Abstract:
A thermal head driver system cyclically and independently drives at least two thermal heads having each a plurality of electric resistance elements. A storage system cyclically stores an image information data, the image information data cyclically being each of at least two types of image information data, respectively corresponding to the thermal heads. A selector system cyclically and correspondingly selects which thermal head should be driven in accordance with the cyclical storage of the types of image information data in the storage system, such that the electric resistance elements of the thermal head, selected by the selector system, are selectively and electrically energized in accordance with a corresponding type of image information data cyclically stored in the storage system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal head driver system for electrically driving a thermal head, and also relates to an image-forming apparatus having such a thermal head and such a thermal head driver incorporated therein. 
     2. Description of the Related Art 
     A thermal head driver system for electrically driving a thermal head is well known. For example, the thermal head is arranged as a line type of thermal head having a plurality of electric resistance elements aligned with each other, and the thermal head driver system is constituted such that the electric resistance elements are selectively and electrically energized in accordance with a single-line of digital image-pixel signals, thereby producing an image on, for example, a thermal sensitive recording sheet. 
     Usually, the thermal head driver system includes a shift register, and a latch circuit connected in parallel to the shift register. The single-line of digital image-pixel signals is serially inputted to and is temporarily stored in the shift register, and the stored digital image-pixel signals are then shifted to the latch circuit. The shifted digital image-pixel signals are latched by the latch circuit, and are stably held therein. The latch circuit is provided with a plurality of output terminals corresponding to a number of the digital image-pixel signals held therein, and each of the output terminals outputs a high-level signal only when a corresponding digital image-pixel signal has a value “1”. 
     The thermal head driver system also includes a plurality of AND-gate circuits each having two input terminals and an output terminal, and a plurality of switching circuits associated with the AND-gate circuits, respectively. One of the input terminals of each AND-gate circuit is connected to a corresponding one of the output terminals of the latch circuit, and the other input terminal of each AND-gate circuit is wired so as to receive a strobe signal having a predetermined pulse width. The output terminal of each AND-gate circuit is connected to the switching circuit associated therewith. Each of the electric resistance elements of the line thermal head is connected to an electric power source through a corresponding switching circuit. 
     With this arrangement of the thermal head driver system, when one of the digital image-pixel signals held in the latch circuit has a value “1”, so that a high-level signal is outputted from a corresponding output terminal of the latch circuit, a corresponding AND-gate circuit is opened so that a corresponding switching circuit is turned ON, whereby a corresponding electric resistance element is electrically energized over a period corresponding to the pulse width of the strobe signal so as to be heated to a predetermined temperature. On the other hand, when one of the digital image-pixel signals held in the latch circuit has a value “0”, a corresponding AND-gate circuit is maintained at a closed state, so that a corresponding switching circuit also maintains an OFF state, whereby a corresponding electric resistance element cannot be electrically energized. 
     Conventionally, one thermal head necessarily involves one thermal head driver system as mentioned above, and these two elements are inseparably related to each other. In other words, a thermal head driver system is provided for the purpose of driving only a single thermal head. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a novel thermal head driver system arranged to selectively drive at least two thermal heads in accordance with at least two types of image information data, respectively, without a thermal head driver system being necessary for each thermal head. 
     Another object of the present invention is to provide a thermal image-forming apparatus including at least two thermal heads, which are selectively driven by the above-mentioned novel thermal head driver system in accordance with at least two types of image information data. 
     In accordance with an aspect of the present invention, there is provided a thermal head driver system that cyclically and independently drives each of at least two thermal heads having each a plurality of electric resistance elements. The thermal head driver system comprises a storage system that cyclically stores an image information data, the image information data cyclically being each of at least two types of image information data, respectively corresponding to the at least two thermal heads, and a selector system that cyclically and correspondingly selects which thermal head should be driven in accordance with the cyclical storage of the at least two types of image information data in the storage system, such that the electric resistance elements of the thermal head, selected by the selector system, are selectively and electrically energized in accordance with a corresponding type of image information data cyclically stored in the storage system. Preferably, the thermal head driver system further comprises a determiner system that determines a time period over which the thermal head, selected by the selector system, is driven. 
     The selector system may comprise a signal generator that generates at least two selection-control signals, each of which changes between a first level and a second level, and the cyclical selection of the driving of the at least two thermal heads is performed in accordance with a combination of the levels of the at least two selection-control signals. In this case, when at least one of the at least two selection-control signals is changed from the first level to the second level, one of the thermal heads is correspondingly selected to be driven by the selector system. Also, when the at least two selection-control signals are kept at the first level, none of the thermal heads are selected to be driven by the selector system. 
     In accordance with another aspect of the present invention, there is provided an image-forming apparatus that forms an image on an image-forming substrate that includes a base member and a layer of microcapsules, coated over the base member, containing a first type of microcapsule filled with a first monochromatic dye, and a second type of microcapsule filled with a second monochromatic dye, the first type of microcapsule exhibiting a first pressure/temperature characteristic such that, when the first type of microcapsule is squashed under a first pressure at a first temperature, the first type of microcapsule breaks discharging the first dye, the second type of microcapsule exhibiting a second pressure/temperature characteristic such that, when the second type of microcapsule is squashed under a second pressure at a second temperature, the second type of microcapsule breaks discharging the second dye. The image-forming apparatus comprises a first pressure applicator that locally exerts the first pressure on the layer of microcapsules, a second pressure applicator that locally exerts the second pressure on the layer of microcapsules, a first thermal head that is driven such that a first localized area of the layer of microcapsules, on which the first pressure is exerted by the first pressure applicator, is heated to the first temperature in accordance with a first image-information data, such that the first type of microcapsule in the first localized area is selectively broken, a second thermal head that is driven such that a second localized area of the layer of microcapsules, on which the second pressure is exerted by the second pressure applicator, is heated to the second temperature in accordance with a second image-information data, such that the second type of microcapsule in the second localized area is selectively squashed, and a thermal head driver system that cyclically and independently controls the driving of the first and second thermal heads, and that is arranged in accordance with the first-mentioned aspect of the present invention. 
