from the horizontal and vertical synchronization signals of the input video signal. Quintuple-ganged switch 19 switches the five memories simultaneously by means of the main- and sub-display switching signal. Histogram memory 12 stores the histograms of the luminance levels of main- and sub-displays independently. Lookup table arithmetical processing circuit 13 accumulatively stores the histograms and normalizes the maximum accumulated frequency so that the frequency thereof becomes equal to the maximum value of the output luminance signal. Lookup table memory 14 stores the normalized data for the main-display and the sub-display independently, and outputs the correction signal. Delay circuit 15 delays the input video signal. Although a correction by using a delay circuit is desirable, the boundaries generated by the gradation processing can be made less conspicuous by employing this practical and simple method shown in the embodiment of the invention. Adder 16 adds the correction signal to the delayed input video signal. The relationship between these above-explained elements of the gradation correcting apparatus and the operation of these are now explained below by referring to Figure 5. Like the -first exemplary embodiment, the window pulse is generated by the timing pulse generator 11. However, the difference from the first exemplary embodiment is that five switches 19a to 19e incorporated within the quintuple-ganged switch 19 are simultaneously switched by the main- and sub-displays switching signal so that the histogram memory 12 and the lookup table memory 14 are switched to the main-display and the sub-display independently. That is, the quintuple-ganged switch 19 consists of two switches 19a and 19c which switch two respective inputs to the histogram memories 12a and 12b, and the lookup table memories 14a and 14b, two switches 19b and 19e which switch two respective outputs from the histogram memories 12a and 12b, and the lookup table memories 14a and 14b, and the switch 19d which connects the output of the lookup table arithmetical processing circuit 13 to either of the two inputs of the lookup table memories 14a or 14b. Thus, the period of the sub-display is switched to use both the histogram memory 12a for the sub-display and the lookup table memory 14a by means for the main- and sub-display switching signal as shown in Figure 5(a). The period of the other is switched to use the histogram memory 12b for the main-display and the lookup table memory 14b. Figure 5(b) shows the operation which takes place during the period of the main-display. Figure 5(b) also shows a graph of the frequencies of the input luminance level histogram produced by the histopram memory 12b, plotted against the input luminance levels. The two frequencies are individually accumulated for each luminance level, and the graph of the frequencies is normalized so that the maximum accumulated frequency becomes equal to the maximum value of the output luminance signal. The normalized data is then stored in the lookup table memory 14b, and a correction signal becomes available as the output of the lookup table memory 14b. Likewise, the five circuits are simultaneously switched by the quintuplely-ganged switch 19 during the period of sub-display and, by this, a correction signal becomes available from the lookup table memory 14a as shown in Figure 5(c). By the processes described above, the correction signals dependent on the individual scenes of the main- and sub-displays can be derived and, by this, the gradation of the main- and sub-displays are corrected at their respective optimums. According to the second exemplary embodiment of the invention, the gradation correction of the main-display and the gradation correction of the sub-display are independently performed according to the respective individual scenes. Therefore, both the main- and the sub-displays are corrected to show easy-to-see pictures. As understood from the above explanations, the luminance information of the sub-display disposed within a domain of the main-display and the luiminance information of the main-display excluding the domain of the sub-display is individually inputted into the gradation correction apparatus. Therefore, the relationship between these two displays can be derived arithmetically. At the same time, since the delay of the signal caused by the arithmetical processing is compensated by the delay means, and the gradations of the main- and sub-displays are properly corrected according to the respective luminance levels so that easy-to-see sub-displays are obtained on the respective displays. By applying the first exemplary embodiment of the invention to an input signal comprised of a sub-display signal superposed on a main-display signal for a double-display, the histograms of the gradation of sub-display are excluded from the input signal by utilizing the main- and sub-display switching signal outputted in synchronizing with the sub-display. Thus, only the gradation of the main-display is corrected according to the scene shown on the main-display, and no gradation correction of the sub-display is made so that the gradation correction apparatus, by which easy-to-see gradation of both the main- and sub-displays, is realized. By applying the second exemplary embodiment of the invention to an input signal comprised of a sub-display signal superposed on a main-display signal for double-display, the histogram of gradation of sub-display and the histogram of gradation of main-display are independently processed by combining the input signal with the window pulse utilizing the main- and sub-display switching signal outputted in synchronizing with the sub-display. Thus, as a result of this, the gradation of the main-display is corrected according to the scene shown on the main-display, and the gradation of the sub-display is corrected according to the scene shown in the sub-display, so that the gradation correction apparatus realizing easy-to-see gradations of both the main- and sub-displays is realized.

