JPH07314758A - Method and device for drivingnly controlling thermal head - Google Patents

Abstract

PURPOSE:To reduce uneven density due to the errors of the values of the resistances of respective heat generating elements. CONSTITUTION:The values Ri of the resistances of respective heat generating elements are measured by the detecting circuit 37 of the value of a resistance. The errors DELTARi of the values of the resistances are obtained by subtracting the values Ri of the resistances from the standard value R0 of a resistance. A basic bias DB0i incorporated with bias correcting data for correcting bias heat energy error is obtained on the basis of the errors DELTARi of the values of the resistances. Gradation correcting data DELTAB1i for correcting gradation representing heat energy errors developing on the basis of the errors DELTARi of the values of the resistances are obtained on the basis of the image data DGB at the recording of medium density, at which uneven density becomes conspicuous, and the errors DELTARi of the values of the resistances. By adding the basic bias data DB0i and the gradation correcting data DB1i with an adder-limiter 39, corrected bias data DBCi are obtained. By means of the corrected bias data DBCi corrected bias heating is performed. Accordingly, since the correction of uneven heat generation is performed on the standard of the medium density, at which uneven density becomes conspicuous, the uneven density becomes inconspicuous.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal head drive control method and apparatus used in a thermal printer,
More specifically, the present invention relates to a thermal head drive control method and apparatus for generating a bias heating drive pulse and a gradation expression heating drive pulse.

[0002]

2. Description of the Related Art Thermal printers include thermal transfer printers and thermal printers. In the former thermal transfer printer,
There are a melt type and a sublimation type, which are ones in which an ink film is superposed on a recording paper, a thermal head is pressed against the back of the ink film to heat it, and the ink of the ink film is transferred to the recording paper. In the latter thermal printer, the thermal recording paper is heated by a thermal head, and the thermal recording paper is colored to perform thermal recording. These thermal heads have a heating element array in which a large number of heating elements (resistive elements) are arranged in a line, and a driver that drives each heating element.

For example, in a color thermosensitive printer, as described in JP-A-61-213169, a color thermosensitive recording material in which a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer and a yellow thermosensitive coloring layer are sequentially provided on a base. Is used. In order to selectively color each thermosensitive coloring layer, each thermosensitive coloring layer has different coloring heat energy. The deeper the heat-sensitive coloring layer, the higher the coloring heat energy required. Also, when the next thermosensitive coloring layer is heat-recorded, the heat-recorded thermosensitive coloring layer on the heat-recorded thermosensitive coloring layer is irradiated with a specific electromagnetic wave so as not to be heat-recorded again. To do.

Each heating element of the thermal head applies color-developing heat energy based on the characteristic curve of the thermosensitive color-developing layer to be recorded to the color thermosensitive recording material, and among the recording pixels virtually divided into squares on the color thermosensitive recording material. Then, an ink dot having a desired density is formed. This color-developing heat energy is the heat energy immediately before the thermosensitive coloring layer to be recorded (hereinafter,
This is referred to as bias heat energy) and heat energy for developing a color to a desired density (hereinafter, this is referred to as gradation expression heat energy). The bias heat energy changes depending on the type of the thermosensitive coloring layer. The image data represents the gradation level of the recording pixel. The higher the gradation expression heat energy, the higher the color density of the ink dots. To express high gradation, heat generation control is performed in fine steps to generate gradation expression heat energy.

Bias heat energy is generated by driving a heating element with one bias heating drive pulse (hereinafter referred to as a bias pulse). The gradation expression heat energy is generated by driving the heating elements with gradation expression heating drive pulses (hereinafter referred to as gradation pulses) in a number corresponding to the image data. Generally, the width of the bias pulse is several ms to several tens of ms, and the width of the gradation pulse is several μs to several tens μs. Also, during the bias heating period,
In addition to continuously driving the heating element with one bias pulse,
It is also possible to drive the heating element intermittently with a plurality of bias pulses.

Also, in the case of a sublimation type thermal transfer printer,
At least one bias pulse for heating to a recording pixel on the recording paper (image receiving paper) at the time of transferring the ink having a desired density, until just before the ink transfer starts;
A number of gradation pulses corresponding to the image data is used to adjust the transfer amount of ink. Further, in the case of the fusion type thermal transfer printer, the gradation is expressed by changing the area of the ink dot transferred in the recording pixel.
At this time, a bias pulse for heating to a temperature at which ink transfer starts and a plurality of gradation pulses for maintaining this temperature are used. The gradation pulse is supplied to the heating element every time the image receiving paper moves by one sub line, and transfers the ink to each sub line. One recording pixel is composed of a plurality of sub-lines, and the gradation of this recording pixel is expressed by the number of sub-lines to which ink is transferred.

By the way, in order for the fine heat generation control to be accurately reflected in the printing result, it is necessary that the resistance values of all the heat generating elements are uniform. However, the resistance value of the heating element generally has a variation of about 5 to 10%, which causes an inconvenient phenomenon such as uneven density or uneven color in a recorded image. The resistance value of each heating element is, for example, 2400
120 to 24 because it is made to be ohm
There is a resistance error of 0 ohm.

In order to improve this, for example, Japanese Patent Laid-Open No. 2-2
As described in Japanese Patent No. 48262, a thermal printer has been proposed which measures all the resistance values of several hundreds of heating elements provided in a thermal head, corrects image data based on the measurement result, and prints. ing. Further, instead of correcting the image data, Japanese Patent Application Laid-Open No. 2-292060
As described in the publication, there is proposed a thermal printer in which a density correction drive pulse corresponding to a resistance value is inserted between gradation pulses to correct density unevenness.

[0009]

However, the method of directly multiplying the image data by the correction data to change the number and width of the gradation pulses requires a large amount of arithmetic processing corresponding to the number of pixels in one frame. This requires a high-speed arithmetic circuit, which increases the manufacturing cost. In particular, when the calculation result is quantized and converted into the number of gradation pulses, this quantization error becomes large, so that a pseudo contour is generated in the print image, and the print quality deteriorates.

Further, in the method of inserting the density correction driving pulse between the grayscale pulses, a density correction driving pulse generating circuit is newly required, which complicates the structure and increases the manufacturing cost. Further, since the density correction driving pulse is interrupted between the gradation pulses, the printing time becomes long.

The present invention is a simple circuit and data processing,
It is an object of the present invention to provide a thermal head drive control method and device that suppresses the occurrence of density unevenness and color unevenness due to the resistance value error of each heating element and prevents the density unevenness and the like from becoming conspicuous. .

