JPH03182367A - Color thermal printer of density gradation controlling type - Google Patents

Abstract

PURPOSE:To correct the non-linearity of the conductive time-density characteristic curve and to correct the inconsistency of characteristics of colors, thereby to obtain the highly accurate density gradation approximately uniform among the colors by controlling the conductive duty ratio for every conductive unit cycle through a conductive interval in accordance with the kind of a printing color. CONSTITUTION:A CPU 40 refers to the data preliminarily set in a predetermined table within a memory 42 so as to variably control the conductive duty ratio for every unit cycle. The time corresponding to the conductive duty ratio is counted inside according to a program or the like based on this data, or the data is set in a timer counter built in a strobe signal generating circuit 38, so that a strobe signal ST is generated from the circuit 38. The conductive duty ratio when cyan color is printed is set slightly smaller than that when magenta color is printed, and the conductive duty ratio when yellow color is printed is set further smaller than that for the cyan color. Therefore, the inconsistency of the conductive time-density characteristic curve among colors is corrected to be overlapped, whereby the approximately uniform density gradation is achieved among the colors.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a density control system in which the density gradation of each pixel is controlled by energizing each heating resistor element for an energizing time that is an integral multiple of the unit energizing time. This invention relates to a gradation control type color thermal printer.

[Prior art] In general, the density gradation control method applies a constant voltage to each heating resistor element of a thermal heater and controls the energization time, thereby giving gradation to the heat generated energy and adjusting the temperature of the pixel. I'm trying to give it some gradation.

FIG. 7 shows an energization time-density characteristic curve used in the density gradation control method. In the solid curve (M), when the heat generating resistor element is energized for the energizing time TI corresponding to the unit energizing time ΔT, a temperature of gradation level dl is obtained, and only for the energizing time T2 corresponding to twice the unit energizing time ΔT. When energized, a density of gradation level d2 is obtained.

In this example, a maximum gradation level d64 near the saturation temperature can be obtained by energizing for a energizing time Tl1i4 corresponding to 64 times the unit energizing time ΔT.

[Problems to be Solved by the Invention] However, as shown in FIG. 7, the proportional relationship between the current application time and the concentration is not linear. Therefore, conventionally, a special arithmetic circuit was provided to convert density data into time-axis data (current application time data) in seven stages of signal processing before printing, and linearization correction was performed during the conversion process. However, this circuit was not only expensive, but also increased the number of data bits for correction, making subsequent processing and circuits complicated.

In the case of a color thermal printer, one print is generally created by overlapping the three colors of yellow (Y), magenta (M), and cyan (C), but as shown in Figure 7, each color The energization time-concentration characteristic curves are not the same between the two, and there are some differences. That is, magenta (
Based on the characteristic curve (solid line) of M), cyan (C)
Characteristic song! 1 (dotted line) is shifted somewhat to the left. Therefore, Y.

If the printing energization time for M and C colors is set at the same rate, the temperature gradation of the three colors will be unbalanced, and a highly accurate full-color image cannot be reproduced. Regarding this point, in the past, the energization time when printing each color of Y, M, and C was relatively adjusted in the density-current-on-time conversion process;
This requires a more expensive conversion circuit, increases the number of data bits, and makes subsequent processing more complicated.

Furthermore, if heat accumulates in the thermal head during printing, both of the characteristic curves (FIG. 7) shift to the left, so it is necessary to detect the temperature of the head and perform temperature compensation. Regarding this point as well, conventionally, temperature compensation has been performed during the concentration-current application time conversion process, which has aggravated the above-mentioned problems.

The present invention was made in view of these problems, and it corrects the non-linearity of the energization time-density characteristic curve without using an expensive special signal conversion processing circuit, and at the same time corrects the inconsistency of the characteristic curves between each color. It is an object of the present invention to provide a density gradation control type color thermal printer which can obtain uniform density gradations for each color.

