Correction apparatus for thermal printer

A correction apparatus for a thermal printer includes a memory for storing the deviation quantity of each heating element of a thermal print head or a temperature correction quantity, a first computer means for adding introduced image data and the deviation quantity wherein the gradation value of the image data is reflected when the heating element for printing the introduced image data is lower than the average resistance value or the current temperature, a second computer means for subtracting the deviation quantity wherein the gradation value of image data is reflected, from the introduced image data when the heating element for printing the introduced image data is higher than the average resistance value or the current temperature. Thus, the resistance correction according to the deviation of the heating element resistance and the temperature correction are realized by a simple circuit, thus reducing the memory capacity, which decreases the amount of required hardware.

BACKGROUND OF THE INVENTION 
The present invention relates to a correction apparatus for a thermal 
printer, and more particularly, to a correction apparatus for a thermal 
printer which performs corrections in accordance with the resistance 
deviation of a heating element, and performs temperature and color 
corrections, wherein the correction apparatus uses simple hardware 
construction. 
Generally, the sublimation type thermal printer prints using a thermal 
print head (TPH). Such a printer prints the desired image by using the 
energy emitted by the TPH to sublimate dye deposited on a film and thereby 
deposit the dye on recording paper. 
The block diagram of the general thermal printer is shown in FIG. 1. An 
analog-to-digital (A/D) converter 10 inputs the analog image signal 
transmitted from a signal input source, for example, a video camera or 
television, as red (R), green (G) and blue (B) signals which are then 
converted into digital signal form. 
A first selector 20 selects a signal output from A/D converter 10 or from 
digital image data transmitted through such protocols as GP-IB, SCSI or 
Centronics, by digital signal input sources such as a personal computer or 
graphics display computer. 
The image signal selected by first selector 20 is stored in screen memory 
30 by screen units of frames or fields under the control of memory 
controller 40 which controls data read/write timing. 
A second selector 50 constituted by multiplexers selects one signal among 
the R, G and B data stored in screen memory 30, and a color converter 60 
converts the selected signal into the complementary color signal, i.e., 
the B signal is converted into a yellow (Y) signal, the G signal is 
converted into a magenta (M) signal, and the R signal is converted into a 
cyan (C) signal. 
Additionally, a corrector 70 performs gamma correction, color correction, 
resistance correction and temperature correction on the output of color 
converter 60, which is then written in a line memory 80 by line units. 
The data read out from line memory 80 by line units is compared in terms of 
its gradation levels using a predetermined gradation value from a middle 
gradation converter 90. Then a strobe signal corresponding to the heating 
time is generated in the units of the compared gradation. TPH 100 is 
driven during the heating time interval, thereby performing a color 
printing operation. 
The color printing is performed by printing the three colors Y, M and C 
respectively on one recording paper according to the following process. 
When the data is read from screen memory 30, a vertical line of data is 
read by a second selector 50 for an initial B signal, and is then written 
into line memory 80 through color converter 60 as a Y signal. The data 
written into line memory 80 is modified by a middle gradation conversion 
in middle gradation converter 90, and then is transmitted to TPH 100, to 
thereby complete the printing of one line. Thus, when approximately 
500-600 lines are printed for one screen, the printing of one color (the Y 
color which is the complement of B color) is completed. 
Second selector 50 transfers data corresponding to one screen of the G 
signal from screen memory 30 to line memory 80 by vertical lines. Next, 
the printing of one screen of the M color (the complement of G color) is 
completed through the above-described process. Then the R signal for one 
screen is selected by second selector 50 and read from line memory 80 by 
vertical lines in the same manner. Then the printing of the C color (the 
complement of R color) is completed through the above process. 
In FIG. 1, A/D converter 10 to memory controller 40 make up image signal 
processing circuit 1, and second selector 50 to TPH 100 make up print 
control circuit 2. Also provided in the present invention is an image 
display circuit (not shown) for processing the output of image signal 
processing circuit 1 for display on a display device, e.g., a monitor. 
