JPH074943B2 - Thermal image recording device capable of expressing gradation - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention prints one line of pixels on a heat-sensitive recording material by selectively heating a heating element dot of a thermal head to generate heat. By moving the material by one line and printing pixels on each line in sequence,
The present invention relates to improvement of a thermal image recording system for obtaining a three-dimensional image.

[Background of the Invention]

Actively researching methods for reproducing images in the form of high-quality hard copies (also called pictorial and hard copies) from electrical image signals obtained from televisions, video cameras, video discs, electronic still cameras, etc. Has been done. The most influential method among them is the thermal image recording method, which is a thermal recording material (thermosensitive coloring type) on a line type thermal head consisting of a group of heating element dots (corresponding to one line) arranged in a horizontal line. Not only that, but also includes, for example, one in which an image receiving sheet and a thermal transfer ink sheet are superposed) are moved, and only a predetermined heating element dot is selectively energized and heated based on an input image signal. One line of pixels is printed on the recording material by heat, and by repeating this, pixels of each line are sequentially printed to complete one two-dimensional image. FIG. 1 is an example. Reference numeral I is a heat transferable ink sheet, on the upper surface of which is thinly coated a mixture of a sublimable dye and a suitable binder. The image-receiving sheet R has an image-receiving layer that receives the sublimable dye and develops a color, which is applied to the lower surface in the figure. When the thermal transfer sheet I and the image receiving sheet R are brought into close contact with each other as shown in the figure, the platen P is rotated to the right while being selectively heated in accordance with an image input signal applied from the terminal E by the thermal head TH from below. The sublimated dye on the heat transferable ink sheet I is transferred to the image receiving layer of the image receiving sheet R to form a color on the formed portion, and an image is printed. In the thermal head TH, minute dot-shaped heating elements HE are arranged in an example in a direction perpendicular to the plane of the drawing.

In this case, the number of heating element dots corresponding to the pixels for one line may reach 1280, for example, depending on the size of the image. However, when the number of dots is large like this, the maximum current value that flows during batch printing that heats all dots at the same time when printing the same color for all one line becomes extremely large, and therefore, A capacity power supply is required.

Therefore, as shown in FIG. 2A, a block printing method has been proposed in which the heating element dot group is divided into two blocks Ba and Bb from the center and the blocks are sequentially driven at different times.
With this method, the time required to print pixels for one line is doubled, but the required maximum current value is
1/2 is enough.

By the way, the temperature distribution when one heating element dot is heated shows a mountain-shaped distribution as shown in Fig. 3. If each block is heated, if all the dots in the block are heated, As shown in Fig. 4A, heat-generating block Ba (dotted area with hatching) and non-heat-generating block Bb
The heat generation temperature at the boundary with (dot area without hatching) is low. This is because the non-exothermic block Bb outside the boundary has a low temperature and therefore heat escapes, and this phenomenon cannot be avoided. This phenomenon is blocked by Ba
This also occurs when the next block Bb is driven after the end of the drive of Fig. 4B, and the temperature distribution is as shown in Fig. 4B. Therefore, the heat generation temperature is not sufficiently high at the boundary part when dividing the blocks in both the first drive and the next drive (see Fig. 4C), so the printed pixels become discontinuous at the boundary and white dots appear. . As a matter of course, these white dots appear in the printed image as white streaks (referred to as white line by the present inventor) in the moving direction of the recording paper or the like. This white line is a fatal drawback when producing high quality images comparable to photographs.

Therefore, instead of performing block division in the center of the dot group, as shown in FIG. 2B, alternately, that is, assuming that the dot groups are numbered sequentially from the end, the first block B ₁ to which the odd numbered dots belong An alternate block printing method has been proposed, in which each block B ₁ and B ₂ is divided into a _second block B ₂ to which an even numbered dot belongs, and printing is performed for each block B ₁ and B ₂ .

By the way, not only this alternating method but also other methods, the print density in each pixel, that is, the amount of colorant such as dye to be transferred, is obtained in order to obtain a print image of high quality comparable to that of a photograph. It is necessary to express gradation by controlling.

Then, it is important how many steps the pixel density is divided into, and generally 16 gradations, 32 gradations, 64 gradations, 128 gradations or 256 gradations
It is divided into gradations. The most effective method of forming pixels having such a gradation density using a thermal head is a method of changing the length of printing (energizing) time. There is generally a relationship between the printing time and the optical density of the print as shown in FIG.