[0012]

In order to achieve the above object, a thermal head drive control method according to a first aspect of the present invention uses heat generation unevenness so as to eliminate heat generation unevenness caused by a resistance error of each heating element. Based on the intermediate density where density unevenness is conspicuous, the density unevenness correction amount when recording this intermediate density is obtained in advance for each heating element, and the number of bias heating drive pulses is determined based on this density unevenness correction amount. It is changed for each heating element. When the bias heating is performed by one bias heating driving pulse, the pulse width is changed for each heating element.

In the thermal head drive controller according to the third aspect of the present invention, the first line memory for storing one line of image data and the bias heating due to the resistance value error of each heating element. Means for storing, for each heating element, pulse number data of the bias heating drive pulse train in which bias correction data for correcting the bias thermal energy error generated at the time is added as basic bias data, and the basic bias data for one line. Second to remember
Of the line memory and the gradation correction data for correcting the gradation expression thermal energy error caused by the resistance value error of each heating element during the gradation expression heating when recording the reference intermediate density. Means for storing for each heating element as pulse number data of bias heating drive pulse, third line memory for storing this gradation correction data for one line, and basic bias data from the second line memory The adder that obtains the corrected bias data by adding the tone correction data from the third line memory, the corrected bias data from the adder, and the image data from the first line memory are switched to change the corrected bias. Bias heating drive based on data After heating each heating element with corrective bias using a pulse train, gradation drive heating drive based on image data from the first line memory It is obtained by a controller for heating gradation expression by pulse train.

The above-mentioned reference intermediate density is 0.
By setting it to 6 to 0.8, it is possible to suppress the occurrence of density unevenness at this intermediate density. 0.6 to this optical density
The density unevenness can be most easily detected in the vicinity of 0.8. Therefore, if density unevenness occurs in this portion, it becomes a conspicuous result, but this will be eliminated. Also,
In order to suppress the density unevenness around the middle density, the density unevenness correction based on the end of bias heating is
Since the density unevenness due to the resistance value unevenness of the heating element is reduced by half at the maximum density and the minimum density, it is possible to perform printing in which the density unevenness is more inconspicuous.

Further, the bias heating drive pulse train is heated by being brought close to the gradation expression heating drive pulse train, or the bias heating drive pulse train is thinned out at regular intervals when the number of pulses is reduced. The rest period after that does not become long, cooling in this period can be suppressed to prevent a decrease in thermal efficiency, and accurate bias heating can be performed.

[0016]

Since the resistance values of the heating elements are different, the generated heat energy is different even if they are driven in the same state. For example, when the heating element is driven by a fixed number of bias pulses or one bias pulse having a long pulse width to perform bias heating, a heating element having a regular resistance value also has a bias heat energy immediately before color development. However, the heat energy generated in the heating element having a resistance error is larger or smaller than the bias heat energy.

In order to correct the bias thermal energy error caused by the resistance value error, the number or pulse width of the bias pulse is adjusted according to the resistance value error. For example, when the heating elements are continuously driven by one bias pulse, the width of the bias driving pulse is adjusted so that all the heating elements generate predetermined bias heat energy. When each heating element is driven by a plurality of bias drive pulses, the number of bias pulses is changed to generate bias heat energy from all heating elements. The resistance value error is the difference between the resistance value determined by design and the resistance value of each heating element. When the standard resistance value in design is used as a reference, the maximum value of the number of bias pulses is different in each thermal printer due to the dispersion of the resistance value. This leads to a change in bias heating software for each thermal printer. In order to prevent this, a predetermined maximum value of the bias pulse may be assigned to the heating element having the maximum resistance value that generates the least amount of heat. Then, the difference between the resistance value of the heating element and this maximum resistance value is used as a resistance value error, and a correction value corresponding to this resistance value error is subtracted from the maximum value. In this case, the thermal energy generated by the bias data may have a value near the color development point, but since it is only slightly different from the normal bias thermal energy immediately before the color development, this thermal energy is also the bias thermal energy. Can be regarded as By using this non-regular bias heat energy, it is possible to eliminate uneven density and uneven color.

The bias heat energy is substantially the same as the gradation expression heat energy in yellow, which is inconspicuous in color, but the bias heat energy is considerably large in magenta and cyan, in which colors are conspicuous. Therefore, the bias thermal energy error due to the resistance value error has a great effect on the density unevenness and color unevenness as well as the density of the printed image. Therefore, by correcting this bias thermal energy error, the print quality is greatly improved. On the other hand,
Also in the case of gradation expression heating, a gradation expression heat energy error due to a resistance value error occurs. The value of this gradation expression thermal energy error is small for magenta and cyan where the color is conspicuous, but if this is also corrected, the print quality will improve accordingly. Therefore, it is preferable to correct both the bias thermal energy error and the gradation expression thermal energy error due to the resistance value error.

By the way, when the gradation expression heat energy error is corrected at the time of gradation expression heating, it takes time to heat the gradation due to the calculation processing of the image data. That is, since the grayscale pulse has a short pulse width, it is difficult to perform calculation while the pulse is generated, and as a result, the pulse period becomes long.
This leads to heat loss and slows the print speed. Therefore, it is preferable to correct the gradation expression thermal energy error during bias heating. For this purpose, the width or the number of bias pulses determined in consideration of the bias thermal energy error may be readjusted according to the gradation expression thermal energy error. Further, the gradation expression thermal energy error can be accurately obtained from the resistance value error and the individual image data, but the calculation becomes complicated when the individual image data is used. For this reason,
The gradation expression thermal energy error is calculated with reference to a constant intermediate density where density unevenness due to uneven heating is conspicuous, and a density unevenness correction amount is calculated for each heating element so as to eliminate this error. Since the pulse number or width of the bias heating drive pulse is changed for each heating element based on the density unevenness correction amount, density unevenness at an intermediate density where density unevenness is most noticeable can be suppressed. As a result, a simple calculation process can be performed efficiently.
It is possible to suppress the occurrence of density unevenness due to the resistance value error of each heating element and prevent the density unevenness from being conspicuous.

When bias heating is performed with a plurality of bias driving pulses, the number of bias pulses varies depending on the heating element and the print line for resistance value error and shading correction. The bias heating period is determined by the maximum number of bias pulses. When the number of bias pulses is small, the bias heating is completed earlier, so that the quiescent period between the bias heating and the gradation expression heating becomes longer, causing heat loss due to heat dissipation. In order to reduce the heat loss due to bias heating, it is preferable to bring the bias pulse train closer to the gradation pulse train. This can be done by inverting each bit of the bias data to make it a complement. Further, by thinning out the bias driving pulse at a predetermined interval, it is possible to shorten the pause period between the bias heating and the gradation expression heating.

[0021]

2 shows a color thermal printer, a platen drum 10 is attached to a rotary shaft 11 driven by a pulse motor 9, and rotates in an arrow direction during printing. A color thermosensitive recording material 12 is wound around the outer periphery of the platen drum 10, and its tip end is fixed by a clamper 13. The clamper 13 is controlled to be opened and closed by the cam mechanism 8. A thermal head 14, a magenta fixing ultraviolet lamp 15, and a yellow fixing ultraviolet lamp 16 are arranged on the outer periphery of the platen drum 10.