     The layer of microcapsules may further contains a third type of microcapsule filled with a third monochromatic dye, the third type of microcapsule exhibiting a third pressure/temperature characteristic such that, when the third type of microcapsule is squashed under a third pressure at a third temperature, the third type of microcapsule breaks discharging the third dye. In this case, the image-forming apparatus comprises further comprises a third pressure applicator that locally exerts the third pressure on the layer of microcapsules, and a third thermal head that is driven such that a third localized area of the layer of microcapsules, on which the third pressure is exerted by the third pressure applicator, is heated to the third temperature in accordance with a third image-information data, such that the third type of microcapsule in the third localized area is selectively squashed. Also, the thermal head driver system cyclically and independently controls the driving of the first, second and third thermal heads, the storage system cyclically stores an image information data, the image information data cyclically being each of the first, second and third image information data, and the selector system cyclically and correspondingly selects which thermal head should be driven in accordance with the cyclical storage of the first, second and third image information data in the storage system. 
     The selector system may comprise a signal generator that generates two selection-control signals, each of which changes between a first level and a second level, and the cyclical selection of the driving of the first, second and third thermal heads is performed in accordance with a combination of the levels of the selection-control signals. When at least one of the two selection-control signals is changed from the first level to the second level, one of the first, second and third thermal heads is correspondingly selected to be driven by the selector system. Also, when all of the two selection-control signals are kept at the first level, none of the first, second and third thermal heads is selected to be driven by the selector system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These objects and other objects of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic conceptual cross-sectional view showing an image-forming substrate, comprising a layer of microcapsules including a first type of cyan microcapsules filled with a cyan dye, a second type of magenta microcapsules filled with a magenta dye and a third type of yellow microcapsules filled with a yellow dye, used in an image-forming apparatus according to the present invention; 
     FIG. 2 is a graph showing a characteristic curve of a longitudinal elasticity coefficient of a shape memory resin; 
     FIG. 3 is a graph showing pressure/temperature breaking characteristics of the respective cyan, magenta and yellow microcapsules shown in FIG. 1, with each of a cyan-developing area, a magenta-developing area and a yellow-developing are indicated as a hatched area; 
     FIG. 4 is a schematic cross-sectional view showing different shell wall thicknesses of the respective cyan, magenta and yellow microcapsules; 
     FIG. 5 is a schematic conceptual cross-sectional view similar to FIG. 1, showing only a selective breakage of a cyan microcapsule in the layer of microcapsules; 
     FIG. 6 a schematic cross-sectional view of an image-forming apparatus, according to the present invention, for forming a color image on the image-forming substrate shown in FIG. 1; 
     FIG. 7 is a partial schematic block diagram of three line-type thermal heads and a thermal head driver circuit therefor incorporated in the color printer of FIG. 6; 
     FIG. 8 is a schematic block diagram of a control circuit board of the color printer shown in FIG. 6; 
     FIG. 9 is a partial schematic wiring diagram of the thermal head driver circuit of FIGS. 7 and 8; 
     FIG. 10 is a table for explaining how one of the three thermal heads to be driven is selected by a combination of levels of two selection-control signals inputted to the thermal head driver circuit; 
     FIG. 11 is a part of a flowchart of a thermal-head-driver control routine executed in a printer control circuit of FIG. 8; 
     FIG. 12 is the remaining part of the flowchart of the thermal-head-driver control routine executed in the printer control circuit of FIG. 8; and 
     FIG. 13 is a timing chart used to explain the thermal-head-driver control routine shown in FIGS.  11  and  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an image-forming substrate, generally indicated by reference  10 , which is used in an image-forming apparatus according to the present invention. The image-forming substrate  10  is produced in a form of a paper sheet. Namely, the image-forming substrate or sheet  10  comprises a sheet of paper  12 , a layer of microcapsules  14  coated over a surface of the paper sheet  12 , and a sheet of protective transparent film  16  covering the microcapsule layer  14 . 
     The microcapsule layer  14  is formed from three types of microcapsules: a first type of microcapsules  18 C filled with cyan liquid dye or ink, a second type of microcapsules  18 M filled with magenta liquid dye or ink, and a third type of microcapsules  18 Y filled with yellow liquid dye or ink, and these microcapsules  18 C,  18 M and  18 Y are uniformly distributed in the microcapsule layer  14 . In each type of microcapsule ( 18 C,  18 M,  18 Y), a shell wall of a microcapsule is formed of a synthetic resin material, usually colored white. Also, each type of microcapsule ( 18 C,  18 M,  18 Y) may be produced by a well-known polymerization method, such as interfacial polymerization, in-situ polymerization or the like, and may have an average diameter of several microns, for example, 5 μm to 10 μm. 
     Note, when the paper sheet  12  is colored with a single color pigment, the resin material of the microcapsules  18 C,  18 M and  18 Y may be colored by the same single color pigment. 
     For the uniform formation of the microcapsule layer  14 , for example, the same amounts of cyan, magenta and yellow microcapsules  18 C,  18 M and  18 Y are homogeneously mixed with a suitable binder solution to form a suspension, and the paper sheet  12  is coated with the binder solution, containing the suspension of microcapsules  18 C,  18 M and  18 Y, by using an atomizer. 
     Note, in FIG. 1, for the convenience of illustration, although the microcapsule layer  14  is shown as having a thickness corresponding to the diameter of the microcapsules  18 C,  18 M and  18 Y, in reality, the three types of microcapsules  18 C,  18 M and  18 Y overlay each other, and thus the microcapsule layer  14  has a larger thickness than the diameter of a single microcapsule  18 C,  18 M or  18 Y. 
     In the image-forming sheet  10 , for the resin material of each type of microcapsule ( 18 C,  18 M,  18 Y), a shape memory resin may be utilized. As is well known, for example, the shape memory resin is represented by a polyurethane-based-resin, such as polynorbornene, trans-1,4-polyisoprene polyurethane. As other types of shape memory resin, a polyimide-based resin, a polyamide-based resin, a polyvinylchloride-based resin, a polyester-based resin and so on are also known. 