FIG. 2 is a detailed circuit diagram of the middle gradation converter 
shown in FIG. 1. Referring to FIG. 2, when the data of one vertical line 
is written into line memory 80, a gradation level generator 92 is enabled 
by an address generator 91, and the read address is output so that line 
memory 80 can perform a reading operation. 
Gradation level generator 92 outputs data for a first gradation level, 
i.e., "0000 0001", to an erasable programmable ROM (EPROM) 93 and to a 
gradation comparator 96. Assume that first gradation level (i.e., an 
optical density expressed as "0000 0001") is 0.2. In order to apply energy 
E1 to the printing film, as shown in FIG. 3, the strobe signal for the 
duration of electrification time t1 which corresponds to energy E1 is 
generated from a time interval generator 95 and is applied to a latch 
register 102. Thus, heating elements 103 emit heat for expressing the 
first gradation level. 
Accordingly, heat energy corresponding to a gradation level of an optical 
density is emitted in proportion to the optical density, as shown in 
S-shaped curve of FIG. 3, and heating time becomes longer as the optical 
density increases, as shown in FIG. 4. 
In the heating of second gradation level, gradation level generator 92 
outputs "0000 0010," and gradation comparator 96 compares the image data 
of line memory 80 which is input to first input terminal A with the 
gradation data input to second input terminal B. The operation is 
performed 256 times, .i.e., once for each gradation (0-255), whereby a 
logic "high" is output when the image data of line memory 80 is higher 
than the gradation data of gradation level generator 92, and when lower, a 
logic "low" is output. Then, the output data of gradation comparator 96 is 
sequentially delivered to shift register 101. For example, approximately 
512 data bits are shifted and stored for the thermal printer which prints 
on A6 size recording paper. This example assumes a case where 512 TPH 
heating elements (103 of FIG. 2) are needed for printing one line of A6 
size paper. 
In EPROM 93, the heating time is preprogrammed corresponding to the 
gradation data generated from gradation level generator 92, and a strobe 
signal is generated for each gradation from a time interval generator 95 
which corresponds to the time constant determined by the capacitance of 
capacitor C1 and the resistance value of one of resistors (rl-rm) selected 
by driving electronic switch 94. The generated strobe signal is also 
applied to latch register 102. 
The output of shift register 101 is delivered to latch register 102, to 
thereby cause the heating of heating elements 103 during the time interval 
t2 generated from time interval generator 95. 
Thus, when the heating is completed according to the above-described 
process for the 255th gradation, printing for one line is completed. In 
like manner, heating for 500-600 lines in one screen of a video printer 
for use with A6 size paper is performed. Heating for the three 
complimentary colors Y, M and C is performed the same as in the above 
process, thereby performing a color printing. 
Meanwhile, a color correction for the YMC dam, a temperature correction for 
the heating element by each gradation, and a resistance correction 
according to the deviation of the resistive heating elements are all 
performed in corrector 70. Ideally, the resistance correction should be 
based on the same resistance values for each heating element, but 
generally speaking each resistance has a variation depending on specific 
production conditions. 
Here, energy (E) can be expressed according to the following equation (1). 
##EQU1## 
To illustrate how varying resistances affect image quality, assume that 525 
heating elements are required for printing one line and the reference 
resistance value (the average resistance value) thereof is 3 K, and that 
the heating time (T) and the applied voltage (V) are fixed. When the 
resistance value is larger than 3 K due to the deviation of each 
resistance value of heating elements 103, the heating energy decreases, as 
shown in Equation (1). As a result, the image quality is degraded in the 
main scanning direction as shown in FIG. 5, by the generation of a dim 
trace in the horizontal direction. 
Accordingly, since the energy emitted by each resistance differs from that 
of the others, the same density cannot be obtained even though the 
electrification is performed for the same duration so as to obtain images 
having the same density. Therefore, the desired color of an image is 
difficult to achieve. 
To solve this problem, a TPH manufacturer estimates each resistance of the 
thermal print head and provides this information to the various hardware 
manufacturers of image processors, print controllers and image displays 
for driving a TPH. The hardware manufacturer then changes the estimated 
data, such that corrector 70 can correct for the deviation of each 
resistance. 