A heating element array 17 is provided on the lower surface of the thermal head 14. As shown in FIG. 1, the heating element array 17 includes a large number of heating elements 17 _{1 to} 17 _n.
Are formed in a line shape in the main scanning direction. Each of the heating elements 17 _{1 to} 17 _n is composed of a resistance element, and when the thermal recording is performed on one pixel, the bias thermal energy for heating until just before color development and the gradation expression thermal energy according to the color density are color-coded. It is applied to the heat-sensitive recording material 12. The magenta fixing ultraviolet lamp 15 emits ultraviolet rays having an emission peak near 365 nm, and the yellow fixing ultraviolet lamp 16 emits ultraviolet rays having an emission peak around 420 nm.

As shown in FIG. 3, the color thermosensitive recording material 1
Reference numeral 2 denotes a cyan thermosensitive coloring layer 21, a magenta thermosensitive coloring layer 22 having a light fixing property of about 365 nm of ultraviolet rays, a yellow thermosensitive coloring layer 23 having a light fixing property of about 420 nm of ultraviolet rays, and a protective layer on the support 20. And 24 are sequentially layered. These thermosensitive coloring layers 21 to 23 are arranged in the order of thermal recording. In FIG. 3, the yellow thermosensitive coloring layer 23 is represented by “Y”, the magenta thermosensitive coloring layer 2 is shown in order to make the intercolor thermosensitive layers 21 to 23 easy to understand.
2 is marked with "M", and cyan thermosensitive coloring layer 21 is marked with "C". Although not shown in FIG. 3, an intermediate layer for adjusting the thermal sensitivity of the magenta thermosensitive coloring layer 22 and the cyan thermosensitive coloring layer 21 is formed between the thermosensitive coloring layers 21 to 23. An opaque coated paper or a plastic film is used as the support 20,
A transparent plastic film is used when producing an OHP sheet.

FIG. 4 shows the coloring characteristics of each thermosensitive coloring layer. In the color thermosensitive recording material 12 of this example, the yellow thermosensitive coloring layer 23 has the lowest coloring thermal energy, and the cyan thermosensitive coloring layer 21 has the highest coloring thermal energy. When the yellow (Y) pixel is subjected to thermal recording, the coloring thermal energy obtained by adding the gradation thermal energy GY _J to the bias thermal energy BY is applied to the color thermosensitive recording material 12. The bias heat energy BY is the heat energy immediately before the yellow heat-sensitive coloring layer 23 develops color, and is applied to the color heat-sensitive recording material 12 during the bias heating period at the beginning of recording one pixel. The gradation expression thermal energy GY _J is determined according to the gradation level J corresponding to the color density of the pixel to be recorded, and the color thermosensitive recording material 12 is supplied during the gradation expression heating period following the bias heating period. Given to. Since magenta M and cyan C are the same, only symbols are attached.

FIG. 1 shows an electric circuit of the color thermal printer. In the frame memory 30, for example, one frame of image data taken by an electronic still camera is written in a state of being separated for each color. This image data represents a gradation level, and the density of the ink dot formed in the recording pixel is determined according to this image data.
At the time of heating for gradation expression, the system controller 31 reads the image data of the color to be printed line by line from the frame memory 30 and writes it in the image data line memory 32.

The system controller 31 is composed of a microcomputer and controls each part in sequence. In addition to the image data line memory 32, a bias data line memory 33 and a resistance value error data line memory 34 are connected to the system controller 31, and the system controller 31 connects these line memories 33 and 34 to each other. Write the data. Further, the system controller 31 includes a multiplier 38, an addition / limiter 3
9, memory controller 42, print controller 4
3 to control the corrected bias data DBC _i (i = 1 to
n (n is the total number of heating elements) is calculated, and this is calculated by the selector 4
Send to 0.

An example of the corrected bias data DBC _i is shown in the following formula 1. The corrected bias data DBC _i is composed of basic bias data DB0 _i and gradation correction data DB1 _i . In this embodiment, since 8-bit data is used, the maximum number of pulses of the corrected bias data DBC _i is “256”. It will be individual. Therefore, for example, a maximum of "224" bias pulses are allocated as the basic bias data DB0 _i , and a maximum of "32" bias pulses are allocated as the gradation correction data DB1 _i . Although this allocation ratio changes depending on the size and type of the heating element, the type of the thermosensitive coloring layer, etc., it is preferable to allocate about 30% of the maximum number of used pulses to the gradation correction data.

[0028]

## EQU1 ## DBC _i = DB0 _i + DB1 _i = (BO + ΔR
1 _i + BHshade + BVshade) + (ΔR2 _i · DG
B) Here, DB0 _{i is} basic bias data, which is obtained by the following mathematical formula 2.

[0029]

[Number 2] _{DB0 i = BO + ΔR1 i +} BHshade + BVshade DB1 i: a gradation correction data, determined by Equation 3 below.

[0030]

[Formula 3] DB1_i= ΔR2_i・ DGB B0: Standard resistance value R₀ (For example, the maximum of the heating elements
It is basic data determined based on the resistance value. ΔR1_i: A bar for correcting bias thermal energy error.
It is the bias correction data, and the resistance value R of each heating element_iFrom
Standard resistance value R₀ Resistance error ΔR_i(= (R_i−
R ₀ )) Multiplied by a coefficient k1 (k1 is the bias pulse)
Coefficient for conversion into the number of lines) BHshade: H shading correction data. BVshade: V shading correction data,
Since the heat escapes at both ends of the round head, the concentration decreases.
Correct ΔR2_i: Coefficient data for correcting gradation thermal energy error
Resistance error ΔR_iTimes the coefficient k2
(K2 is a coefficient for converting into the bias pulse number) DGB: Image data for recording an optical density of 0.6

Operation elements BO, k1, k2, BHshade,
BVshade and DGB are determined in advance by experiments or the like, and are written in the memory of the system controller 31.

FIG. 5 is a basic bias data DB when the color thermal recording material shown in FIG. 1 is used for recording line by line while moving the color thermal recording material in the sub-scanning direction.
The H shading correction data BHshade in 0 _i is shown. Due to the heat accumulated in the thermal head, the color density gradually increases toward the final line, and in order to correct this, the H shading correction data BHshade decreases toward the final line. Since the basic bias data is represented by the number of bias pulses as described above, this H shading correction data is also represented by the number of bias pulses. The V shading correction data BVshade is also represented by the number of bias pulses.

