Abstract:
A character display control circuit comprises a selection circuit to select and output, as one of the RGB signals, one of a first voltage, a second voltage lower than the first voltage, and one or more third voltages existing between the first and second voltages; a holding circuit in which first data, consisting of a plurality of bits, for the selection circuit to select and output one of the first, second, and third voltages, is set in response to display timings of the display characters; and a selection control circuit to supply the selection circuit with selection signals for the selection circuit to select and output one of the first, second and third voltages depending on the first data, second data associated with the selection of the first voltage or the second voltage, and third data associated with the selection/unselection of the third voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority upon Japanese Patent Application No. 2003-338038 filed on Sep. 29, 2003, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a character display control circuit. 
   2. Description of the Related Art 
   There are times when various information such as current channel, time and volume is displayed at a given position on a television receiver. Such an information display is carried out by generating video signals for information display with an OSD (On Screen Display) circuit, controlled by a microcomputer, and by switching between analog video signals for television images and video signals for information display as appropriate. See, e.g., Japanese Patent Application Laid-Open Publication No. Hei. 10-108088. 
     FIG. 3  is a block diagram of a television receiver for describing a conventional OSD output. 
   The conventional television receiver comprises a microcomputer  500 , a mixing circuit  600 , a signal processing circuit  200  and a display  300 . 
   The microcomputer  500  performs various information processing. The microcomputer  500  contains an OSD output circuit for OSD display of information, and the OSD output circuit outputs, to the mixing circuit  600 , red, blue and green (herein after referred respectively to as R, G and B) data and data I (intensity) for adjusting RGB data levels. 
   The data I, intended to adjust the level between “0” and “1” of the RGB data, can be adjusted to, for example, an intermediate level. 
   The mixing circuit  600  adjusts the RGB data levels with the data I, thus outputting the result thereof. 
   The signal processing circuit  200  performs signal processing based on the output result of the mixing circuit  600 , outputting signals R′, G′ and B′ obtained to the display  300 . 
   As above, in conventional OSD display, the operation for adjusting R, G and B data levels, output by the microcomputer, according to the data I is handled by the externally attached mixing circuit  600 . If the data I is not used, there are two possible display colors (“HIGH” (hereinafter H) and “LOW” (hereinafter L)) for each of R, G and B produced by the character display control circuit, resulting in 2×2×2=8 or eight possible combinations in total. That is, eight colors can be displayed, and the number of display colors can be further increased using the data I as shown in  FIG. 3 . 
   When the input data I is “L” (logic value of “0”), the mixing circuit  600  outputs data as is as normal R, G or B color. On the other hand, when the input data I is “H” (logic value of “1”), the mixing circuit  600  makes adjustment so that the color is lighter than normal—at an intermediate level, for example. In this case, there are, for example, 8+8−1=15 or 15 possible combinations of colors produced, making it possible to display 15 colors. 
   The reason for subtracting 1 in the above equation is that if RGB data are all “0” (black), the output is “0” despite adjustment of the display signal by the data I, causing the display color to remain unchanged. 
   Thus, the conventional OSD circuit adjusts the RGB data levels according to the data I using the mixing circuit  600  attached externally of the microcomputer  500 , with up to 15 display colors available. 
   With the conventional OSD circuit, creation of colors other than the eight colors leads to the problem of increased scale as a result of use of an externally attached circuit for adjusting the RGB data and the data I. In the case of games and the like, on the other hand, the conventional 15 colors are short of meeting the demand for OSD display using more than 15 colors, for example. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a character display control circuit capable of increasing OSD display colors without increasing the circuit scale. 
   In order to achieve the above object, according to a major aspect of the present invention there is provided a character display control circuit for generating RGB signals to display characters on screen on a display device, the circuit comprising a selection circuit to select and output, as one of the RGB signals, one of a first voltage, a second voltage lower than the first voltage, and one or more third voltages existing between the first and second voltages; a holding circuit in which first data, consisting of a plurality of bits, for the selection circuit to select and output one of the first, second, and third voltages, is set in response to display timings of the display characters; and a selection control circuit to supply the selection circuit with selection signals for the selection circuit to select and output one of the first, second and third voltages depending on the first data, second data associated with the selection of the first voltage or the second voltage, and third data associated with the selection/unselection of the third voltages. 