Normally, the following method is used to control the printing time. That is, the time required to achieve the highest density is divided into the above-mentioned number of grayscales to be reproduced, or more pulses, and the total time width of these pulses is required to achieve the above-mentioned highest density. The time width of each pulse is not necessarily the same. Then, the desired gradation is realized depending on how many of these pulses are actually added to the thermal head dot.

In the previously proposed alternating method, the applied pulse is controlled up to the highest gradation level n (eg, n = 64) at the first block B ₁ , and then at the next block B ₂ , similarly, the highest gradation level is controlled. The applied pulse up to n was controlled and this was repeated alternately. Therefore, in the case of batch printing without block printing, if it takes T milliseconds to complete the pixels for one line, in the case of block printing, it takes twice as much 2 × T milliseconds.

However, according to the follow-up experiment conducted by the present inventors, in the previously proposed alternate block printing method, the pixels formed from the dots belonging to the block to be printed later are adjacent to the heat storage effect of the previously printed one. Due to the effect, it was darker than the previous one, so it was found that dark-striped lines appear in the obtained two-dimensional image every other pixel in the moving direction of the recording paper.

[Object of the Invention]

An object of the present invention is to shorten the printing time for one line, which takes 2 × T milliseconds, and solve the problem of dark streaks in the alternating block printing method as described above.

[Outline of Invention]

A feature of the present invention is that in a thermal image recording apparatus that forms a two-dimensional image having gradation by controlling the driving time of the heating element dots based on the print data, the thermal image recording apparatus has a plurality of heating element dots and Are divided into a first group to which odd-numbered dots belong and a second group to which even-numbered dots belong, print data supply means for supplying print data, and a reference for driving heating element dots. The pulse signal for setting the time is divided into a first pulse signal and a second pulse signal whose phases are opposite to each other, and the first pulse signal is supplied to the first group of the heating element dots, A pulse signal supply means for supplying a second pulse signal to the second group, and a plurality of changes in the print data and a plurality of pulses during the printing of one line. By providing the control means for controlling the driving time of each of the heating element dots by repeating the application of the scanning signal, the printing time is halved to enable high quality gradation expression.

Hereinafter, the present invention will be specifically described with reference to examples. In the following embodiment, the "print data supply means" is a level data signal generation circuit (61), a clock signal generation circuit (62), a load signal generation circuit (63), a shift register (65) and a latch circuit (66). The "pulse signal supply means" consists of
Made from the thermal head drive signal generating circuit (64), the "control means" consists of NAND gate G ₁ ~G _10.

(Embodiment) FIG. 6 shows a circuit diagram of this embodiment. Here, the number of heating element dots arranged in a line on the thermal head is 10 for simplicity.
The description will be given assuming that the number of gradation levels to be reproduced is 6 and the number of gradation levels to be reproduced is 6.

The thermal head is provided with heating element dots D _{1 to} D ₁₀ in one row, and one of these terminals is grouped together as a power supply terminal V and the other terminal is a corresponding NAND gate circuit.
It is connected to the output terminal of G ₁ ~G _10. The image signal is converted into level data signals LD11 to LD15 by the level data signal generation circuit 61 in a manner described later in detail, and sequentially applied to the input of the shift register 65. A clock signal is added to the shift register 65 by the clock signal generation circuit, and the level data signals LD11 to LD15 are sequentially sent. Each bit output of the shift register 65 is connected to a latch circuit 66, and when the load signal L from the load signal generating circuit 63 is generated, the contents of the shift register are transferred to the latch circuit 66. ing. The output of each bit of the latch circuit 66 is the NAND gate circuits G _{1 to} G ₁₀ described above.
Is connected to each one of the input terminals. As described above, the heating element dots D _{1 to} D ₁₀ are blocks B _{1 of} odd-numbered dots (D ₁ , D ₃ , D ₅ , D ₇ , D ₉ ) and even-numbered dots (D ₂ , _{D 4, D 6, D 8} , D 10 is divided into the block B ₂ of), NAND gate circuit connected to the heating element dots of the B ₁ block, i.e. _{G 1, G 3, G 5} , G 7 , The other input of G ₉ is the strobe signal SS from the thermal head drive signal generation circuit 64.
₁ is the strobe signal SS ₂ from the thermal head drive signal generation circuit 64 to the other input of the NAND gate circuits G ₂ , G ₄ , G ₆ , G ₈ and G ₁₀ connected to the heating element dot of the B ₂ block. Each is added.