FIG. 6 is a graph showing the relationship between the thermal energy and the color density of two heating elements having different resistance values. In the case where two heating elements having resistance values RA and RB (RA> RB) are used to develop a color at an intermediate density of 0.6, the heating element of RB has the same resistance because the resistance is smaller. The amount of heat generated increases under the driving conditions. Therefore, the heating element having the resistance value of RB reaches the target optical density of 0.6 with a smaller number of drive pulses. The difference ΔE (= EA−EB) in heat energy at this time is converted into the number of bias pulses, and the heat generation amount of each heat generating element is controlled by this difference, whereby each heat generating element has a target density “0.6”. Can generate heat. Therefore, it is possible to suppress the occurrence of density unevenness in the vicinity of the intermediate density “0.6” where density unevenness is easily noticeable. As a result, uneven heat generation is effectively suppressed, and uneven density is less noticeable.

As shown in FIG. 1, each heating element 17 ₁ to 1
The resistance value R _{i of} 7 _n (i is 1 to n) is detected by the resistance value detection circuit 37 at the time of factory shipment of the thermal printer or at the time of initial setting after replacement of the thermal head 14, and this is detected by the RAM 3
It is written in 6. The resistance value detection circuit 37 selects each of the heating elements 17 _{1 to} 17 _n one by one by the transistors 48 _{1 to} 48 _n , applies a predetermined voltage thereto, and detects the resistance value R _i from the voltage drop time thereof. This resistance value detecting method is described in detail in, for example, Japanese Patent Application No. 4-233626 filed by the present applicant. The system controller 31 detects the detected resistance value R _i.
Based on the conversion table stored in the ROM 35, the bias data DB0 _i and the resistance value error data ΔR _i are obtained for each heating element and stored in a predetermined area in the RAM 36, and these data are stored in the line memories 33 and 3.
Write to 4. The resistance value detection circuit 37 may be incorporated in the device, or a resistance value detection device may be separately provided to detect each resistance value.

The system controller 31 first writes the image data for one line to the image data line memory 32 as described above at the time of initial setting after the power is turned on, and also to the bias data line memory 33 and the RAM 3.
Basic bias data DB of each heating element read from 6
Write 0 _i for one line. Further, the resistance value error data line memory 34 writes the resistance value error data ΔR _i for one line. The basic bias data DB0 _{i which} is addressed and read from the line memory 33 by the memory controller 42 is sent to the addition / limiter 39.

The resistance value error data ΔR _i read from the line memory 34 by being addressed by the memory controller 42 is sent to the multiplier 38. Image data DGB for recording an intermediate density of 0.6 is input from the system controller 31 to the multiplier 38. The multiplier 38 receives the image data DGB, the resistance value error data ΔR _i, and the coefficient data. The gradation correction data DB1 _i is calculated by multiplying by k2, and this is sent to the addition / limiter 39.

The adder / limiter 39 is used in the line memory 33.
The corrected bias data DBC _i is calculated by adding the basic bias data DB0 _i from the multiplier 38 to the gradation correction data DB1 _i from the multiplier 38, and this is sent to the comparator 41 via the selector 40. In addition, modified bias data DB
When calculating C _i , the address is controlled so that the data of the same heating element of each line memory 33, 34 is read. Further, the addition / limiter 39 sets the protruding data as the upper limit data so that the data does not exceed 8 bits due to the addition. For example, the bias data DB0 _i
When the addition result of the gradation correction data DB1 _i and the gradation correction data DB1 _i exceeds “FF”, the upper limit “FF” is output.

The system controller 31 generates a system 1 line start signal in response to the print start signal,
This is output to the memory controller 42. When the memory controller 42 receives the system 1-line start signal, it switches the selector 40, connects the addition / limiter 39 to the comparator 41, and outputs the 1-line start signal to the print controller 43. Thereby, the bias heating sequence is executed.
In this bias heating sequence, the memory controller 42 reads the basic bias data DB from the line memory 33.
0 _i is read and this is sent to the addition / limiter 39.
Further, the resistance value error data ΔR _i is read from the line memory 34, sent to the multiplier 38, the gradation correction data DB1 _i is calculated by the multiplier 38, and then sent to the addition / limiter 39. The adder / limiter 39 adds the basic bias data DB0 _i and the gradation correction data DB1 _i to calculate the corrected bias data DBC _i , which is sent to the comparator 41 via the selector 40. The correction bias data DBC _i for one line is calculated 256 times. The comparator 41 is the print controller 4
The corrected bias data for one line is compared 256 times with the bias comparison data of “0” to “FF” from 3.

Upon completion of 256 comparisons with the corrected bias data for one line, the print controller 4
3 sends a 1 line end signal to the memory controller 42. When the memory controller 42 receives the 1-line end signal, it connects the selector 40 to the image data line memory 32 and executes the gradation expression heating sequence. In this gradation expression heating sequence, the memory controller 4
2 reads the image data for one line 128 times from the image data line memory 32 and sends it to the comparator 41. The comparator 41 compares the image data for one line 128 times with the gradation comparison data of “0” to “7F” from the print controller 43.

The print controller 43 sends a 1-line end signal to the memory controller 42 not only when the comparison based on the bias comparison data is completed but also when the comparison based on the gradation comparison data is completed. The memory controller 42 is a 1-bit counter (flip-flop) 42 that counts the number of 1-line end signals from the print controller 43.
a, the system 1 line end signal is sent to the system controller 31 every time the 1 line end signal is input twice, that is, when the bias heating and the gradation heating are completed.

The print controller 43 includes a 1-bit counter 43a, and switches the type of comparison data to be created each time it receives a 1-line print start signal from the memory controller 42. That is, when the first 1-line print start signal is received, "0"
~ 8-bit bias comparison data representing "FF" is sequentially output. When the second 1-line print start signal is received, 8-bit gradation comparison data representing "0" to "7F" is sequentially generated. The same applies hereinafter. In this embodiment, 128 gradations are expressed by using the gradation comparison data of "0" to "7F" in hexadecimal notation, but the gradation expression number is the characteristic of each thermosensitive coloring layer. It can be changed by increasing or decreasing the gradation comparison data as appropriate.

The comparator 41 compares the corrected bias data with the bias comparison data, or compares the image data with the gradation comparison data. If the comparison data is smaller, the drive data of "H" is output, and otherwise, the drive data of "L" is output. For example, in the bias heating, when the comparison data “0” is sent from the print controller 43, the corrected bias data for one line is sequentially compared with the comparison data. This gives 1
The comparison result of the lines is sent from the comparator 41 to the shift register 45 as a serial signal. When the comparison of the corrected bias data for one line is completed, the print controller 43 generates comparison data of "1" and sends it to the comparator 41. Therefore, by using the comparison data of "0" to "FF", each corrected bias data is compared 256 times, and as a result, it is converted into driving data of 256 bits. Then, the 256-bit drive data is sequentially sent to the shift register 45 bit by bit. Similarly, the image data and the gradation comparison data are also compared and converted into 128-bit drive data.

