JPH01159266A - Thermal head drive system - Google Patents

Abstract

PURPOSE:To permit the relationship between the number of recording dots and an image density to reach an ideal level as close as possible by providing a gradation level holding means and a specific pulse application means. CONSTITUTION:A gradation level holding means 12 to hold the number (n) of recording dots which represents the gradation level of an image and a pulse application means 15 to apply a recording pulse to a heat generating element provided on a thermal head 14 in accordance with the number of recording dots to be output from the gradation level holding means 12 and at the same time, to apply an auxiliary pulse in accordance with the number of recording dots of an image matrix 11 within an application time of the recording pulse against an adjacent heat generating element to the heating heat generating element, are provided. Under this constitution, the recording dots are influenced by an adjoining heat generating element and the area of the recording dots increases to perform printing. Therefore, the value of image density for the number (n) of the recording dots represents an ideal property by presetting an auxiliary pulse to be applied in accordance with the number of recording dots.

Description

[Detailed Description of the Invention] [Summary] The present invention relates to a method for driving a thermal head of a thermal printer that uses heat to record images and other sentiments, and particularly relates to a method for driving a thermal head of a thermal printer that uses heat to record images and other sentiments, and in particular for a method for driving a thermal head of a thermal printer using heat. Regarding the driving method of the thermal head that controls In thermal printers that control the number of recorded dots to express the gradation of an image and print by driving linearly arranged heating elements, gradation level maintenance maintains the number of recorded dots that represent the gradation level of the image. A recording pulse is applied to a heating element provided in the thermal head in accordance with the number of recording dots output from the gradation level holding means and the recording pulse is applied to a heating element adjacent to the heating element. and pulse application means for applying an auxiliary pulse according to the number of recorded dots in the pixel matrix within the pulse application time.

[Industrial application field]

The present invention relates to a method for driving a thermal head of a thermal printer that uses heat to record information such as images, and particularly relates to a method for driving a thermal head that controls the dot area of an area gradation type recording method. .

[Conventional technology]

In conventional thermal printers of thermal type or thermal transfer type, binary printing is performed for a large number of pixels using a thermal head. It faces the platen 4 with the recording paper 3 interposed therebetween. The ink sheet 2 is heat-fusible, and by heating the ink sheet 2 with the thermal head 1, the ink on the ink sheet 2 is melted and transferred onto the recording paper 3 for recording. The thermal head 1 has heating elements for one line arranged along the direction perpendicular to the plane of the drawing, and recording for one line is performed almost simultaneously. When one line of recording is completed, the recording paper 3 and the ink sheet 2 are simultaneously transported in the direction of the arrow.

In such a recording method, as shown in FIG. 13, a plurality of recording dots 5 (in this case, 4×4 16 recording dots) are grouped together to form a pixel matrix 6, and the pixels within the pixel matrix 6 are The gradation (intermediate tone) is changed by changing the number of recording dots 7. In this case, the number of dots 7 in the pixel matrix 6 can be changed depending on the shading of the part of the pixel matrix 6 in the image to be recorded. The shading of the image is expressed and recorded by increasing and decreasing it. However, in this case, the area of each dot 17 is constant, and depending on the gradation, the number of dots 7 is increased when the gradation is dark, and the number of dots 7 is decreased when the gradation is light.

(Problem to be Solved by the Invention) By the way, in such a conventional area gradation type recording method, taking as an example a case where the pixel matrix size is NXN (N is a natural number) dots, the number of recorded dots is Number n and pixel density (sensitivity of light reflected from an image to the human eye) O
,D, which relationship is as shown in Figure 3. That is, the number of recorded dots n and the density 0. D means the number of dots is O and the pixel density is O.
Therefore, the pixel density becomes Do with the number of dots N2, but in the middle there is no linear relationship; in areas with high image density, even if the number n of recorded dots changes slightly, the pixel density of that area becomes 0. Since D changes significantly, the number of dots n
The pixel density is 0. Since the change in D is too large, there is a problem in that the gradation becomes discontinuous and a false contour occurs. Note that when recording halftone images, it is not ideal for the middle part to be linear, and the optimal input/output density curve varies depending on the nature of the image, but in general, the characteristics shown in Figure 4 are used. is considered desirable. Since the characteristics shown in FIG. 3 are significantly different from those shown in FIG. 4, there has been a problem in that image quality deteriorates due to this difference in characteristics.

The present invention was made in view of these conventional problems, and aims to provide a thermal head driving method that can bring the relationship between the number of recorded dots and image density closer to the ideal relationship. That purpose.