[Means for Solving the Problems] In order to achieve the above object, the color thermal printer of the present invention provides one-to-one printing for a plurality of pixels on one printing line.
By energizing each of the plurality of corresponding heating resistive elements for a time that is an integral multiple of the unit energization cycle, a predetermined density gradation is given to each pixel, and three colors of yellow, cyan, and magenta or black are added to them. In a density gradation control type color thermal printer that creates one print by overprinting four additional colors, the energization duty ratio is controlled for each unit energization cycle according to the type of color to be printed. The structure is equipped with means.

Furthermore, in order to perform temperature compensation, the present invention is configured to include means for controlling the energization duty ratio for each unit energization cycle according to the type of color to be printed and the temperature of the thermal head.

[Function] According to the present invention, the energization duty ratio within a unit energization cycle is not constant (fixed) throughout the energization interval, but is changed for each unit energization cycle, and is also changed depending on the type of color to be printed.

The energization duty ratio within each energization cycle may be set in advance as data based on the energization time-concentration characteristic curve, statistical values, etc. In the energization interval, it gradually decreases at the beginning, remains approximately constant in the middle, and at the end. The colors to be printed are magenta (M), cyan (C), and yellow (Y).
The energization duty ratio within each energization cycle is selected to be the smallest value in the order of .

With these characteristics, the energization duty ratio is controlled for each energization cycle for each energization interval for each color, thereby correcting the nonlinearity of the energization time-density characteristic curve, and at the same time correcting the deviation in characteristics between each color. The correction is performed, and highly accurate density gradations that are approximately uniform for each color can be obtained.

Therefore, there is no need for a special circuit that converts density data into energization time data and performs the above correction at the stage of signal processing before energization, and the number of data bits does not increase.

In addition, at the same time as the control for each color, the influence of heat accumulation in the head can be compensated for by variable control of the energization duty ratio within a unit energization cycle according to the temperature of the head, and therefore special signal conversion processing and compensation can be performed. No more circuit is needed.

[Embodiment] FIG. 1 shows the main structure of a density gradation control type thermal printer according to this embodiment.

In FIG. 1, the thermal head 10 includes, for example, 51
The heating resistor 12 is formed by arranging two heating resistance elements R1 to R512 in a row, and the same number of heating resistance elements (51
A shift register 14 and a latch circuit 16 having a bit capacity of 2) are provided. Furthermore, a thermistor 18 for temperature compensation is provided close to the heating resistor 12, and its output signal is converted into digital temperature detection data DT by an A/D converter 44 and then supplied to the CPU 40.

During the printing time of each printing line, the data comparison circuit 28
512-bit serial gradation data [CK PIJ-
CK P512J] multiple times at regular intervals, e.g. 64
The signal is continuously applied to the shift register 14 times (K=1 to 64). Here, the n-th bit CKPnj specifies that the n-th heating resistance element Rn has a unit energization cycle Δ.
Contains information on whether or not electricity should be applied during T. In other words “
If it is 1", it instructs energization, and if it is "&IQ", it instructs de-energization.

Thus, when each gray scale data is loaded into the shift register 14 in synchronization with the clock signal GK from the clock circuit 34, each bit CK PIJ- is then loaded at the timing of the latch signal LA from the latch signal generation circuit 36. C.K.
P512J is sent as an electric pulse to the heat generating resistor 12 via the latch circuit 16, and the heat generating resistive elements R1 to R512 are selectively energized and generate heat during a unit energization cycle Δτ according to the information content of the corresponding bit.

This unit energization cycle ΔT consists of an energization time tE in which current actually flows through the heating resistor element and a time tC in which current does not flow.
Defined by T. In this embodiment, the ratio of the energization time tE to the unit energization cycle ΔT, that is, the energization duty ratio, is not constant (fixed) throughout the energization interval, but is set in units according to the type of color to be printed under the control (Color) of the CPU 40. Changes with each energization cycle.

FIG. 2 shows a unit energization cycle according to this embodiment.

As shown, unit energization cycle ΔT n +ΔT n
The length of the energization time tE, n t tE, n+1, and thus the energization duty ratio change for each +1.

FIG. 3 shows a line printing cycle according to this embodiment. TE, N, TE, N+1 are energization intervals,
It consists of 64 (times) unit energization cycles ΔTl to ΔTG4. TC,N, TC,N+1 are cooling intervals.