     In general, as is apparent from a graph of FIG. 2, the shape memory resin exhibits a coefficient of longitudinal elasticity, which abruptly changes at a glass-transition temperature boundary T g . In the shape memory resin, Brownian movement of the molecular chains is stopped in a low-temperature area “a”, which is less than the glass-transition temperature T g , and thus the shape memory resin exhibits a glass-like phase. On the other hand, Brownian movement of the molecular chains becomes increasingly energetic in a high-temperature area “b”, which is higher than the glass-transition temperature T g , and thus the shape memory resin exhibits a rubber elasticity. 
     The shape memory resin is named due to the following shape memory characteristic: after a mass of the shape memory resin is worked into a shaped article in the low-temperature area “a”, when such a shaped article is heated over the glass-transition temperature T g , the article becomes freely deformable. After the shaped article is deformed into another shape, when the deformed article is cooled to below the glass-transition temperature T g , the other shape of the article is fixed and maintained. Nevertheless, when the deformed article is again heated to above the glass-transition temperature T g , without being subjected to any load or external force, the deformed article returns to the original shape. 
     In the image-forming sheet  10 , the shape memory characteristic per se is not utilized, but the characteristic abrupt change of the shape memory resin in the longitudinal elasticity coefficient is utilized, such that the three types of microcapsules  18 C,  18 M and  18 Y can be selectively squashed and broken at different temperatures and under different pressures, respectively. 
     As shown in a graph of FIG. 3, a shape memory resin of the cyan microcapsules  18 C is prepared so as to exhibit a characteristic longitudinal elasticity coefficient, indicated by a solid line, having a glass-transition temperature T 1 ; a shape memory resin of the magenta microcapsules  18 M is prepared so as to exhibit a characteristic longitudinal elasticity coefficient, indicated by a single-chained line, having a glass-transition temperature T 2 ; and a shape memory resin of the yellow microcapsules  18 Y is prepared so as to exhibit a characteristic longitudinal elasticity coefficient, indicated by a double-chained line, having a glass-transition temperature T 3 . 
     Note, by suitably varying compositions of the shape memory resin and/or by selecting a suitable one from among various types of shape memory resin, it is possible to obtain the respective shape memory resins, with the glass-transition temperatures T 1 , T 2  and T 3 . For example, the glass-transition temperatures T 1 , T 2  and T 3  may be set to 70° C., 110° C. and 130° C., respectively. 
     As shown in FIG. 4, the microcapsule walls of the cyan microcapsules  18 C, magenta microcapsules  18 M, and yellow microcapsules  18 Y have differing thicknesses W C , W M  and W Y , respectively. Namely, the thickness W C  of cyan microcapsules  18 C is larger than the thickness W M  of magenta microcapsules  18 M, and the thickness W M  of magenta microcapsules  18 M is larger than the thickness W Y  of yellow microcapsules  18 Y. 
     Also, the wall thickness W C  of the cyan microcapsules  18 C is selected such that each cyan microcapsule  18 C is compacted and broken under a breaking pressure that lies between a critical breaking pressure P 3  and an upper limit pressure P UL  (FIG.  3 ), when each cyan microcapsule  18 C is heated to a temperature between the glass-transition temperatures T 1  and T 2 ; the wall thickness W M  of the magenta microcapsules  18 M is selected such that each magenta microcapsule  18 M is compacted and broken under a breaking pressure that lies between a critical breaking pressure P 2  and the critical breaking pressure P 3  (FIG.  3 ), when each magenta microcapsule  18 M is heated to a temperature between the glass-transition temperatures T 2  and T 3 ; and the wall thickness W Y  of the yellow microcapsules  18 Y is selected such that each yellow microcapsule  18 Y is compacted and broken under a breaking pressure that lies between a critical breaking pressure P 1  and the critical breaking pressure P 2  (FIG.  3 ), when each yellow microcapsule  18 Y is heated to a temperature between the glass-transition temperature T 3  and an upper limit temperature T UL . 
     Note, for example, the breaking-pressures P 1 , P 2 , P 3  and P UL  may be set to 0.02, 0.2, 2.0 and 20 MPa, respectively, and a wall thickness of a microcapsule ( 18 C,  18 M,  18 Y) concerned is selected such that it is compacted and broken under a given breaking pressure when it is heated to a given temperature. Also, note, the upper limit temperature T UL  is suitably set to, for example, 150° C. 
     Thus, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming sheet  10 , it is possible to selectively squash and break the cyan, magenta and yellow microcapsules  18 C,  18 M and  18 Y. 
     For example, if the selected heating temperature and breaking pressure fall within a hatched cyan-developing area C (FIG.  3 ), defined by a temperature ranging between the glass-transition temperatures T 1  and T 2  and by a pressure ranging between the critical breaking pressure P 3  and the upper limit pressure P UL , only the cyan microcapsules  18 C are squashed and broken, as representatively shown in FIG.  5 . Also, if the selected heating temperature and breaking pressure fall within a hatched magenta-developing area M, defined by a temperature ranging between the glass-transition temperatures T 2  and T 3  and by a pressure ranging between the critical breaking pressures P 2  and P 3 , only the magenta microcapsules  18 M are squashed and broken. Further, if the selected heating temperature and breaking pressure fall within a hatched yellow-developing area Y, defined by a temperature ranging between the glass-transition temperature T 3  and the upper limit temperature T UL  and by a pressure ranging between the critical breaking pressures P 1  and P 2 , only the yellow microcapsules  18 Y are squashed and broken. 
     Accordingly, if the selection of a heating temperature and a breaking pressure, which should be exerted on the image-forming sheet  10 , are suitably controlled in accordance with a series of digital color image-pixel signals: digital cyan image-pixel signals, digital magenta image-pixel signals and digital yellow image-pixel signals, it is possible to form a color image on the image-forming sheet  10  on the basis of the digital color image-pixel signals. 