Serial drive data is shifted in the shift register 45 by a clock and converted into a parallel signal. The drive data converted into the parallel signal by the shift register 45 is latched in the latch array 46 in synchronization with the latch signal. The AND gate array 47 outputs the signal of "H" when the drive data is "H" within the period in which the strobe signal is input.

Transistors 48 _{1 to} 48 _n are connected to the respective output terminals of the AND gate array 47. These transistors 48 _{1 to} 48 _n are turned on when the output of the AND gate is “H”. Heating elements 17 _{1 to} 17 _n are connected in series to these transistors 48 _{1 to} 48 _n , respectively.

The strobe signal generating circuit 44 is controlled by signals from the system controller 31 and the print controller 43, and turns on the heating elements 17 _{1 to} 17 _n .
Generates a strobe signal for determining time and OFF time. There are two types of strobe signals, one for bias heating and one for gradation expression. In addition, the strobe signal has a pulse width that varies with the color to be printed. For this reason,
The setting data of each strobe signal is obtained in advance for each color and is stored in a predetermined area of the RAM 36. The strobe signal generating circuit 44 is described in detail in, for example, Japanese Patent Application No. 5-74508 filed by the present applicant.

FIG. 7 shows an example of a drive pulse train for each heating element during yellow recording. In addition, BHshad
e and BVshade are set to "0". For example, the first heating element shows the maximum resistance value, which is the standard resistance value R _0.
And 256 bias pulses are assigned to this. 176 bias pulses are assigned to the basic data B0, and 48 bias pulses are assigned as the bias correction data ΔR1 ₁ for correcting the bias thermal energy error. Further, 32 bias pulses are assigned to the gradation correction data DB1 ₁ for correcting the gradation expression thermal energy error. Therefore, when the pixel of the gradation level “128” is recorded by the first heating element, as shown in FIG. 7, DB0 ₁ = 224, DB1
_{Since 1} = 32, DB ₁ = 256, and DG ₁ = 1
28.

The second heating element has a lower resistance than the first heating element. This second heating element has a gradation level of "128"
When recording pixels of, BO = 176, ΔR1 ₂ =
18, DB0 ₂ = 194, DB1 ₂ = 20, DG ₂ = 1
28. Further, the third heating element has the same resistance value as the second heating element, and the gradation level "64" is generated by this third heating element.
When recording the pixels of, BO = 176, ΔR1 ₃ =
18, DB0 ₃ = 194, DB1 ₃ = 20, DG ₃ = 6
It is 4. Further, the fourth heating element has a slightly lower resistance than the first heating element, and the gradation level "13" is generated by the fourth heating element.
When recording the pixels of, BO = 176, ΔR1 ₄ =
25, DB0 ₄ = 201, DB1 ₄ = 28, DG ₄ = 1
It becomes 3.

Next, the operation of the above embodiment will be described with reference to FIGS. When a print start switch (not shown) is operated, the system controller 3
1 feeds the color thermosensitive recording material 12. At the same time, the system controller 31 causes the frame memory 3
The yellow image data on the first line from 0 to 0 is read out for each pixel and written in the image data line memory 32. Further, in order to reduce the variation in the amount of heat generated due to the resistance value error of each heating element, the basic bias data DB0 _i is read from the RAM 36 and written in the bias data line memory 33. Further, the system controller 31 resets the counter 31a (p = 0).

At the time of sheet feeding, the platen drum 10 is stopped in a state where the clamper 13 is vertical in FIG. 2, so when the cam mechanism 8 is operated, the clamper 13 is shifted to the unclamping position. After that, the color thermosensitive recording material 12 is fed to the platen drum 10, and when the tip of the color thermosensitive recording material 12 is positioned in the clamp, the cam mechanism 8 operates and the tip of the color thermosensitive recording material 12 is pressed by the clamp 13. Since the platen drum 10 rotates in this state, the color thermosensitive recording material 12 is wound around the platen drum 10.

When the platen drum 10 rotates intermittently by a constant step and the leading end of the recording area of the color thermosensitive recording material 11 reaches the heating element array 17, the thermal recording of the first line of the yellow image becomes possible. The system controller 31 generates a system 1 line start signal,
This is sent to the memory controller 42. At this time, the counter 31a updates by adding 1 to the count value (p =
p + 1). The memory controller 42 connects the selector 40 to the adder / limiter 39 according to the system 1 line start signal, and corrects bias data DBC of each heating element.
_i is sent to the comparator 41.

Further, the memory controller 42 designates the address and sequentially from the first heating element 17 ₁ to the n-th heating element 1.
Resistance error data ΔR from line memory 34 up to 7 _n
i is sent to the read multiplier 38. The multiplier 38 receives the resistance error data ΔR _i , the coefficient data k2, the optical density 0.
The gradation correction data DB1 _i is obtained by multiplying it with the image data DGB for recording the intermediate density of 6, and this is sent to the addition / limiter 39. Further, the memory controller 42 specifies the bias data D from the line memory 33 by addressing.
B0 _i is read out and sent to the addition / limiter 39. Adding limiter 39 calculates the corrected bias data DBC _i adds the bias data DB0 _i and the gradation correction data DB1 _i. This modified bias data DBC
_i is sent to the comparator 41 via the selector 40.

The memory controller 42 also generates a 1-line start signal and sends it to the print controller 43. Upon receiving the 1-line start signal, the print controller 43 sends the bias comparison data of “0” to the comparator 41. At this time, the number of times the 1-line start signal is generated is counted by the 1-bit counter 43a, and when the count value is "1", the bias comparison data is sent. When the count value is "0", the gradation comparison data is sent.

The count value of the 1-bit counter 43a is
Since the one 1-line start signal is received, the count value is "1". Therefore, the print controller 43 sends the bias comparison data to the comparator 41. The comparator 41 compares the corrected bias data (DBC _i ) of each heating element with the bias comparison data (DC) output from the print controller 43,
When the latter is smaller than the former, the bias drive data of "1" is output. The serial bias drive data for one line output from the comparator 41 is sent to the shift register 45 and is shifted in the shift register 45 by the clock to be converted into parallel bias drive data. The system controller 31 confirms that the first line is in a state capable of thermal recording, and then latches the parallel bias drive data in the latch array 45. The latched bias drive data is sent to the AND gate array 47.

On the other hand, in response to the bias heating strobe request signal from the print controller 43, the strobe signal generating circuit 44 outputs the bias heating strobe signal AN.
It is sent to the D gate array 47. AND gate array 47
Is when the bias drive data is "H",
While the strobe signal is being input, the "H" signal is output. By this signal, for example, the transistor 48 ₁
There since turned on, the bias pulse having a pulse width corresponding to the strobe signal is supplied to the heating elements 17 _1, heating element 17 ₁ generates heat.