   According to the present invention, it is possible to increase OSD display colors in number without increasing the circuit scale. 
   Features and objects of the present invention other than the above will become clear by reading the description of the present specification with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a block diagram of a television receiver including a character display control circuit according to the present invention; 
       FIG. 2  is a configuration diagram for describing the character display control circuit according to the present invention; and 
       FIG. 3  is a block diagram of a television receiver for describing a conventional OSD output. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   At least the following matters will be made clear by the explanation in the present specification and the description of the accompanying drawings. 
   &lt;Configuration of a Character Display Control Circuit&gt; 
     FIG. 1  is a block diagram for describing a character display control circuit according to an embodiment of the present invention. As shown in the figure, a television receiver has a microcomputer  400  that includes a character display control circuit  100  and handles various information processing, the signal processing circuit  200  for handling signal processing of RGB signals output from the character display control circuit  100 , and the display  300  for displaying images based on R′, G′ and B′ signals output from the signal processing circuit  200 . 
     FIG. 2  shows a configuration for describing the character display control circuit  100  in the microcomputer  400 . In the figure, the portion enclosed by the dot and dash line is the same in configuration for all colors of R, G and B. In the embodiment of the present invention, therefore, a description will be made only on the case of R for convenience of description, and so the description of the cases of G and B will be omitted. 
   The character display control circuit  100  according to the embodiment of the present invention has an OSD output circuit  10 , an output level register (“holding circuit”)  20 , a decoder  30 , an output level selector  40  and a ladder output switching circuit (“selection circuit”)  50 . 
   The OSD output circuit  10  outputs data R (“second data”) and data I (“third data”) for adjusting the level of the data R. The OSD output circuit  10  also has a timing detection circuit  60  (“setting circuit”)—a circuit to which a horizontal synchronizing signal HSync, a vertical synchronizing signal VSync and a clock CLK, higher in frequency than the HSync, are input. The timing detection circuit  60  sets, for example, two-bit data (“first data”) in the output level register  20  for a screen position corresponding to the HSync and VSync. In the output level register  20 , data of (L, H) or (L, L) is stored as (a 1 , a 2 ). It does not matter which combinations of “H” 0  and “L” for (a 1 , a 2 ) are used as long as the output (b 1 , b 2 , b 3 ) of the decoder  30  produces the values described below. 
   The decoder  30  converts the two-bit data stored in the output level register  20  to a three-value output. For example, the output (b 1 , b 2 , b 3 ) of the decoder  30  is (L, H, L) when the input is (L, H) and (L, L, H) when the input is (L, L). That is, either of b 2  and b 3  is “H”, whereas all others are “L.” 
   AND circuits  71 ,  72  and  73  enable or disable the output of the data R according to b 1 , b 2  and b 3  of the output of the decoder  30 . Since b 1  is “L”, the AND circuit  71  outputs “L” irrespective of the data R. The AND circuit  72  outputs the data R as is when b 2  is “H” and outputs “L” irrespective of the value of the data R when b 2  is “L.” The AND circuit  73  outputs the data R as is when b 3  is “H” and outputs “L” irrespective of the value of the data R when b 3  is “L.” 
   An inverter  74  receives the data R as input and outputs a level opposite to the data R. 
   The output level selector  40 , provided with multiplexers  41 ,  42 ,  43  and  44 , controls the ladder output switching circuit  50  on the basis of the levels of the data I, data R and the decoder  30 . 
   The multiplexer  41  selectively outputs the output of the AND circuit  71  or the data R depending on the value of the data I. Specifically, the multiplexer  41  outputs the data R when the data I is “L” and outputs the output of the AND circuit  71  when the data I is “H.” 
   The multiplexer  42  selectively outputs the output of the AND circuit  72  depending on the value of the data I. The multiplexer  42  outputs “L” when the data I is “L” and outputs the output of the AND circuit  72  when the data I is “H.” 