     With reference to FIG. 6, the image-forming apparatus according to the present invention is schematically shown, and is constituted as a line color printer so as to form a color image on the aforementioned image-forming sheet  10 . 
     The color printer comprises a rectangular parallelopiped housing  20  having an entrance opening  22  and an exit opening  24  formed in a top wall and a side wall of the housing  20 , respectively. The image-forming sheet  10  (not shown in FIG. 6) is introduced into the housing  20  through the entrance opening  22 , and is then discharged from the exit opening  24  after the formation of a color image on the image-forming sheet  10 . Note, in FIG. 6, a path  26  for movement of the image-forming sheet  10  is indicated by a chained line. 
     A guide plate  28  is provided in the housing  20  so as to define a part of the path  26  for the movement of the image-forming sheet  10 , and a first thermal head  30 C, a second thermal head  30 M and a third thermal head  30 Y are securely attached to a surface of the guide plate  28 . Each thermal head ( 30 C,  30 M,  30 Y) is formed as a line thermal head perpendicularly extended with respect to a direction of the movement of the image-forming sheet  10 . 
     As conceptually shown in FIG. 7, the line thermal head  30 C includes a plurality of heater elements or electric resistance elements R c1  to R cn  (where n=1, 2, 3, . . . ), and these electric resistance elements R c1  to R cn  are linearly aligned with respect to each other along a length of the line thermal head  30 C. Also, the line thermal head  30 M includes a plurality of electric resistance elements R m1  to R mn  (where n=1, 2, 3, . . . ), and these electric resistance elements R m1  to R mn  are linearly aligned with respect to each other along a length of the line thermal head  30 M. Similarly, the line thermal head  30 Y includes a plurality of electric resistance elements R y1  to R yn  (where n=1, 2, 3, . . . ), and these resistance elements are linearly aligned with respect to each other along a length of the line thermal head  30 Y. 
     According to the present invention, each of the electric resistance elements (R c1  to R cn ; R m1  to R mn ; and R y1  to R yn ) is selectively energized by a thermal head driver circuit  31  in accordance with a corresponding monochromatic (cyan, yellow, magenta) digital image-pixel signal in a manner as stated in detail hereinafter. Of course, when a digital cyan image-pixel signal has a value “1”, a corresponding electric resistance element R cn  is heated to a temperature, which falls in the range between the glass-transition temperatures T 1  and T 2 ; when a digital magenta image-pixel signal has a value “1”, a corresponding electric resistance element R mn  is heated to a temperature, which falls in the range between the glass-transition temperatures T 2  and T 3 ; when the digital yellow image-pixel signal has a value “1”, the corresponding electric resistance element R yn  is heated to a temperature, which falls in the range between the glass-transition temperature T 3  and the upper limit temperature T UL . 
     Note, the line thermal heads  30 C,  30 M and  30 Y are arranged in sequence so that the respective heating temperatures increase in the movement direction of the image-forming substrate  10 . 
     As shown in FIG. 6, the color printer further comprises a first roller platen  32 C, a second roller platen  32 M and a third roller platen  32 Y associated with the first, second and third thermal heads  30 C,  30 M and  30 Y, respectively, and each of the roller platens  32 C,  32 M and  32 Y may be formed of a suitable hard rubber material. The first roller platen  32 C is provided with a first spring-biasing unit  34 C so as to be elastically pressed against the first thermal head  30 C at a pressure between the critical compacting-pressure P 3  and the upper limit pressure P UL ; the second roller platen  32 M is provided with a second spring-biasing unit  34 M so as to be elastically pressed against the second thermal head  30 M at a pressure between the critical compacting-pressures P 2  and P 3 ; and the third roller platen  32 Y is provided with a third spring-biasing unit  34 Y so as to be elastically pressed against the second thermal head  30 Y at a pressure between the critical compacting-pressures P 1  and P 2 . 
     During a printing operation, the respective roller platens  32 C,  32 M and  32 Y are intermittently rotated in a counterclockwise direction (FIG. 6) with a same peripheral speed. Accordingly, the image-forming sheet  10 , introduced through the entrance opening  22 , intermittently moves toward the exit opening  24  along the path  26 . Thus, the image-forming sheet  10  is subjected to pressure ranging between the critical breaking-pressure P 3  and the upper limit pressure P UL  when passing between the first line thermal head  30 C and the first roller platen  32 C; to pressure ranging between the critical breaking-pressures P 2  and P 3  when passing between the second line thermal head  30 M and the second roller platen  32 M; and to pressure ranging between the critical breaking-pressures P 1  and P 2  when passing between the third line thermal head  30 Y and the third roller platen  32 Y. Namely, the roller platens  32 C,  32 M and  32 Y are arranged in sequence so that the respective pressures, exerted by the platens  32 C,  32 M and  32 Y on the line thermal heads  30 C,  30 M and  30 Y, decrease in the movement direction of the image-forming substrate  10 . 
     Note, the introduction of the image-forming sheet  10  into the entrance opening  22  of the printer is carried out such that the transparent protective film sheet  16  of the image-forming sheet  10  comes into contact with the thermal heads  30 C,  30 M and  30 Y. 
     With the arrangement of the above-mentioned line printer, for example, when one of the electric resistance elements R cn  is heated to a temperature in the range between the glass-transition temperatures T 1  and T 2 , a cyan dot, having a dot size (diameter) of 50 μm to 100 μm, is developed on the microcapsule layer  14  of the image-forming sheet  10 , because only the cyan microcapsules  18 C are squashed and broken at a dot area heated by the resistance element (R cn ) concerned. Of course, although a plurality of cyan, magenta and yellow microcapsules  18 C,  18 M and  18 Y are uniformly included in a dot area (50 μm to 100 μm) to be developed on the microcapsule layer  14 , it is possible to squash and break only the cyan microcapsules  18 C, because the heating temperature is within the range between the glass-transition temperatures T 1  and T 2 . 
     In FIG. 6, reference  36  indicates a control circuit board for controlling a printing operation of the color printer, and reference  38  indicates an electrical main power source for electrically energizing the control circuit board  36 . 