Hereinafter, the corrected bias data are compared line by line using the bias comparison data of "1" to "FF", and the heating elements 17 _{1 to} 17 _n are driven according to the comparison result. Therefore, the number of bias pulses in consideration of the resistance value of each heating element is applied to each heating element, and each heating element is corrected and bias-heated.

"FF" is displayed in the print controller 43.
When the comparison of the corrected bias data for one run by the bias comparison data of 1 is completed, the 1 line end signal is sent to the memory controller 42. The memory controller 42 is
The selector 40 is switched by the 1-line end signal, the line memory 32 is connected to the comparator 41, and the 1-line start signal is sent to the print controller 43. After switching the selector 40, the memory controller 42 sequentially reads the yellow image data for one line written in the image data line memory 32 one by one and sends the yellow image data to the comparator 41.

Since the second one-line start signal is input to the print controller 43, the count value of the counter of the print controller 43 changes from "1" to "0". As a result, the print controller 43
Compares the gradation comparison data “0” to “7F” with the comparator 4
It outputs to 1 in order. The comparator 41 first compares the gradation comparison data of “0” with the yellow image data of the first line from the line memory 32. When the former is smaller than the latter, the comparator 41 outputs "H".
The gradation expression drive data of is output.

The drive data for one line output from the comparator 41 is sent to the AND gate array 47 after passing through the shift register 45 and the latch array 46.
Then, in the AND gate array 46, the output of the AND circuit whose grayscale drive data is "H" becomes "H" during the input of the short strobe signal. Since the transistor to which the signal of "H" is input is turned on, the heating element connected to the turned-on transistor is driven by the gradation pulse having a short width to generate heat. Similarly, the gradation comparison data “1” to “7F” are compared, and the heating elements are driven by the number of gradation pulses corresponding to the yellow image data.

The print controller 43 sends a 1-line end signal to the memory controller 42 after the generation of the gradation pulse train of the first line is completed. Memory controller 4
2 outputs the system 1 line end signal to the system controller 31 in response to the second 1 line end signal. The system controller 31 confirms the completion of printing of the first line by the system 1 line end signal, and thereby rotates the platen drum 10 by one line to feed the paper. During this paper feeding, the system controller 31 sends a system 1 line start signal to the memory controller 42. The memory controller 42 switches the selector 40 and sends the corrected bias data from the addition / limiter 39 to the comparator 41. Further, the system controller 31 reads the yellow image data of the second line from the frame memory 40 by the system 1 line start signal, and writes it in the image data line memory 32.

After that, as described above, the correction bias heating is performed with a plurality of bias pulses, and then the gradation is performed with the number of gradation pulses corresponding to the yellow image data of the second line from the image data line memory 32. Expression heating. Then, K (K is 256 at maximum) P and J
(J is a maximum of 128) gradation pulses are applied to each heating element to thermally record the second line. In the same way,
Thermal recording is sequentially performed on the third and subsequent lines. When the count value p of the counter 31a reaches the final line number, the line recording for one frame of the yellow image ends.

During the thermal recording of the yellow image, as shown in FIG. 2, the portion of the color thermosensitive recording material 12 on which the yellow image is thermally recorded reaches the yellow fixing ultraviolet lamp 16 as the platen drum 10 rotates. The yellow fixing ultraviolet lamp 16 irradiates the color thermosensitive recording material 12 with near ultraviolet rays having a wavelength of about 420 nm. As a result, the diazonium salt compound contained in the yellow thermosensitive recording material 12 is decomposed and the coloring ability is lost.

When the platen drum 10 makes one rotation and the recording area comes to the position of the thermal head 14 again, the magenta image is thermally recorded line by line by the same process as the yellow image. Even in the bias heating during recording of the magenta image, the same number of bias pulses as in the thermal recording of the yellow image are used. However, since the bias heat energy BM of the magenta thermosensitive coloring layer is large, a strobe signal having a long pulse width is used, and thus a bias pulse having a wide pulse width is created. Then, by adjusting the number of bias pulses, all the heating elements are subjected to the correction bias heating so that the heating value becomes almost the same when recording the intermediate density of the optical density of 0.6. Also, the gradation pulse used for recording a magenta image has a wider pulse width than that for yellow recording. After recording the magenta image,
After fixing the magenta thermosensitive coloring layer, a cyan image is recorded line by line.

The maximum value of the bias pulse is "256", but depending on the heating element, it becomes smaller than this due to resistance value error and shading correction. However, even if the bias pulse is smaller than "256",
Since the bias heating period is "256" bias pulses, an idle period occurs between the bias heating and the gradation expression heating. If the rest period is long, the heat energy for bias heating is lost. 9 and 10
Is to bring the bias pulse train closer to the gradation pulse train to shorten the rest period between the bias heating and the gradation expression heating,
An example in which heat loss is reduced will be described. The comparison data generation circuit 50 is connected to the comparator 41 to which the image data DG _i and the corrected bias data DBC _i are selected and input. The comparison data generation circuit 50 includes a comparison data counter 51 and eight exclusive OR circuits 52 _a , 52 _b , ...
・ It has 52 _h . In this example,
The same components as those in the embodiment shown in FIG. 1 are designated by the same reference numerals. In this embodiment, the number of gradations is "256". In this way, by setting the number of bias pulses and the number of gradation pulses to be “256” in common, since common comparison data can be used, the bias comparison and gradation comparison sequences are the same. In this case, the counter 43a becomes unnecessary, and when printing one line, the print controller is operated for two lines.

[0064] Comparative data counter 51 sends the count output of the 8-bit by counting the clock signal to one input terminal of the exclusive OR circuit 52 _a to 52 _h. bias/
The gradation control signal is “L” in bias heating and “H” in gradation expression heating. Comparison data counter 51
Is reset to "0" by a reset signal and then counts up by "1" by a clock signal.

[0065] In the bias heating, since the bias / tone control signal is "L", the count value of the 8 bits from the comparison data counter 51, eight exclusive-OR circuit 52 _a
Inverted at ~ 52 _h . For example, when the count value is "3" in decimal, it becomes "252". Therefore, when the corrected bias data is "252", the output of the comparator 42 becomes "H" when the count value of the comparison data counter 51 is in the range of "3" to "255", and the heating element can be driven. Become. As a result, as shown in FIG. 11, the bias pulse train is closer to the gradation pulse train side and the rest period is shorter.

In the gradation expression heating, since the bias / gradation control signal is "L", the count value of the comparison data counter 97 is sent to the comparator 42 as it is. In addition,
The comparison data counter 97, at the beginning of the counting operation,
It is reset to "0". The processing procedure in the comparison data generating circuit is shown in FIG. Where "q" is
This is the count value of the counter that counts the line start signal.