   The multiplexer  43  selectively outputs the output of the AND circuit  73  depending on the value of the data I. The multiplexer  43  outputs “L” when the data I is “L” and outputs the output of the AND circuit  73  when the data I is “H.” 
   The multiplexer  44  selectively outputs the output of the AND circuit  74  or the data R depending on the value of the data I. The multiplexer  44  outputs a level opposite to the data R when the data I is “L” and outputs the output of the inverter  74  when the data I is “H.” 
   The multiplexers  41 ,  42 ,  43  and  44  described above are an example for implementing the present invention. Other configurations may be employed as long as the input-to-output relationship of the input level selector  40  is the same. 
   Having transmission gates  51 ,  52 ,  53  and  54 , the ladder output switching circuit  50  outputs a value obtained by resistance-dividing the voltage between power supply Vcc and ground Vss according to the output of the output level selector  40 . As shown in the figure, resistors R 1 , R 2 , R 3 , R 4  and R 5  are connected in series between the power supply Vcc (e.g., 5V) and the ground Vss (e.g., 0V). 
   The transmission gates  51 ,  52 ,  53  and  54  are connected commonly at one end, with the other end of the transmission gate  51  connected between the resistors R 1  and R 2 . The transmission gate  51 , controlled in terms of conduction by the output of the multiplexer  41 , outputs, at its conduction, a value (e.g., 3.8V) (“first voltage”), obtained by resistance-dividing the supply voltage Vcc between (R 2 +R 3 +R 4 +R 5 ) and R 1 , as an R signal. 
   Similarly, the transmission gate  52  is connected between the resistors R 2  and R 3  at the other end. The transmission gate  52 , controlled in terms of conduction by the output of the multiplexer  42 , outputs, at its conduction, a value (e.g., 2.9V), obtained by resistance-dividing the supply voltage Vcc between (R 3 +R 4 +R 5 ) and (R 1 +R 2 ), as the R signal. 
   The transmission gate  53  is connected between the resistors R 3  and R 4  at the other end. The transmission gate  53 , controlled in terms of conduction by the output of the multiplexer  43 , outputs, at its conduction, a value (e.g., 2.4V), obtained by resistance-dividing the supply voltage Vcc between (R 4 +R 5 ) and (R 1 +R 2 +R 3 ), as the R signal. 
   The transmission gate  54  is connected between the resistors R 4  and R 5  at the other end. The transmission gate  54 , controlled in terms of conduction by the output of the multiplexer  44 , outputs, at its conduction, a value (e.g., 1.0V) (“second voltage”),obtained by resistance-dividing the supply voltage Vcc between R 5  and (R 1 +R 2 +R 3 +R 4 ), as the R signal. It should be noted that one or more third voltages are defined between the maximum voltage and the minimum voltage in the output of the R signal. For example, in the present embodiment, two voltages of 2.9V and 2.4V are set as the third voltage. In this case, the “number of the third voltages” is two. 
   With the above configuration, it is possible to set the R signal to four different levels according to the data I, the data R and the register setting. Using the same configuration, four different output levels can also be set for data G and B. 
   &lt;Operation of the Character Display Control Circuit&gt; 
   A description will be given next of the operation of the character display control circuit  100  shown in  FIG. 2 . 
   First, two-bit data (a 1 , a 2 ) is set in the output level register  20  based on HSync, VSync and CLK, and then the two-bit data is converted to three values (b 1 , b 2 , b 3 ) by the decoder  30 . The b 1  is “L” as described above, whereas one of b 2  and b 3  is “H” and the other “L.” 
   i) When the Data I is “L” and the Data R “H” 
   The output of the multiplexer  41  becomes “H”, whereas the outputs of the multiplexers  42 ,  43  and  44  become “L”, turning on the transmission gate  51  of the ladder output switching circuit  50 . Therefore, a value (e.g., 3.8V), obtained by resistance-dividing the supply voltage Vcc between (R 2 +R 3 +R 4 +R 5 ) and R 1 , is output as the R signal. That is, the first voltage is output in this case irrespective of the output of the decoder  30 . 