(Means for Solving the Problem) In order to achieve this object, the present invention has a structure in which the number n of recorded dots in the pixel and matrix 11 is controlled as shown in FIG. to express the gradation of the image,
A thermal printer 10 that performs printing by driving linearly arranged heating elements includes a gradation level holding means 12 that holds the number n of recording dots representing the gradation level of an image, and an output from the gradation level holding means 12. A recording pulse is applied to a heating element provided in the thermal head 14 in accordance with the number n of recording dots to be recorded, and the pixel matrix is applied to a heating element adjacent to the heating element within the application time of the recording pulse. A pulse applying means 15 is provided for applying an auxiliary pulse according to the number of recorded dots.

(Function) Next, to explain the present invention in detail, the number n of recording dots of the data of the gradation level to be recorded is temporarily held in the gradation level holding means 12 from the input data signal of the thermal printer 10. be done. And in the pulse application means 15,
Corresponding to the number n of recorded dots inputted from the gradation level holding means 12, the heating element provided in the thermal head 14 is controlled so that the relationship between the number n of recorded dots and the pixel density becomes an ideal curve. While applying a recording pulse, an auxiliary pulse is applied to a heating element adjacent to the heating element within the application time of the recording pulse according to the number of recording dots in the pixel matrix.

Recorded dots are affected by adjacent heating elements, and the area of the recorded dot increases when printed. Therefore, by setting the auxiliary pulse to be applied in advance according to the number of recorded dots, the number of recorded dots n can be increased. pixel density 0. The value of D has the ideal characteristics shown in FIG.

(Example) Embodiments of the present invention will be described below based on the drawings.

FIG. 2 is a block diagram showing an embodiment of the present invention, and shows a case where the number n of recording dots in the pixel matrix 11 is controlled to express the gradation of an image in a thermal printer 10 of a thermal transfer recording type. There is a transmission line 17 to which image data is input, and a recording driver decimal signal representing the gradation level of the pixel matrix of the image inputted from the transmission line 17 is transmitted by the rising edge of the clock signal from the data clock generator 18.
There is a latch circuit 20 as gradation level holding means 12 for holding the gradation level.

If the pixel matrix in this case is 4x4 dots, there are 17 gradation levels from level 0, where nothing is printed in the pixel matrix, to level 16, where all dots are printed, but generally all dots are printed. Since the gradation level is not used, there are 16 gradation levels from gradation level 0 to gradation level 15, and the gradation level is designated by a 4-bit signal.

Also, if there are 3456 dots in one thermal head line, the power consumption will be 1k if you drive them simultaneously.
Since the number of dots exceeds W, the dots are driven in parts, and as shown in FIG. 5, the dots from ■ to ■ are divided into eight times. If the dots are divided and driven in this manner, there will be no influence of adjacent dots, the dot shape will be small, and an image with high resolution can be recorded. An example of the driving timing is shown in FIG. 6, and as shown in the figure, the printing is performed by driving in the order of ■■■. Therefore, in order to change the area of the recording dot, we decided to apply an auxiliary pulse to the dots adjacent to the driven dot, and applied the auxiliary pulse at the timing shown in Figure 7 instead of the driving timing shown in Figure 6. do. That is, when dot (2) is driven, an auxiliary pulse is applied to adjacent dots (2) and (2), and although these adjacent dots do not print, they generate heat by being driven. Therefore, as shown in FIG. 8, the shape of the recording dot is such that when the auxiliary pulse is applied, the recording dot 31 is different from the recording dot 32 when the auxiliary pulse is applied, and the size of the recording dot is thermal. It spreads in the direction of the pitcher's force of the head, increasing the recording dot area. The spread of the area of the recording dot changes depending on the pulse width of the auxiliary pulse, and the larger the pulse width, the larger the area of the dot. Therefore, by controlling the pulse width, the area of the recording dot can be finely controlled.

In addition, the thermal head is driven at the timing shown in Figure 7, and one line is divided into 8 parts, and further divided into 2 parts.
Since the auxiliary pulse is divided, a clock obtained by multiplying the data clock by 16 times is used to drive the thermal head. In addition, in the timing shown in FIG. 7, the application timing of the auxiliary pulse to the dots on the right and left of the recording dot is shifted, so that the dot on the left is applied in the first half of the recording pulse, and the dot on the right is applied in the second half of the recording pulse. pulses are applied to reduce power consumption. However, if there is sufficient power supply capacity, the auxiliary pulses may be generated on both sides at the same timing, or the pulse length may be set to an arbitrary length that is half or more of the print pulse.