The image signal to be printed starts from the first line and is divided into the last line and input over time.
As an example, the image signal of the first line is shown in FIG. This image signal is divided into 1 to 10 sections along the time axis, and each section corresponds to each pixel, that is, each heating element dot D _{1 to} D ₁₀ .

This image signal is sequentially compared with the comparison levels L _{1 to} L ₅ shown on the right side of FIG. 7, and the level data signals LD _{11 to} LD ₁₅ shown in FIG.
Is converted to. This level data signal is set to "1" when the image signal is equal to or higher (darker) than the comparison level, and is set to "0" in other cases. For example, in the case of comparison with the level L ₁ , the signals of lower level are only the signals of the 6th section and the others are “1”.
The signal becomes as shown by LD ₁₁ , and in comparison with the level L ₃ , the signals in each of the 1st, 4th, 5th, 8th, and 10th sections are larger than this level, so LD _{13 in} FIG. A level data signal such as

FIG. 9 is a diagram showing the waveforms of the strobe signals SS ₁ and SS ₂ generated by the thermal head drive signal generation circuit 64.

As can be seen in Fig. 5, of the energizing time of the thermal head,
During the first few milliseconds (part a in FIG. 5), the thermal head itself and the image-receiving layer are heated to a certain temperature level and do not substantially contribute to the printing operation. Therefore, the strobe signals SS ₁ and SS ₂ are also provided with a corresponding preheating period tp. Then, subsequently, strobe signals corresponding to the respective density levels are sequentially output from the lowest density level, but these are configured as follows. Here, 5 groups of pulses corresponding to 5 gradation levels are output. Of these, the time from the rising edge of the first pulse of SS ₁ of the first group to the falling edge of the last pulse of SS ₂ is t _{The same time as the 1st} and 2nd groups is t ₂ , the third is t ₃ , the 4th is t ₄ , and the 5th is t ₅
When determined so on, print density corresponding to a concentration of between tp + t ₁ only when energized heating elements of the thermal head level data signal LD ₁₁ That comparison level L ₁ and L ₂ can be obtained and at tp + t ₁ + t ₂ LD A concentration equivalent to _12, and so on
Set t ₁ to t ₅ so that a print density equivalent to LD ₁ n can be obtained by tp + t ₁ ... + tn (n = 1 to 5). Is composed of a further time t ₁ ~t ₅ of each group is divided into eight in this embodiment, mutually phase inverted four each of pulses of the SS ₁ and SS ₂ within that time.

Next, the operation of this embodiment will be described. First, a clock signal is generated from the clock signal generation circuit 62, and the signal of LD ₁₁ is sequentially added to the shift register 65 from the section 10 side by the level data signal generation circuit. When all the signals for 10 dots have been added, the load signal L is output from the load signal generating circuit 63 (FIG. 9), and the data in LD ₁₁ is transferred to the latch circuit 66. Then, this time, the level data signal generation circuit 61 outputs the level data signal of the LD ₁₂ , and the clock signal generation circuit 62 starts the work of sending this to the shift register 65.

On the other hand, the thermal head is driven based on the level data signal LD ₁₁ by the strobe signals SS ₁ and SS ₂ from the thermal head drive signal generation circuit 64. Only preheating is performed. At this time, the logical product of SS ₁ and SS ₂ and the level data signal LD ₁₁ stored in the latch circuit 66 is performed, so that the level data signal is “0”, that is, other than D ₆ here. All of the dots are preheated. Subsequently, the level data signal LD is transmitted by the strobe signals SS ₁ and SS _2.
A print equivalent to ₁₁ is made, but at this time SS
₁ and SS ₂ and each other because the inverted signal of the phase added is by D ₁ belonging to block _{B 1, D 3, D 5} , D 7, D 9 and block D _2, D ₄ belonging to the B _2, D ₈ , D ₁₀ will be driven alternately. Accordance connexion certain dots, for example, focusing on D _3, the sum of in conduction time during this period is half of t _1, but not enough time t ₁ required to obtain a concentration corresponding to the LD ₁₁ , SS ₁
Is 0 and D ₃ is not energized, SS ₂ is 1 so that adjacent D ₂ and D ₄ are energized. As a result, due to the adjacency effect, D ₃ Since the portion is also heated, the print density is substantially the same as that when the current is continuously applied for the time of t ₁ .