     FIG. 8 shows a schematic block diagram of the control circuit board  36 . As shown in this drawing, the control circuit board  36  comprises a printer control circuit  40  including a microcomputer. The printer control circuit  40  receives a series of digital color image-pixel signals from a personal computer or a word processor (not shown) through an interface circuit (I/F)  42 . The received digital color image-pixel signals are suitably processed and are converted into a frame of digital cyan image-pixel signals, a frame of digital magenta image-pixel signals, and a frame of digital yellow image-pixel signals, and these frames of digital color image-pixel signals are once stored in a memory  44 . 
     Also, the control circuit board  36  is provided with a motor driver circuit  46  for driving three electric motors  48 C,  48 M and  48 Y, which are used to rotationally drive the roller platens  32 C,  32 M and  32 Y, respectively. In this embodiment of the color printer, each of the motors  48 C,  48 M and  48 Y is a stepping motor, which is driven in accordance with a series of drive pulses outputted from the motor driver circuit  46 , the outputting of drive pulses from the motor driver circuit  46  to the motors  48 C,  48 M and  48 Y being controlled by the printer control circuit  40 . 
     As shown in FIG. 8, the thermal head driver circuit  31  for the line thermal heads  30 C,  30 M and  30 Y is included in the control circuit board  36 , and is controlled by a set of selection-control signals “ST1” and “ST2”, a series of clock pulses “CLK”, a low-active latch signal “LATCH” and a series of digital color image-pixel signals “DATA”, which are outputted from the printer control circuit  40 . 
     FIG. 9 partially shows an arrangement of the thermal head driver circuit  31 . As is apparent from this drawing, the thermal head driver circuit  31  comprises a shift register  50  including a plurality of D-type flip-flops  50   1  to  50   n  (where n=1, 2, 3, . . . ), and a latch circuit  52  including a plurality of D-type latches  52   1  to  52   n  (where n=1, 2, 3, . . . ). During a printing operation, a single-line of monochromatic (cyan, magenta, yellow) digital image-pixel signals “DATA” is read from the memory  44 , and is then inputted to the shift register  50 . 
     While the series of monochromatic digital image-pixel signals “DATA” is inputted to the shift register  50 , these digital image-pixel signals are successively shifted to the flip-flops  50   1  to  50   n  in accordance with the series of clock pulses “CLK”. Then, the respective monochromatic image-pixel signals held by the flip-flops  50   1  to  50   n  are simultaneously shifted to the latches  52   1  to  52   n  of the latch circuit  52 , and are latched by outputting the low-active latch signal “LATCH” from the printer control circuit  40  to the latch circuit  52  through an invertor  53  (FIG.  9 ), whereby the respective digital image-pixel signals are stably held in the latches  52   1  to  52   n . Thus, either a high-level signal or a low-level signal is stably outputted from a Q-terminal of each latch ( 52   1 , . . . ,  52   n ) in accordance with binary values of a corresponding monochromatic digital image-pixel signal held therein. Namely, when the digital image-pixel signal has a value “1”, the high-level signal is outputted from the Q-terminal of the corresponding latch ( 52   1 , . . . ,  52   n ), and, when the digital image-pixel signal has a value “0”, the low-level signal is outputted from the Q-terminal of the corresponding latch ( 52   1 , . . . ,  52   n ). 
     The thermal head driver circuit  31  further comprises a plurality of driver circuit elements  54   1  to  54   n  (where n=1, 2, 3, . . . ). Each of the driver circuit elements  54   1  to  54   n  includes a set of AND-gate circuits  56 C,  56 M and  56 Y, a set of field-effect transistors (FET)  58 C,  58 M and  58 Y, and a pair of invertors  60 A and  60 B, all being wired in a manner as shown in FIG.  9 . 
     In particular, each of the AND-gate circuits  56 C,  56 M and  56 Y has three input terminals, one of which is connected to the Q-terminal of the corresponding latch ( 52   1 , . . . ,  52   n ), and the respective remaining input terminals of each AND-gate circuit ( 56 C,  56 M,  56 Y) are connected to two signal lines SL 1  and SL 2 , through which the selection-control signals “ST1” and “ST1” are fed, respectively. Note, as shown in FIG. 9, the invertor  60 A is interposed between the signal line SL 1  and the corresponding input terminal of the AND-gate circuit  56 C, and the inverter  60 B is interposed between the signal line SL 2  and the corresponding input terminal of the AND-gate circuit  56 M. 
     Also, each of the AND-gate circuits  56 C,  56 M and  56 Y has an output terminal, which is connected to a gate (G) of the corresponding FET ( 58 C,  58 M,  58 Y) A source (S) of each FET ( 58 C,  58 M,  58 Y) is connected to an electric power source (V h ), and respective drains (D) of the FETs  58 C,  58 M and  58 Y are connected to the electric resistance elements R cn , R mn  and R yn . Of course, when an output level of each AND gate circuit ( 56 C,  56 M,  56 Y) is changed from a low-level to a high-level, the corresponding FET ( 58 C,  58 M,  58 Y) is turned ON, so that the corresponding electric resistance element (R cn , R mn , R yn ) is electrically energized. 
     With the arrangement of the aforementioned thermal head driver circuit  31 , usually both the selection-control signals “ST1” and “ST2” are maintained at a low-level under control of the printer control circuit  40 , so that all the output levels of the AND-gate circuit ( 56 C,  56 M and  56 Y) are also maintained at the low-level, whereby all the electric resistance elements R cn , R mn  and R yn  cannot be electrically energized. 
     When the digital cyan image-pixel signals included in the single-line are held in the respective latches  52   1  to  52   n , and when these latches  52   1  to  52   n  are latched, only an output level of the selection-control signal “ST2” is changed from the low-level to a high-level, so that only the respective electric resistance elements R c1  to R cn  are selectively energized in accordance with the digital cyan image-pixel signals held in the latches  52   1  to  52   n . Namely, for example, when the digital cyan image-pixel signal held in the latch  52   1  has a value “1”, the output level of the corresponding AND-gate circuit  56 C is changed from the low-level to the high-level, whereby the corresponding electric resistance element R c1  is electrically energized. On the other hand, when the digital cyan image-pixel signal held in the latch  52   1  has a value “0”, the output level of the corresponding AND-gate circuit  56 C is maintained at the low-level, whereby the corresponding electric resistance element R c1  cannot be electrically energized. 