Corrector 70, as shown in FIG. 6, functions as follows: Uncorrected m-bit 
image data generated from color converter 60 is input as the address 
signal of the lower m-bits of a second ROM 72. In a first ROM 71, the 
resistance location address of k-bits of TPH 100 corresponding to the 
heating element to which the current image data is being applied is input 
from address generator 91 of middle gradation converter 90. Then first ROM 
71 outputs n-bits which represent a stored quantized value corresponding 
to the degree of deviation between the resistance value of the input 
address and the reference value (an average resistance value). Then the 
output data of first ROM 71 is input as the address signal of the upper 
n-bits of second ROM 72. 
Here, each resistance location address bit corresponds to a power of two, 
e.g., when the number of resistive elements of TPH 100 is 512, "k" 
consists of nine bits, because 512 is two to the ninth power. In the same 
manner, if the number of elements is 2048 (2.sup.11), the resistance 
location address signal k consists of eleven bits. The bit number of k is 
larger than that of n. 
When the total number of resistances is 2048, k is 11 bits since k-bits is 
a resistance location address of TPH 100. The capacity of second ROM 72 
can be greatly extended by reducing the k-bit data to n-bit data by using 
first ROM 71. 
This can be understood by considering that the maximum number of gradation 
levels which can be expressed by m-bit image data is 2.sup.m. Therefore, 
the memory capacity required for each resistor of TPH 100 is 2.sup.m. 
Accordingly, to store all the data, a large capacity (2.sup.m .times.2048) 
is needed for the total of 2048 resistances. Since the m-bit image data 
consists of eight bits, about 1 M bytes of memory capacity is needed. 
In first ROM 71, since the amount of resistance deviation between an 
arbitrary resistance and the adjacent resistance in TPH which consists of 
a plurality of resistances is small, 11-bit data can be converted into 6 
bits by grouping several adjacent elements. Then the output (6-bits data) 
of first ROM 71 can be input to the address of second ROM 72. Accordingly, 
the capacity of second ROM 72 can be decreased using first ROM 71. 
In second ROM 72, a more desirable thermal print head can be obtained as 
the variation of the resistance value of TPH becomes smaller. This is 
impractical, however, due to semiconductor manufacturing process 
limitations. With the allowable value of the maximum variance fixed as 
"111111" in binary form, a 6-bit signal is output whose most significant 
bit (MSB) is a sign bit. 
When the MSB is "1," the relevant resistance value is larger than the 
average resistance value. Therefore, the address of second ROM 72, wherein 
the data whose gradation is lower than that of the currently input data is 
stored, is accessed. If the MSB is "0," the relevant resistance value is 
smaller than the average resistance value. Therefore, accessing of second 
ROM 72 is carried out for the data whose gradation is higher than that of 
the currently input image data. Here, 256 eight-bit data strings 
constitute one block, and second ROM 72 is composed of 64 blocks in total. 
To explain this in more detail, assume a first element of TPH resistance is 
3.4 K and the average resistance value of TPH is 3.5 K, and the resistance 
location address is input to first ROM 71 as "00000000001." Then, 6-bit 
data of "100011" is stored into the corresponding resistance location 
address of first ROM 71 so as to print using a gradation level which is 
three levels lower than average, since energy E increases when the 
resistance R decreases according to Equation (1). 
Further, when the data is input to the upper bit address of second ROM 72, 
the substantial compensative data whose gradation is lower by three 
gradations than that of the image data output from color converter 60 is 
output. 
When these two ROMs (71 and 72) are used, 2,048 (2.sup.11) bytes are 
required for the capacity of first ROM 71 and 16 K bytes (64.times.256) 
are required for the capacity of second ROM 72 if TPH has 2048 heating 
elements. As a result, memory capacity is decreased. 
The quantity of hardware decreases as the capacity of the memory decreases, 
thereby reducing the cost. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
correction apparatus for a thermal printer which realizes temperature, 
color, and resistance corrections using simple hardware construction in a 
sublimation type thermal printer. 