The maximum value of the corrected bias data is "25".
When it is smaller than 5 ”, in order to shorten the pause period between the bias heating and the gradation expression heating and reduce the heat loss,
It is preferable to thin the bias drive pulse at regular intervals. Figure 1
2 and 13 show this embodiment. When the bias data is created, the discrete comparison data is sent to the comparator, and when the gradation pulse train is created based on the image data, the comparison data increased in order from the minimum value is sent to the comparator. Discrete comparison data is generated by the comparison data generation circuit 70 shown in FIG. The comparison data generation circuit 70 includes a comparison data counter 71 including an up / down counter and a count value of the comparison data counter 71 for bias / count.
Scrambler 7 for scrambling based on gradation control signal
2 and.

The comparison data counter 71 is in a subtraction mode from the preset value "FF" when the bias / gradation control signal is "H", and subtracts each time the clock signal is counted, and outputs an 8-bit down-count. "FF" ~
“0” is sequentially sent to the scrambler 72. Further, when the bias / gradation control signal is “L”, the addition mode from “0” is set, and the addition is performed every time the clock signal is counted,
The 8-bit up count outputs “0” to “FF” are sequentially output to the scrambler 72.

The scrambler 72 outputs the scrambled bias comparison data to the comparator 41 as shown in Table 1 when the bias / gradation control signal is "H". When the bias / gradation control signal is "L", the count value of the comparison data counter 71 is output as it is.

[0070]

[Table 1]

As shown in Table 1, when the bias drive data for which the bias / gradation control signal is "H" is created, the down-count output obtained by subtracting "8" from "255" in decimal notation by the scrambler 72. "255", "2"
47 ”,“ 239 ”,“ 231 ”, ...“ 15 ”,
"7", "254", "246", ... "8",
“0” is output to the comparator 41. Further, when the grayscale drive data in which the bias / grayscale control signal becomes “L” is created, the comparison data of “0” to “255” are sequentially output to the comparator 41. FIG. 13 shows a processing procedure in the comparison data generating circuit 70.

As described above, when the bias drive data is created, the comparison data is generated by subtracting "8" from "255" and scrambling, so that the tail end of each bias pulse train is generated as shown in FIG. It is said that the pulses have been almost completed and are just packed. Also, when creating gradation drive data,
Since the comparison data is generated in the order of “0” to “255”, the frontmost pulse of each gradation pulse train is justified. Therefore, since no pause period is generated between the bias pulse train and the gradation pulse train and the gradation expression heating is started immediately after the bias heating, the heat loss at the bias heating can be reduced.

Since the multiplication by the multiplier 38 uses the fixed point system, a large calculation error occurs due to the truncation below the decimal point. FIG. 15 shows an embodiment in which the arithmetic error is prevented from increasing even if the fixed point method is used. The resistance value error data line memory 34 stores the resistance value error data Δ for correcting the gradation expression thermal energy error.
A value obtained by multiplying R _i by X is written by the system controller 31. The multiplier 38 outputs the image data DG for recording an intermediate density of ΔR _i · X and an optical density of 0.6.
B is multiplied by the coefficient k2. After this, in the divider 60,
After dividing the output from the multiplier 38 by X, add this value
Send to the limiter 39. In this way, since the value multiplied by X is multiplied by the image data and the rounding operation after the decimal point is performed, the error due to the rounding operation is reduced as compared with the case where the multiplication is not performed by X. Therefore, even if the fixed-point arithmetic is adopted, the arithmetic error does not increase, so that it is not necessary to adopt the floating-point arithmetic, which makes the hardware configuration complicated, and the configuration can be simplified.

In the above embodiment, when the bias data is determined based on the resistance value of each heating element, the detected maximum resistance value is used as a reference to determine the difference from the standard resistance value. And 256 for the maximum resistance
The number of bias pulses was reduced based on the resistance value error of each heating element with this bias pulse given as a reference, but in addition to this, the average resistance value was calculated based on the detected resistance value. The number of bias pulses may be increased or decreased with respect to the average resistance value. Furthermore, the bias data for each heating element may be determined based on the minimum resistance value.

In the above embodiment, the line memory for writing the image data is one, but a plurality of line memories are provided.
While recording using the image data of one line memory, the image data of the next line may be written in the image data of the other line memory. Further, in the above embodiment, three line memories are provided and switched by the selector. However, in addition to this, the area of the memory is divided to configure each line memory, which is read by the memory controller. You may divide. Further, in the above embodiment, the gradation correction data DB1 _i is multiplied by the image data DGB for recording the intermediate density of “0.6”, the resistance value error data ΔR _i and the correction coefficient data k2 by the multiplier 38. , But instead of this, gradation correction data DB
1 _i may be calculated in advance and stored in the memory as lookup table data. In this case, since it is not necessary to perform the calculation each time, it is possible to quickly perform the uneven density correction based on the uneven resistance value. Further, not only the multiplication result but also the addition result may be stored as lookup table data. Also, instead of multiplying by the multiplier 38, the pre-multiplied data ΔR _i · k2 · DGB is
Alternatively, the multiplier 38 may be omitted. In this case, the multiplier 38 may be omitted. Moreover,
As shown in FIG. 16, one line of image data DG _i from the frame memory 30 is written in the first line memory 80, and the corrected bias data DB calculated in Equation 1 is calculated.
C _i is written in the second line memory 81, and these are read alternately by the system controller 82 and sent to the comparator 84 via the selector 83 to obtain the drive data for the bias pulse and the gradation pulse. Good.

The present invention can be applied to sublimation type thermal transfer recording as well as thermal recording. Further, the present invention can be applied not only to color recording but also to recording of one color halftone image. Further, although the line printer moves the color thermal recording material and the thermal head relatively one-dimensionally, the present invention can be applied to a serial printer in which both the color thermal recording material and the thermal head move. . Further, although the color thermosensitive recording material is rotated by the platen drum, the color thermosensitive recording material may be linearly reciprocated by a pair of conveying rollers. Further, instead of changing the ON / OFF time of the strobe signal for each color, the number of bias pulses and gradation pulses may be changed for each color. Further, the length of the strobe signal is changed between the bias heating and the gradation heating, but the same strobe signal may be used. In this case, the number of pulses is changed according to the heating amount. Further, in the above embodiment, the correction amount is calculated by the linear function based on the resistance value error ΔR _i , but the correction amount may be calculated by using the quadratic function or the like. Further, instead of using a large number of bias pulses, the correction bias heating may be performed with one bias pulse. In this case, the pulse width is changed for each heating element.