     When the digital magenta image-pixel signals included in the single-line are held in the respective latches  52   1  to  52   n , and when these latches  52   1  to  52   n  are latched, only an output level of the selection-control signal “ST1” is changed from the low-level to a high-level, so that only the respective electric resistance elements R m1  to R mn  are selectively energized in accordance with the digital magenta image-pixel signals held in the latches  52   1  to  52   n . Namely, for example, when the digital magenta image-pixel signal held in the latch  52   1  has a value “1”, the output level of the corresponding AND-gate circuit  56 M is changed from the low-level to the high-level, whereby the corresponding electric resistance element R m1  is electrically energized. On the other hand, when the digital magenta image-pixel signal held in the latch  52   1  has a value “0”, the output level of the corresponding AND-gate circuit  56 M is maintained at the low-level, whereby the corresponding electric resistance element R m1  cannot be electrically energized. 
     When the digital yellow image-pixel signals included in the single-line are held in the respective latches  52   1  to  52   n , and when these latches  52   1  to  52   n  are latched, both output levels of the selection-control signals “ST1” and “ST2” are changed from the low-level to the high-level, so that only the respective electric resistance elements R y1  to R yn  are selectively energized in accordance with the digital yellow image-pixel signals held in the latches  52   1  to  52   n . Namely, for example, when the digital yellow image-pixel signal held in the latch  52   1  has a value “1”, the output level of the corresponding AND-gate circuit  56 Y is changed from the low-level to the high-level, whereby the corresponding electric resistance element R y1  is electrically energized. On the other hand, when the digital yellow image-pixel signal held in the latch  52   1  has a value “0”, the output level of the corresponding AND-gate circuit  56 Y is maintained at the low-level, whereby the corresponding electric resistance element R y1  cannot be electrically energized. 
     In short, by a combination of the levels of the selection-control signals “ST1” and “ST2”, it is possible to select which thermal head ( 30 C,  30 M,  30 Y) should be driven so that the electric resistance elements (R c1  to R cn ; R m1  to R mn ; R y1  to R yn ) included in the corresponding thermal head ( 30 C,  30 M,  30 Y) are selectively and electrically energized, as shown in a TABLE of FIG.  10 . 
     Whenever the electric resistance elements R c1  to R cn  are selectively and electrically energized, the electrical energization is continued until the electrically-energized electric resistance elements (R cn ) are heated to a temperature between the glass-transition temperatures T 1  and T 2 , and the electrical energization is stopped by returning the high-level of the selection-control signal “ST2” to the low-level when the heated resistance elements (R cn ) have reached the temperature between the glass-transition temperatures T 1  and T 2 . For example, a period of the electrical energization of the electric resistance elements (R cn ) may be set to 3 ms. 
     Whenever the electric resistance elements R m1  to R mn  are selectively and electrically energized, the electrical energization is continued until the electrically-energized electric resistance elements (R mn ) are heated to a temperature between the glass-transition temperatures T 2  and T 3 , and the electrical energization is stopped by returning the high-level of the selection-control signal “ST1” to the low-level when the heated resistance elements (R mn ) have reached the temperature between the glass-transition temperatures T 2  and T 3 . For example, a period of the electrical energization of the electric resistance elements (R mn ) may be set to 4 ms. 
     Whenever the electric resistance elements R y1  to R yn  are selectively and electrically energized, the electrical energization is continued until the electrically-energized electric resistance elements (R yn ) are heated to a temperature between the glass-transition temperature T 3  and the upper limit temperature T UL , and the electrical energization is stopped by returning the high-levels of the selection-control signals “ST1” and “ST2” to the low-levels when the heated resistance elements (R yn ) have reached the temperature between the glass-transition temperature T 3  and the upper limit temperature T UL . For example, a period of the electrical energization of the electric resistance elements (R cn ) may be set to 5 ms. 
     FIGS. 11 and 12 show a flowchart of a thermal-head-driver control routine executed by the printer control circuit  40 . This thermal-head-driver control routine is constituted as a time-interruption routine which is repeatedly executed at regular intervals of, for example, 5 μs, and the execution of this routine is started when the printer control circuit  40  receives a printing-operation-start signal from a personal computer or a word processor (not shown) through the interface circuit (I/F)  42 . 
     In this embodiment, the execution of the thermal-head-driver control routine is performed under the following conditions: 
     (a) During the printing operation, three single-lines three-primary color (cyan, magenta and yellow) digital image-pixel signals are successively read in a cycle from the memory  44 , and are outputted from the printer control circuit  40  to the shift register  50  in the order of a single-line of cyan digital image-pixel signals, a single-line of magenta digital image-pixel signals and a single-line of yellow digital image-pixel signals, before the cycle is again repeated. Also, the low-active latch signal “LATCH” cyclically produces three latch pulses: a first latch pulse for latching the cyan digital image-pixel signals, a second latch pulse for latching the magenta digital image-pixel signals, and a third latch pulse the yellow digital image-pixel signals; 
     (b) The thermal heads  30 C,  30 M and  30 Y are spaced apart from each other by a distance corresponding to, for example, 200 single-lines of image-dots recorded on the image-forming sheet  10 . For this reason, the single-line of magenta digital image-pixel signals is repeatedly outputted as a dummy single-line of image-pixel signals, all having a value “0”, until the first single-line of cyan image-dots, recorded by the alignment of electric resistance elements R c1  to R cn  of the thermal head  30 C, reaches the alignment of electric resistance elements R m1  to R mn  of the thermal head  30 M, and the single-line of yellow digital image-pixel signals is also repeatedly outputted as a dummy single-line of image-pixel signals, all having a value “0”, until the first single-line of cyan image-dots, recorded by the alignment of electric resistance elements R c1  to R cn  of the thermal head  30 C, reaches the alignment of electric resistance elements R y1  to R yn  of the thermal head  30 Y; and 
     (c) For the same reason, the single-line of cyan digital image-pixel signals is repeatedly outputted as a dummy single-line of image-pixel signals, all having a value “0”, until the last single-line of cyan image-dots, recorded by the alignment of electric resistance elements R c1  to R cn  of the thermal head  30 C, reaches the alignment of electric resistance elements R y1  to R yn  of the thermal head  30 Y, and the single-line of magenta digital image-pixel signals is also repeatedly outputted as a dummy single-line of image-pixel signals, all having a value “0”, until the last single-line of magenta image-dots, recorded by the alignment of electric resistance elements R m1  to R mn  of the thermal head  30 M reaches the alignment of electric resistance elements R y1  to R yn  of the thermal head  30 Y. 