In order to perform such variable control of the energization duty ratio for each unit energization cycle, the CPU 40 refers to data preset in a predetermined table in the memory 42. Then, based on the data, the time corresponding to each energization duty ratio is internally measured by software, or by setting the data in a timer/counter built into the strobe signal generation circuit 38, the circuit 38 generates a A strobe signal ST as shown in FIGS. 2 and 3 is generated.

Figure 4 (A, (B), and (C)) shows each unit cycle ΔTl to ΔT when printing each color (magenta, cyan, yellow).
An example of setting energization duty ratio data for l1i4 is shown. In each figure, the energization duty ratio gradually becomes smaller at the beginning of the energization interval (ΔTl to ΔT 10), becomes - constant at the middle (ΔTll to ΔT55), and becomes constant at the final stage (ΔT10).
'l'5G to ΔT64), it gradually increases.

With these characteristics, the energization duty ratio changes for each unit energization cycle throughout the energization interval, and the seventh
The nonlinearity of the energization time-concentration characteristic curve shown in the figure is corrected, and the relationship between the number of energizations and the concentration becomes a linear proportional relationship as shown in FIG.

Among the colors, the energization duty ratio during cyan printing is set to a somewhat smaller value than the energization duty ratio during magenta printing, and the energization duty ratio during yellow printing is set to an even smaller value.

As a result, the deviation (
Figure 7) is corrected and overlaps as shown in Figure 6)
, substantially uniform density gradation can be obtained for each color.

Further, according to this embodiment, the CPU 40 controls the head temperature based on the head temperature detection data DT from the thermistor 18.
Control (Temp) is performed to change the energization duty ratio within a unit energization cycle according to the temperature of zero. For this control, data of different energization duty ratios are set in a table in the memory 42 for each head temperature. However, the fourth
If the energization duty ratio data in Figure (A) (magenta print) is for a head temperature of 25', for example, for a head temperature of 30', other data as shown in FIG. 5 is referred to. In this way, when the head temperature becomes high, the energization duty ratio is reduced so as to reduce the thermal energy generated in each unit energization cycle ΔTl.

The gradation data [CK
Plj-CK P512j (k=!~64) is created as follows.

Digital video signals S Y, S N, and S C of each color (Y, M, and C) are input to the frame memory 20 as pixel data, respectively. Each row of frame memory 20 corresponds to a horizontal scanning line of a television image, and pixel data is written in an order corresponding to rask scanning. And Y, M, C
When overlapping printing is performed in this order, the following processing is first performed on yellow (Y) pixel data. That is, starting from the first column of the frame memory 20, one column (J)
Pixel data for each l life alJ, a2j,...
....a512J is read out and supplied to the color process circuit 22, where it undergoes image processing such as inverse gamma correction, and then is converted into 6-bit density data blJ.
, b2j.

......converted to b512j. Each of these density data blJ is (0) (minimum density) to (84
) (maximum moisture level) and has a value (gradation level) according to the density of the corresponding pixel.

Density data for one print line outputted from the color process circuit 22 blJ, b2J, -...
- b 512J is once taken into the line buffer y2B and then applied to one input terminal of the data comparison circuit 28. Writing and reading of line buffer y26 is done using DMA.
This is done under the control of the controller 30.

The other input terminal of the data comparison circuit 28 is supplied with a 6-bit comparison reference value DN, which is incremented by 1 at regular intervals, from the gradation counter 32 during the energization interval TE. The data comparison circuit 28 compares this comparison reference value DN with each density data, and sets the bit to "1" when the latter is equal to or larger than the former, and sets the bit to "0" when the latter is equal to or larger than the former. Generate bits as tone bits.

For example, energization time data Blj, B2J,...B
The values of 512j are <10>, <2>, etc., respectively.
Assume that <1>. In this case, in the first comparison, the comparison base l value DN is (1), and the first gradation data [CI PIJ, CI P2J, ...
CIP512jl becomes [1, 1,...1]. In the second comparison, the comparison reference value DI is (2), and the second tone data [C2PIJ, C2P
2J.