To accomplish the above object, there is provided a thermal printer which 
introduces an image signal from a signal input source and performs a 
gradation comparison with respect to a predetermined gradation value by 
line units, and then performs printing using a thermal print head 
consisting of a plurality of heating elements, the printer comprising: 
a line memory for storing the image signal by line units; 
correction means having a memory wherein the deviation information of each 
heating element of the thermal print head is stored, and computing means 
for adding the image data of the line memory to the amount of deviation 
wherein the gradation value of the image data is reflected, by reading the 
deviation information stored in the memory when the resistance value of a 
heating element used for printing image data read from the line memory is 
lower than average resistance value, and for subtracting the amount of 
deviation wherein the gradation value of the image data is reflected, from 
the image data of the line memory by reading the deviation information 
stored in the memory when the resistance value of heating element for 
printing image data read from the line memory is higher than average 
resistance value; and 
TPH control means for performing a gradation comparison between the output 
of the correction means and the predetermined gradation value, and 
outputting the result to the thermal print head. 
In another embodiment of the present invention, there is provided a thermal 
printer which introduces an image signal from a signal input source and 
performs a gradation comparison with a predetermined gradation value by 
line units, and then performs printing using a thermal print head 
consisting of a plurality of heating elements, the printer comprising: 
a line memory for storing the data of the image signal by line units; 
detecting means for detecting the temperature of the current thermal print 
head; 
correction means having a memory wherein the correction information 
corresponding to the difference between the temperature of the current 
thermal print head and a predetermined reference temperature is stored, 
and computing means for summing the image data read from the line memory 
and the amount of correction wherein the gradation value of the image data 
is reflected, by reading the correction information stored in the memory 
when the detected temperature of the current thermal head is lower than 
the predetermined reference temperature, and for subtracting the amount of 
correction wherein the gradation value of the image data is reflected, 
from the image data of the line memory, by reading the correction 
information stored in the memory when the detected temperature of the 
current thermal print head is higher than the predetermined reference 
temperature; and 
TPH control means for performing a gradation comparison between the output 
of the correction means and the predetermined gradation value, and 
outputting the result to the thermal print head.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be described below in more detail with reference 
to the attached drawings. 
A thermal printer to which the correction apparatus according to the 
present invention is applied has the same construction as that of the 
thermal printer shown in FIGS. 1 and 2. 
FIG. 7 is a circuit diagram of an embodiment of a correction apparatus for 
a thermal printer according to the present invention. 
Referring to FIG. 7, an input terminal of a first ROM 171 is connected to 
the output terminal of address generator 91 of middle gradation converter 
90 shown in FIG. 2. The output terminal of first ROM 171 is connected to 
the control contact points of third and fourth control switches SW3 and 
SW4 and to second input terminals of first computing means 172 and second 
computing means 173. 
Input terminals of first and third control switches SW1 and SW3 are 
connected to the output of color converter 60 shown in FIG. 1, and the 
output terminals thereof are connected to first input terminals of first 
computing means 172 and second computing means 173, respectively. Input 
terminals of second and fourth control switches SW2 and SW4 are connected 
to the output terminals of first computing means 172 and second computing 
means 173, respectively, while their output terminals are connected to the 
input of line memory 80 shown in FIG. 1. The input of inverter INV1 is 
connected to the output terminal of ROM 171, and the output thereof is 
connected to both control contact points of first and second control 
switches SW1 and SW2. 
Operation of the correction apparatus shown in FIG. 7 is explained as 
follows. 
When the resistance location address currently being printed is input to an 
input terminal ADDR of first ROM 171, a coefficient is output which 
corresponds to the amount of deviation of the resistance associated with 
the resistance location address. 
The coefficient is stored in first ROM 171 in binary form and is expressed 
by (n-1)-bits, with the most significant bit being a sign bit. 
First through fourth control switches SW1 to SW4 are selectively operated 
depending on the most significant bit (sign bit) among the n-bit data 
output from ROM 171. 