Claims: A gradation correcting apparatus for use in video devices having a picture in picture display, comprising: timing pulse generating means (11) for generating window pulses, said window pulses being derived from a horizontal synchronizing signal and a vertical synchronizing signal of an input video signal and timed relative to said synchronizing signals so as to define a predetermined display domain on a display screen; first switching means (17) for producing a first control signal by using said window pulses and a main and sub-display switching signal, said main and sub-display switching signal being a second control signal timed relative to said window pulses so as to determine a sub-display domain within said predetermined display domain; a histogram memory (12) for storing luminance levels of said input video signal in order to generate a histogram, wherein said histogram is produced by storing, in response to said control signals, the luminance levels of said input video signal either for said sub-display domain or for said predetermined display domain excluding said sub-display domain; correction signal output means (13, 14) for generating a correction signal to be added to the input video signal for gradation correction, said correction signal being calculated from a predetermined relationship between said histogram and desired output luminance levels of the picture to be displayed; delay means (15) for delaying both said input video signal and said main and sub-display switching signal for a predetermined period; second switching means (18), responsive to said delayed main- and sub-display switching signal, for outputting said correction signal from said correction signal output means (13, 14) either during the display of said sub-display domain or during the display of said predetermined display domain excluding said sub-display domain; and adding means (16) for adding said correction signal to said delayed input video signal to produce an output video signal with gradation correction. A gradation correcting apparatus according to claim 1, wherein the delay time of said delay means (15) is equal to an amount of time required to process said video input signal by means of said histogram memory (12) and said correction signal output means (13, 14). A gradation correcting apparatus according to claim 1, wherein said second switching means (18) outputs the correction signal for said predetermined display domain excluding said sub-display domain, and outputs a zero correction signal for said sub-display domain. A gradation correcting apparatus according to claim 1, wherein said first and said second switching means (17, 18) comprise AND circuits. A gradation correcting apparatus according to one of claims 1-4, wherein said correction signal output means (13, 14) comprises: a lookup table arithmetical processing means (13) for accumulatively storing frequencies of occurrence of said luminance levels stored in said histogram memory, thereby forming an accumulative histogram by accumulating said frequencies of said histogram, and for determining said desired output luminance levels by multiplying each frequency of said accumulative histogram by a normalizing coefficient constituted by a maximum input luminance level divided by the maximum frequency of said accumulative histogram, and a lookup table memory (14) for storing the output of said lookup table arithmetical processing means (13) and for producing said correction signal as a difference between said desired output luminance levels and associated input luminance levels to be corrected. A gradation correcting apparatus for use in video devices having a picture in picture display, comprising: timing pulse generating means (11) for generating window pulses, said window pulses being derived from a horizontal synchronizing signal and a vertical synchronizing signal of an input video signal and timed relative to said synchronizing signals so as to define a predetermined display domain on a display screen; switching means (19a-19e) for producing switching signals by using said window pulses and a main and sub-display switching signal, said main and sub-display switching signal being timed relative to said window pulses so as to determine a sub-display domain within said predetermined display domain; histogram memory means (12a, 12b) controlled by said switching means (19a) for independently storing luminance levels of said sub-display domain of said input video signal and luminance levels of said predetermined display domain of said input video signal, and for generating two independent histograms for said sub-display domain and for said predetermined display domain excluding said sub-display domain, respectively; lookup table arithmetical processing means (13) likewise controlled by said switching means (19b) for accumulatively storing for said respective display domains frequencies of occurrence of said luminance levels stored in said histogram memory means (12a, 12b), thereby forming respective accumulative histograms by accumulating said frequencies of said respective histograms, and for determining desired output luminance levels by multiplying each frequency of said respective accumulative histograms,by a normalizing coefficient constituted by a maximum input luminance level divided by the maximum frequency of said respective accumulative histogram; lookup table memory means (14a, 14b) likewise controlled by said switching means (19c, 19d) for storing respective outputs of said lookup table arithmetical processing means (13) and for producing said respective correction signals as a difference between said desired output luminance levels and associated input luminance levels to be corrected; delay means (15) for delaying said input video signals for a predetermined period; and adding means (16) controlled by said switching means (19e) for adding said respective correction signal to said delayed input video signal to produce an output video signal with gradation correction. A gradation correcting apparatus according to claim 6 wherein said switching means comprises a quintuple-ganged switch (19a-19e) which operates five switches. A gradation correcting apparatus according to claim 7, wherein said quintuple-ganged switch comprises; two switches (19a, 19c) for switching two respective inputs of said histogram memory means (12a, 12b) and said lookup table memory means (14a, 14b); two switches (19b, 19e) for switching two respective outputs of said histogram memory means (12a, 12b) and said lookup table memory means (14a, 14b); and one switch (19d) for switching the output of said lookup table arithmetic processing means (13) to an input terminal of said lookup table memory means (14a, 14b), respectively.

Inventors: IZAWA YOSUKE, OKUMURA NAOJI, IZAWA, YOSUKE, OKUMURA, NAOJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H04N5
IPC codes: H04N5