When printing based on the level data signal LD ₁₁ is completed in this way, the load signal L is then issued from the load signal generation circuit, and the level data signal LD ₁₂ of the next density level already stored in the shift register 65 is sent to the latch circuit 66. Be transferred. Then, the next level data signal LD ₁₃ is continuously sent to the shift register 65, while strobe signals SS ₁ and SS ₂ corresponding to the level data signal LD ₁₂ are issued from the thermal head drive signal generation circuit 64, in the same manner as before. L for t ₂
A level of printing corresponding to D ₁₂ is performed. When this process is repeated and printing based on the level data signal LD ₁₅ is completed, one line image is completed. Then, the positional relationship between the thermal head and the thermal recording material is moved by one line and the image signals are exchanged to replace the level data signal. And repeat the above work from the beginning. In this way, printing is completed for a predetermined number of lines, and when this is repeated for each primary color of YMC (B), full-color printing is completed.

With the above configuration, when the adjacent heating element dots are not driven at a certain level, for example, in FIG.
In the case of dots in the 5th section when printing a level equivalent to LD ₁₄ , the adjacency effect of adjoining dots cannot be expected, and there is a risk that the density will be insufficient. It is an example listed in. Here is shown only the portion from the latch circuit 66 for easy until the heating element dots D ₁ to D _10, other configurations are exactly same as the sixth embodiment FIG. The same reference numerals are given to the constituent elements common to those of the embodiment shown in FIG. The difference from FIG. 6 is that the gate circuits GAn, GBn, GCn (n = 1 to 1) are provided between the NAND gate circuits G _{1 to} G ₁₀ and the lines of the strobe signals SS ₁ and SS ₂ from the thermal head drive signal generation circuit 64. The point is that the logic circuit consisting of 10) is added. As an example, focusing on the line of D ₃ , one input of the NAND gate circuit G ₃ connected to this is connected to the latch circuit 66 to input the level data signal in the 3rd section, and the other input terminal. Is connected to the output of the OR gate circuit GA ₃ . The strobe signal SS ₁ is supplied to _one input of the OR gate circuit GA ₃ , and the other input is connected to the output of the AND gate circuit GB ₃ . The other strobe signal SS ₂ is supplied to one input terminal of the AND gate circuit GB ₃ , and the other input terminal is connected to the output terminal of the NOR gate circuit GC ₃ .
The input terminal of this NOR gate circuit GC ₃ is the previous dot,
That is, the dot level data signal for the second zone and the dot after the one, that is, the level data signal for the fourth zone are added.

With such a structure, the strobe signal SS as shown in FIG.
_{As 1} and SS ₂ are added, as long as the level data signal of the dots in the adjacent blocks 2 and 4 is "1", as described above, the dots belonging to the odd-numbered block B ₁ and the even-numbered block B ₁ The dots belonging to B ₂ are driven alternately, but 2
The level data signal dots Gu and 4 wards "0" that is, a state where no printing output of GC ₃ becomes "1", NA
Not only the strobe signal SS ₁ but also SS ₂ is supplied to the ND gate circuit G ₃ , and after that, the dot D ₃ is continuously driven. In this way, it is possible to prevent the print density from being insufficient when both adjacent dots are not driven. This applies not only to D ₃ but also to other heating element dots, but for the dots located at both ends, that is, for D ₁ and D ₁₀ , the level data signal of one adjacent dot is always “0”. because the same thing as may be taken only signals of the other adjacent dots, sub connexion GC ₁
And the GC ₁₀ is a simple inverter.