When the sign bit is "high," which means that the corresponding resistance 
value designated by the resistance location address is higher than the 
average resistance value, third and fourth control switches SW3 and SW4 
are closed. When the sign bit is "low," which means that the corresponding 
resistance value designated by the resistance location address is lower 
than the average resistance value, first and second control switches SW1 
and SW2 are closed via inverter INV1. 
When first and second control switches SW1 and SW2 are closed, the 
uncorrected image data is input to a first input terminal of first 
computing means 172 from color converter 60. First computing means 172 
adds (n-1)-bit coefficient data for the gradation increase according to 
the deviation of the relevant resistance output from ROM 171 to the 
uncorrected image data. As a result, the m-bit compensative data is 
output. 
First computing means 172 is explained in more detail as follows. 
For convenience, assume that the image data before the compensation is "i", 
and (n-1)-bit coefficient output from first ROM 171 is k. Output Q of 
first computing means 172 can be expressed as follows. 
EQU Q=i+ik . . . (1) 
That is, image data (i) before the compensation and coefficient (k) is 
multiplied in first computing means 172. The result (ik) is added to image 
data (i) before the compensation and is output. When, as a result of the 
computation, a carry is generated in the (m+1)-bit of the output of first 
computing means 172, each of the m-bit outputs (Q) output to second 
control switch SW2 is "1". 
Accordingly, output Q assumes 2.sup.m as its value. For example, if m is 
8-bits, the value of Q cannot exceed 255 expressed in decimal form. 
When third and fourth control switches SW3 and SW4 are closed, the 
uncorrected image data is input to a first input terminal of second 
computing means 173 from color converter 60. Second computing means 173 
subtracts (n-1)-bit data for the gradation decrease according to the 
deviation of the relevant resistance output from ROM 171, from the 
uncorrected image data, thereby outputting the compensated data. 
Second computing means 173 is explained in more detail as follows. 
Output Q' of second computing means 173 can be expressed as follows. 
EQU Q'=i-ik . . . (2) 
That is, image data (i) before the compensation and coefficient (k) is 
multiplied in second computing means 173. The result (ik) is subtracted 
from image data (i) before the compensation and is output. 
When, as a result of the computation, a borrow is generated in the 
(M+1)-bit of the output of second computing means 173, each of the m-bit 
outputs (Q') output to fourth control switch SW4 is "0". 
Accordingly, output Q of first computing means 172 and output Q' of second 
computing means 173 are the values wherein the gradation value of the 
image data commonly expressed by 256 gradations incorporates the amount of 
deviation of each resistance. The outputs of first and second computing 
means 172 and 173, i.e., the image data which is actually printed, is 
compensated in accordance with the amount of deviation of the heating 
element resistance and with the gradation value. 
Here, not only the compensation value according to the resistance deviation 
but also the temperature and color-correction data obtained through 
experiment can be stored in ROM 171, as shown in FIG. 8. For example, a 
coefficient of correction data in accordance with the current TPH 
temperature can be stored in first ROM 171 for each of colors Y, M and C. 
The means (not shown) for detecting the current TPH temperature is a 
thermistor or the like, which is commonly known. 
Correction data is stored in first ROM 171, so that when the current TPH 
temperature is higher than the predetermined reference temperature, the 
gradation value of the image data output from line memory 80 can be 
lowered. Correction data for increasing the gradation of the image data 
output from line memory 80 when the current TPH temperature is lower than 
the reference temperature is also stored. 
Here, one bit, i.e., the most significant bit of the correction data, is 
used as a sign bit. When the most significant bit is "1," the current TPH 
temperature is higher than the predetermined reference temperature, and 
when the most significant bit is "0," the current TPH temperature is lower 
than the predetermined reference temperature. 
In first ROM 171, the correction data for varying the image data value 
stored in the line memory according to the difference of the temperature 
of the current thermal print head and the reference temperature is stored 
in a look-up table for yellow, magenta and cyan colors. 
As described above, the correction apparatus of the thermal printer of the 
present invention corrects for resistance deviation of a heating element, 
and performs temperature and color corrections using a simple circuit, to 
thereby reduce the memory capacity and hardware volume.