...C2P512J] becomes [1, 1, ...0. In the third comparison, the comparison reference value DN is (
3), the third gradation data [C3PIJ, C3P2
J.

...C3P512J] becomes [1, O, ...0.

In this way, during the energization interval TE, each time the density gradation comparison reference MDN increments by one step, it is compared with each of the density data b, b2J, b512j, etc. , gradation data according to each comparison result [CI Plj, CI P2J, ... CIP
512J], [C2Plj, C2P2J,...
...C2P512j].

. . . are serially sent to the shift register 14 of the thermal head 10 at a constant cycle, and the heating resistive elements RIR2, RIR2,
...R512 selectively energizes and generates heat.

Then, once an image is printed with yellow,
Next, the same processing as described above is performed for each image data in the order of magenta and cyan, and a full-color print is obtained as a result of overlapping printing of each color.

As described above, in this embodiment, the type of color to be printed (Y
. Since this also compensates for the heat storage effect, no temperature data/energization time conversion circuit is provided between the color process circuit 22 and the line buffer 26. Therefore, the line buffer 26 and the data comparison circuit 28 can be configured with circuits having the same number of bits (6 bits) as the color processing circuit 22.

In addition, although three colors (Y, M, C) were used in this example,
When creating a color print using four colors including black (B), the energization duty ratio for black (B) may be controlled for each unit energization time in the same manner as described above.

[Effects of the Invention] By having the above-described configuration, the present invention provides the following effects.

By controlling the energization duty ratio for each unit energization cycle throughout the energization interval according to the type of color to be printed, it is possible to correct the non-linearity of the energization time-density characteristic curve and at the same time correct the deviation in characteristics between each color. It is possible to obtain approximately uniform and highly accurate density gradation for each color. Therefore, an expensive special circuit for correcting density data by converting it into energization time data at the signal processing stage before energization is unnecessary, and the number of data pits does not increase.

In addition, at the same time as control for each color, the influence of heat accumulation in the head can be compensated for by variable control of the energization duty ratio within a unit energization cycle according to the temperature of the head, and a special signal for temperature compensation can be used. No need for a conversion circuit.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of the main circuit parts of a density gradation control type color thermal printer according to an embodiment of the present invention. FIG. 2 is a diagram showing the timing of a unit energization cycle according to the embodiment. Figure 3 is a diagram showing the timing of the line printing cycle according to the embodiment, and Figures 4 (A), (B), and (C) are the energization duty for each unit cycle when printing each color (magenta, cyan, yellow). A diagram showing an example of setting ratio data, Figure 5, is similar to Figure 4 (
FIG. 6 is a diagram showing an example of setting the energization duty ratio data when the head temperature changes with respect to the set value of A), FIG. The figure shows energization time-density characteristic curves for each color without correction according to the present invention. 10...Thermal head, R1, R2,...R512...Heating resistance element,
38...Strobe signal generation circuit, 40...C
P.U. 42...Memory. Figure 4 (A) Figure 4 (El) Figure 4 (C) Figure 5

Claims (2)

[Claims] (1) A predetermined density gradation is given to each pixel by energizing each of a plurality of heating resistive elements that correspond one-to-one to a plurality of pixels on one printing line for a time that is an integral multiple of a unit energization cycle. In a density gradation control type color thermal printer, which creates one color print by overlapping printing in three colors of yellow, magenta, and cyan, or four colors including black, A color thermal printer characterized in that it is equipped with means for controlling the energization duty ratio for each unit energization cycle according to the type of color. (2) A predetermined density gradation is given to each pixel by energizing each of the plurality of heating resistive elements that correspond one-to-one to the plurality of pixels on one print line for a time that is an integral multiple of the unit energization cycle. In a density gradation control type color thermal printer, which creates one color print by overlapping printing in three colors of yellow, magenta, and cyan, or four colors including black, A color thermal printer comprising means for controlling the energization duty ratio for each unit energization cycle according to the type of color and the temperature of the thermal head.