In the above example, the strobe signals SS ₁ and SS ₂ are added to the same dot only when the level data signals of the adjacent dots are both "0". However, in the embodiment of FIG. Replace the NOR gate circuit GCn (n = 2 to 9) with the NAND gate circuit GCn '(n = 2 to 9), and strobe the level data signal of the adjacent dot if either one is "0". It is configured to add both signals SS ₁ and SS ₂ . Since this condition is always satisfied at dots D ₁ and D ₁₀ at both ends, both strobe signals are always added. In this case, even if one of the level data signals of the adjacent dots is "0" and the other is "1", both strobe signals are added, and due to the effect of adjacency, the dot is slightly more dense than that of the adjacent one. It becomes higher and wins,
This is, so to speak, an effect of edge enhancement, and is rather preferable as an output print of an image.

In each of the above embodiments, for simplification of description, an example of printing an image having 6 gradations using a thermal head having a heating element for 10 dots per column has been described. It goes without saying that the number of dots and the number of gradations can be extended.

〔The invention's effect〕

As described above, according to the present invention, in the alternate block printing method, a high quality image can be obtained, there is no problem of black streaks, and the printing time for one line (that is, the creation time of the entire image) is obtained. It only takes half.

[Brief description of drawings]

FIG. 1 is a conceptual diagram for explaining a general thermal image recording system. FIG. 2A is an explanatory diagram when the heating element dot of the conventional thermal head is divided into two blocks. FIG. 2B is an explanatory diagram when the heating element dot of the thermal head of the present invention is divided into two blocks. FIG. 3 is a conceptual diagram showing the temperature distribution when one heating element dot is made to generate heat. FIG. 4A is a conceptual diagram showing a temperature distribution when the thermal head is caused to generate heat in the block Ba according to the conventional method. Similarly, FIG. 4B is a conceptual diagram of the block Bb. FIG. 4C is a composite view of FIG. 4A and FIG. 4B. FIG. 5 is a graph showing the relationship between the printing time and the optical density of the print. FIG. 6 is a circuit diagram of the recording apparatus according to the embodiment of the present invention. FIG. 7 is a graph showing the waveform or time chart of the image signal of the first row used in the embodiment of the present invention. FIG. 8 is a waveform diagram of the level data signal. FIG. 9 is a pulse waveform diagram. FIG. 10 is a circuit diagram of another embodiment of the present invention. FIG. 11 is a circuit diagram of still another embodiment of the present invention. [Explanation of symbols for main parts] TH: Thermal head HE: Heating element (heating element dot) R: Image receiving sheet (thermosensitive recording material) I: Thermal transfer ink sheet (thermal recording material) P: Platen B ₁ ... 1st block B ₂ ... 2nd block SS ₁ , SS ₂ ... Strobe signal (strobe pulse) E: Electrical image signal input terminal D: Heating element dot 61: Level data signal generation circuit 62: Clock signal generation circuit 63: Load signal generation circuit 64: Thermal head drive signal generation circuit 65: Shift register 66: Latch circuit G: NAND gate circuit

Continuation of front page (56) References JP-A-58-126176 (JP, A) JP-A-58-126175 (JP, A) JP-A-58-187074 (JP, A)

Claims (4)

[Claims] 1. A thermal image recording apparatus for forming a two-dimensional image having gradation by controlling the driving time of heating element dots based on print data, and having a plurality of heating element dots A thermal head divided into a first group to which odd-numbered dots belong and a second group to which even-numbered dots belong, print data supply means for supplying print data, and reference time when driving heating element dots The pulse signal for setting the pulse signal is divided into a first pulse signal and a second pulse signal whose phases are opposite to each other, and the first pulse signal is assigned to the first group of the heating element dots, Pulse signal supply means for supplying two pulse signals to the second group, and a plurality of changes of the print data and a plurality of pulses during one line of printing. A thermal image recording apparatus capable of gradation expression, comprising: a control unit that controls the driving time of each heating element dot by repeating the application of a signal. 2. The sum of the number of repetitions per line of the first pulse signal and the second pulse signal is an even multiple of the number of changes per line of the print data. The thermal image recording device according to claim 1. 3. A means for detecting a state of the print data applied to an adjacent dot of each of the heating element dots, wherein the first pulse signal is supplied when none of the adjacent dots is driven. The thermal image recording apparatus according to claim 1, wherein both the second pulse signals are applied to the dots. 4. A means for detecting a state of the print data applied to an adjacent dot of each of the heating element dots, wherein the first pulse signal is provided when one of the adjacent dots is not driven. The thermal image recording apparatus according to claim 1, wherein both of the first pulse signal and the second pulse signal are applied to the dot.