     With reference to a timing chart shown in FIG. 13, the thermal-head-driver control routine will be now explained below. 
     At step  101 , it is determined whether a flag F 1  is “0” or “1”. At an initial stage in which the printing operation has just begun, since F 1 =0, the control proceeds to step  102 , and it is determined whether a first latch pulse of the low-active latch signal “LATCH”, indicated by reference LAT 1  in the timing chart of FIG. 13, is outputted from the printer control circuit  40  to the latch circuit  52 . If the outputting of the first latch pulse “LAT1” is not confirmed, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, there is no progress until the outputting of the first latch pulse “LAT1” is confirmed. 
     In the beginning of the printing operation, a first single-line of digital cyan image-pixel signals, indicated by reference C 1 (DATA), is inputted in the shift register  50 , and these digital cyan image-pixel signals C 1 (DATA) are successively shifted to the flip-flops  50   1  to  50   n  in accordance with the series of clock pulses “CLK”, as shown in the timing chart of FIG.  13 . Then, the respective digital cyan image-pixel signals C 1 (DATA) held by the flip-flops  50   1  to  50   n  are simultaneously shifted to the latches  52   1  to  52   n  of the latch circuit  52 , and are latched by an outputting of the first latch pulse “LAT1”. 
     At step  102 , when the outputting of the first latch pulse “LAT1” is confirmed, the control proceeds to step  103 , in which the flag F 1  is made to be “1”. Then, at step  104 , it is determined whether a flag F 2  is “0” or “1”. At the initial stage, since F 2 =0, the control proceeds to step  105 , in which the selection-control signal “ST2” is made to be high, whereby only the electric resistance elements R c1  to R cn  of the thermal head  30 C are selectively and electrically energized in accordance with the cyan image-pixel signals C 1 (DATA) held in the latches  52   1  to  52   n  of the latch circuit  52 . 
     At step  106 , it is determined whether a count number of a counter CC has reached a numerical value of 600, which corresponds to a time period of 3 ms (3 ms/5 μs=600). At the initial stage, since CC=0, the control proceeds to step  107 , in which the count number of the counter CC is incremented by “1”. Then, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, the incrementing of the count number of the counter CC is merely carried out until the count number of the counter CC reaches the numerical value of 600 (with F 1 =1 and F 2 =0). 
     At step  106 , when it is confirmed that the count number of the counter CC has reached the numerical value of 600, the control proceeds from step  106  to step  108 , in which the selection-control signal “ST2” is returned to the low-level, so that the selective and electrical energization of the electric resistance elements R c1  to R cn  of the thermal head  30 C is stopped. 
     At step  109 , the counter CC is reset to “0”. Then, at step  110 , the flag F 1  is made to be “0”, and the flag F 2  is made to be “1”. Thus, the routine once ends. 
     When the routine is executed after the time of 5 μs has elapsed, the control proceeds to step  102  via step  101  (F 1 =0 at step  110 ), in which it is determined whether a second latch pulse of the low-active latch signal “LATCH”, indicated by reference LAT 2  in the timing chart of FIG. 13, is outputted from the printer control circuit  40  to the latch circuit  52 . If the outputting of the second latch pulse “LAT2” is not confirmed, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, there is no progress until the outputting of the second latch pulse “LAT2” is confirmed. 
     As is apparent from the timing chart of FIG. 13, during the selective and electrical energization of the electric resistance elements R c1  to R cn  of the thermal head  30 C, a first single-line of digital magenta image-pixel signals, indicated by reference M 1 (DATA), is inputted to the shift register  50 , and these digital magenta image-pixel signals M 1  (DATA) are successively shifted to the flip-flops  50   1  to  50   n  in accordance with the series of clock pulses “CLK”, as shown in the timing chart of FIG.  13 . Then, the respective digital magenta image-pixel signals M 1  (DATA) held by the flip-flops  50   1  to  50  are simultaneously shifted to the latches  52   1  to  52   n  of the latch circuit  52 , and are latched by an outputting of the second latch pulse “LAT2”. 
     At step  102 , when the outputting of the second latch pulse “LAT2” is confirmed, the control proceeds to step  103 , in which the flag F 1  is made to be “1”. Then, the control jumps from step  104  to step  111  (F 2 =1), in which it is determined whether a flag F 3  is “0” or “1”. At the initial stage, since F 3 =0, the control proceeds to step  112 , in which the selection-control signal “ST1” is made to be high, whereby only the electric resistance elements R m1  to R mn  of the thermal head  30 M are selectively and electrically energized in accordance with the magenta image-pixel signals M 1 (DATA) held in the latches  52   1  to  52   n of the latch circuit  52 . 
     At step  113 , it is determined whether a count number of a counter MC has reached a numerical value of 800, which corresponds to a time period of 4 ms (4 ms/5 μs=800). At the initial stage, since MC=0, the control proceeds to step  114 , in which the count number of the counter MC is incremented by “1”. Then, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, the incrementing of the count number of the counter MC is merely carried out until the count number of the counter MC reaches the numerical value of 800 (with F 1 =1 and F 2 =1). 