   ii) When the Data R is “L” 
   The output of the multiplexer  44  becomes “H”, whereas the outputs of the multiplexers  41 ,  42  and  43  become “L”, turning on the transmission gate  54  of the ladder output switching circuit  50 . Therefore, a value (e.g., 1.0V), obtained by resistance-dividing the supply voltage Vcc between R 5  and (R 1 +R 2 +R 3 +R 4 ), is output as the R signal. That is, the second voltage is output in this case irrespective of the data I and the output of the decoder  30 . 
   iii) When Both the Data I and the Data R are “H” 
   First, when (b 1 , b 2 , b 3 )=(L, H, L), the output of the AND circuit  72  becomes “H”, whereas the outputs of the AND circuits  71  and  73  and the inverter  74  become “L.” Then, in the output level selector  40 , the output of the multiplexer  42  becomes “H”, whereas the outputs of the multiplexers  41 ,  43  and  44  become “L”, turning on the transmission gate  52  of the ladder output switching circuit  50 . Therefore, a value (e.g., 2.9V), obtained by resistance-dividing the supply voltage Vcc between (R 3 +R 4 +R 5 ) and (R 1 +R 2 ), is output as the R signal. 
   Next, when (b 1 , b 2 , b 3 )=(L, L, H), the output of the AND circuit  73  becomes “H”, whereas the outputs of the AND circuits  71  and  72  and the inverter  74  become “L.” Then, in the output level selector  40 , the output of the multiplexer  43  becomes “H”, whereas the outputs of the multiplexers  41 ,  42  and  44  become “L”, turning on the transmission gate  53  of the ladder output switching circuit  50 . Therefore, a value (e.g., 2.4V), obtained by resistance-dividing the supply voltage Vcc between (R 4 +R 5 ) and (R 1 +R 2 +R 3 ), is output as the R signal. 
   Thus, when the data I is “L”, the output value of the decoder  30  is not reflected, causing the transmission gate  51  or  54  to be selected depending on the value of the data R. When both the data I and data R are “H”, the output value of the decoder  30  is reflected, causing the transmission gate  52  or  53  to be selected. 
   Therefore, the character display control circuit  100  can set four-value output levels through logic operation of the data R, the data I and the output of the decoder  30 . By employing the same configuration for G and B, it is possible to set four values for each of G and B, thus resulting in 4×4×4=64 OSD display colors as possible combinations. 
   Here, while the data I can be changed in each horizontal scan period during character display, the value of the output level register  20  is fixed and cannot be changed. 
   However, the timing detection circuit  60  can output, based on HSync indicating that horizontal scan lines change at given timings, an interrupt signal—a signal for setting a value in the output level register  20 —to the microcomputer  400 . As the microcomputer  400  accepts the interrupt signal, the character display control circuit  100  can set the value of two-bit data, held in the output level register  20 , to a different value between different horizontal scan lines. Therefore, the above-described 64 colors can be used during on-screen character display, allowing enriched color display. 
   As described above, the character display control circuit  100  according to the present invention handles color adjustment of the respective RGB data and the data I without using the externally attached mixing circuit  400 , thus keeping the circuit scale unchanged. It is also possible to further increase displayable colors in number by using a register storing a two-bit data for each of RGB colors and outputting arbitrary voltage levels from the ladder output level switching circuit  50  with four taps. Additionally, output level is selected through logic operation thanks to its hardware configuration, thus lightening software processing. Further, it is possible to set a different value in the register between horizontal scan lines, thus increasing the number of displayable colors in character display. 
   While a specific description was made on the embodiment of the present invention based on the embodiment, the present invention is not limited thereto, and it should be understood that various modifications can be made without departing from the spirit of the present invention. By making simple changes such as making the output level register three-bit or longer, providing six or more resistors in the ladder output switching circuit, and configuring a decoder and output level selector to handle such an output switching, the character display control circuit can display more than 64 colors. 
   Although the preferred embodiment of the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.