[0077]

According to the present invention, the unevenness of heat generation due to the variation of the resistance value of each heat generating element and the bias thermal energy error generated at the time of bias heating, and the gradation expression generated at the time of gradation heating. It is calculated separately from the thermal energy error, and these errors are eliminated by changing the number or width of the bias heating drive pulses. Therefore, even if the resistance value of each heating element varies. It is possible to eliminate the influence of this variation and suppress the occurrence of color unevenness due to this. Further, since it becomes possible to correct the uneven heat generation due to the uneven resistance value of the thermal head, the tolerance of the uneven resistance value to the thermal head is increased, the yield of the thermal head is improved, and the cost can be reduced. Become.

Further, in order to eliminate uneven heat generation due to variations in resistance value of each heating element, when recording this intermediate density with reference to a fixed intermediate density which makes it easy to detect uneven density due to uneven heat generation. The density unevenness correction amount of is calculated in advance for each heating element, and the pulse number or width of the bias pulse is changed for each heating element based on this density unevenness correction amount. For example, it is possible to eliminate the uneven density and the uneven color due to the uneven resistance value of each heating element with reference to the vicinity of "0.6", so that the uneven density can be made inconspicuous. Further, since the uneven heat generation is not corrected based on the end of the bias heating, the uneven heat generation is corrected based on the intermediate density.
Since the density unevenness due to the resistance unevenness of the heating element is almost halved in the maximum density portion and the minimum density portion, the density unevenness can be made even less noticeable, and the printing quality can be improved. That is, in the low-concentration region, thermal energy that is slightly larger than the original gradation-expressing thermal energy error is given, but the resulting density change is hardly noticeable. On the other hand, in the high-concentration region, the concentration energy which is slightly smaller than the original gradation expression thermal energy error is given, but since the concentration is originally high in the high-concentration region, it is not conspicuous.

Also, with respect to the gradation expression thermal energy error, the intermediate density where the heat generation unevenness is most noticeable, for example, “0.6”.
Since it is determined uniformly based on the above, it is not necessary to individually use the image data of each heating element to perform correction, and the arithmetic processing is simplified. That is, the method of directly correcting the individual image data by the correction data requires a large amount of arithmetic processing, which requires a high-speed arithmetic circuit and raises the manufacturing cost. Can be resolved. Moreover, by driving each heating element with a pulse train composed of a large number of pulses, fine heating control can be performed, and the occurrence of density unevenness can be further reduced.

Further, by heating the bias pulse close to the gradation pulse for heating, or by thinning out at regular intervals when reducing the number of pulses in the bias pulse, the cooling period after bias heating becomes longer. Therefore, it is possible to suppress the decrease in thermal efficiency and perform accurate bias heating.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main part of a thermal head drive control device of the present invention.

FIG. 2 is a schematic diagram of a color thermal printer.

FIG. 3 is an explanatory diagram showing a layer structure of a color thermosensitive recording material.

FIG. 4 is a graph showing coloring characteristics of each thermosensitive coloring layer.

FIG. 5 is a graph showing the number of pulses for shading correction.

FIG. 6 is a graph showing the relationship between heat energy and color density in heating elements having different resistance values.

FIG. 7 is an explanatory diagram showing an example of a bias pulse train and a gradation pulse train.

FIG. 8 is a flowchart showing a processing procedure of a color thermal printer.

FIG. 9 is a block diagram showing a comparison data generation circuit according to another embodiment.

FIG. 10 is a flowchart showing a processing procedure for generating comparison data.

FIG. 11 is an explanatory diagram showing an example of a bias pulse train and a gradation pulse train.

FIG. 12 is a block diagram showing a comparison data generation circuit in another embodiment.

FIG. 13 is a flowchart showing a processing procedure for generating comparison data in the comparison data generation circuit.

FIG. 14 is an explanatory diagram showing an example of a bias pulse train and a gradation pulse train generated by the comparison data generation circuit.

FIG. 15 is a block diagram showing a main part of a thermal head drive control device in which a calculation error is reduced.

FIG. 16 is a block diagram showing another embodiment including line memories for image data and corrected bias data.

[Explanation of symbols]

12 color thermal recording material 14 thermal head 17 _{1 to} 17 _n heating element 31 system controller 32 to 34 line memory 37 resistance value detection circuit 38 multiplier 39 adder / limiter 41 selector 42 memory controller 43 print controller 44 strobe signal generation circuit 50, 70 comparison data generation circuit 51,71 comparison data counter 60 divider

Claims (6)

[Claims] 1. A pixel is used as a recording material by applying a bias heating drive pulse and a number of gradation expression heating drive pulses corresponding to image data to each heating element of a thermal head in which a large number of heating elements are arranged in a line. In a thermal head drive control method of a thermal printer for recording, the intermediate density is recorded with reference to an intermediate density at which density unevenness due to unevenness of heat generation is conspicuous so as to eliminate uneven heat generation due to resistance value error of each heating element. A thermal head drive control characterized in that a density unevenness correction amount at this time is obtained in advance for each heating element, and the pulse number or width of the bias heating drive pulse is changed for each heating element based on this density unevenness correction amount. Method. 2. The thermal head drive control method according to claim 1, wherein the reference intermediate density is an optical density of 0.6 to 0.8. 3. A thermal head drive controller for a thermal printer for recording pixels by applying a bias heating drive pulse train and a gradation expression heating drive pulse train to each heating element of a thermal head in which a large number of heating elements are arranged in a line. A first line memory for storing one line of image data, and bias correction data for correcting a bias thermal energy error generated during bias heating due to a resistance value error of each heating element. Means for storing the pulse number data of the bias heating driving pulse train as basic bias data for each heating element, the second line memory for storing this basic bias data for one line, and the reference intermediate density Gradation expression when recording the gradation expression heat energy that occurs due to the resistance value error of each heating element during heating. Means for storing gradation correction data for correcting an error as pulse number data of bias heating drive pulses for each heating element; and a third line memory for storing this gradation correction data for one line, An adder for adding the basic bias data from the second line memory and the gradation correction data from the third line memory to obtain corrected bias data, the corrected bias data from the adder, and the first line memory The image data from the first line memory is switched, and the heating elements are corrected and bias-heated by the bias heating drive pulse train based on the corrected bias data, and then the gradation is expressed by the heating drive pulse train based on the image data from the first line memory. A thermal head drive control device comprising a controller for heating. 4. The thermal head drive controller according to claim 3, wherein the reference intermediate density is 0.6 in optical density.
A thermal head drive controller characterized in that the thermal head drive controller is set to 0.8. 5. The thermal head drive controller according to claim 3 or 4, wherein heating is performed by bringing the bias heating drive pulse train close to the gradation expression heating drive pulse train. 6. The thermal head drive control device according to claim 3 or 4, wherein when the number of bias heating drive pulse trains is reduced, the thermal head drive control device is thinned out at regular intervals.