     At step  113 , when it is confirmed that the count number of the counter MC has reached the numerical value of 800, the control proceeds from step  113  to step  115 , in which the selection-control signal “ST1” is returned to the low-level, so that the selective and electrical energization of the electric resistance elements R m1  to R mn  of the thermal head  30 M is stopped. 
     At step  116 , the counter MC is reset to “0”. Then, at step  117 , the flag F 1  is made to be “0”, and the flag F 3  is made to be “1”. Thus, the routine once ends. 
     When the routine is executed after the time of 5 μs has elapsed, the control proceeds to step  102  via step  101  (F 1 =0 at step  117 ), in which it is determined whether a third latch pulse of the low-active latch signal “LATCH”, indicated by reference LAT 3  in the timing chart of FIG. 13, is outputted from the printer control circuit  40  to the latch circuit  52 . If the outputting of the third latch pulse “LAT3” is not confirmed, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, there is no progress until the outputting of the third latch pulse “LAT3” is confirmed. 
     As is apparent from the timing chart of FIG. 13, during the selective and electrical energization of the electric resistance elements R m1  to R mn  of the thermal head  30 M, a first single-line of digital yellow image-pixel signals, indicated by reference Y 1 (DATA), is inputted to the shift register  50 , and these digital yellow image-pixel signals Y 1 (DATA) are successively shifted to the flip-flops  50   1  to  50   n  in accordance with the series of clock pulses “CLK”, as shown in the timing chart of FIG.  13 . Then, the respective digital yellow image-pixel signals Y 1 (DATA) held by the flip-flops  50   1  to  50   n  are simultaneously shifted to the latches  52   1  to  52   n  of the latch circuit  52 , and are latched by outputting the third latch pulse “LAT3”. 
     At step  102 , when the outputting of the third latch pulse “LAT3” is confirmed, the control proceeds to step  103 , in which the flag F 1  is made to be “1”. Then, the control jumps from step  104  to step  111  (F 2 =1), and further jumps from step  111  to step  118  (F 3 =1), in which the selection-control signals “ST1” and “ST2” are made to be high, whereby only the electric resistance elements R y1  to R yn  of the thermal head  30 Y are selectively and electrically energized in the accordance with the magenta image-pixel signals Y 1 (DATA) held in the latches  52   1  to  52   n  of the latch circuit  52 . 
     At step  119 , it is determined whether a count number of a counter YC has reached a numerical value of 1000, which corresponds to a time period of 5 ms (5 ms/5 μs=1000). At the initial stage, since YC=0, the control proceeds to step  120 , in which the count number of the counter YC is incremented by “1”. Then, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, the incrementing of the count number of the counter YC is merely carried out until the count number of the counter YC reaches the numerical value of 1000 (with F 1 =1, F 2 =1 and F 3 =1). 
     At step  119 , when it is confirmed that the count number of the counter YC has reached the numerical value of 1000, the control proceeds from step  119  to step  121 , in which the selection-control signals “ST1” and “ST2” are returned to the low-level, so that the selective and electrical energization of the electric resistance elements R y1  to R yn  of the thermal head  30 Y is stopped. 
     At step  122 , the counter YC is reset to “0”. Then, at step  123 , the flag F 1  is made to be “0”, the flag F 2  is made to be “0”, and the flag F 3  is made to be “0”. Thus, the routine once ends. Thereafter, although the routine is repeatedly executed at regular intervals of 5 μs, there is no progress until the outputting of the first latch pulse “LAT1” is again confirmed. 
     As is apparent from the timing chart of FIG. 13, during the selective and electrical energization of the electric resistance elements R y1  to R yn  of the thermal head  30 Y, a second single-line of digital cyan signals, indicated by reference C 2 (DATA), is inputted to the shift register  50 , and these digital cyan image-pixel signals C 2 (DATA) are successively shifted to the flip-flops  50   1  to  50   n  in accordance with the series of clock pulses “CLK”, as shown in the timing chart of FIG.  13 . 
     On the other hand, as soon as the selective and electrical energization of the electric resistance elements R y1  to R yn  of the thermal head  30 Y is completed (step  121 ), the motors  48 C,  48 M and  48 Y are driven in accordance with the series of drive pulses outputted from the motor driver circuit  46 , such that the image-forming sheet  10  is intermittently fed by a distance corresponding to the single-line of image-dots recorded on the image-forming sheet  10 . 
     After the intermittent movement of the image-forming sheet  10  is completed, once the first latch pulse “LAT1” is again outputted from the printer control circuit  40  to the latch circuit  52 , the selective and electrical energization of the electric resistance elements (R cl  to R cn ; R ml  to R mn ; and R y1  to R yn ) are cyclically repeated in accordance with the aforesaid execution of the routine in FIGS. 11 and 12 until a color image is completely recorded on the image-forming sheet  10 . 
     As is apparent from the foregoing, according to the present invention, plural thermal heads ( 30 C,  30 M,  30 Y) have a common single shift register ( 50 ) and a common single latch circuit ( 52 ). Accordingly, in comparison to a conventional case where a thermal head driver system is provided for each thermal head, it is possible to reduce a production cost of the thermal head driver system according to the present invention. 
     In the aforesaid embodiment of the present invention, although the three thermal heads  30 C,  30 M and  30 Y are selectively driven by the combination of the levels of the two selection-control signals “ST1” and “ST2”, of course, it is possible to perform a selective driving of two thermal heads by the combination of the levels of the two selection-control signals “ST1” and “ST2”. On the other hand, in a case where a combination of levels of three selection-control signals are utilized, it is possible to selectively drive at least seven thermal heads in accordance with at least seven types of digital image-pixel signals. Namely, when n selection-control signals are utilized, it is possible to selectively drive a number of thermal heads, being (2 n −1). 
     Finally, it will be understood by those skilled in the art that the foregoing description is of preferred embodiments of the system and the apparatus, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof. 
     The present disclosure relates to a subject matter contained in Japanese Patent Application No. 10-106137 (filed on Apr. 16, 1998) which is expressly incorporated herein, by reference, in its entirety.