In order to apply such an auxiliary pulse, the number of recording dots, which is gradation level data from the latch circuit 20, is 4.
There is a ROM 25, which is one of the pulse application means 15, into which the bit signal, the horizontal dot position signal from the X address signal generator 21, and the vertical dot position signal from the Y address signal generator 22 are input. In response to the signal from the latch circuit 20, the ROM 25 stores a recording pulse presence/absence signal M indicating whether a recording pulse is present at the corresponding dot position. (1 bit) and an auxiliary pulse signal S (4 bits) depending on whether an auxiliary pulse is required for that recording dot. Next, there is a line buffer 26 into which the signal from the ROM 25 is input. The line buffer 26 is 1
A memory for 3456 line dots and a capacity of 16 x 3456 bits are required to temporarily store the clock pulses multiplied by 16 for divided driving, and a 6-bit pulse width signal to drive the thermal head and a buffer. Since double the capacity is required to make it a double buffer, it is necessary to have a capacity of 16x3456x6x2. The line buffer 26 is the next thermal head 1
4 (same as the thermal head 1 shown in FIG. 12) to drive the thermal head 14. In this way, the ROM 25 and the line buffer 26 form the pulse application means 15.

Image data is input from the transmission line 17 to the latch circuit 20,
A signal indicating the number of recorded dots representing the gradation level of the pixel matrix of the image is held in the latch circuit 20 by the rising edge of the clock signal from the data clock generator 18. The latch circuit 20 outputs a 4-bit signal indicating the number of recording dots, which is gradation level data, and the horizontal dot position signal is output from the X address signal generator 21, and the vertical dot position signal is output from the Y address signal generator. 22, the ROM 25 serves as one of the pulse application means 15.
Enter. Corresponding to the signal from the latch circuit 20, a recording pulse presence signal M0 (1 bit) indicating whether a recording pulse exists at the corresponding dot position and whether an auxiliary pulse is required for the recording dot are generated. , outputs an auxiliary pulse signal S (4 bits) to the line buffer 26. The line buffer 26 outputs one data clock.
Using the clock multiplied by 6 times, a recording pulse is applied to the heating element provided in the next thermal head 14, and the heating element adjacent to the heating element is applied to the heating element of the pixel matrix at the timing shown in FIG. Auxiliary pulses are applied depending on the number of recorded dots. Thus usually the third
By applying an auxiliary pulse, the image density characteristics shown in the figure can be changed to have any input/output characteristics, including the ideal density characteristics shown in Figure 4. be able to.

Next, a method for determining the recording pulse presence/absence signal Mo and the auxiliary pulse width signal, which should be written in the ROM 25 in advance in order to change the characteristics, will be explained. The relationship between the recording pulse width and the dot area in binary recording varies somewhat depending on the type of thermal paper used as the recording medium and the type of thermal ink sheet and recording paper, but the relationship is generally as shown in FIG. 9. As shown in FIG. 9, when the pulse width is less than tmin, the recording dot area is not constant with respect to the output of the pulse width, so stable recording dots cannot be obtained, and the pulse width is less than tmin.
to t□8, the recorded dot area increases monotonically as the pulse width increases. At t□8, the area ratio of the recording dots is 100%, and even if the pulse width exceeds this, the area ratio will not improve and the energy for printing will only increase unnecessarily, so t□8 should be ignored. Pulse widths exceeding this are not used.

Next, set the recording pulse width to tmi. When the relationship between the auxiliary pulse width and the recording dot area is determined under this condition, the characteristics shown in FIG. 10 are obtained. That is, the recording dot area ratio increases monotonically as the auxiliary pulse width increases, and an area ratio of 100% is obtained when the auxiliary pulse width is t8. Therefore, in order to determine the auxiliary pulse width so as to obtain the image density characteristics shown in FIG. It is possible to obtain a characteristic curve as shown in FIG. Next, this curve is superimposed on the ideal characteristic curve shown in FIG. 4 as shown in FIG. 11, and the area ratio to be increased or decreased for each number of recorded dots is determined. Once the degree of increase and decrease of the area ratio is determined, the auxiliary pulse width for each number of recording dots can be determined from the relationships shown in FIGS. 9 and 10.

In the above embodiment, the recording dot area is controlled by increasing or decreasing the auxiliary pulse width, but the recording dot area may be controlled by changing the number of pulses while keeping the auxiliary pulse width constant.

(Effects of the Invention) As explained above, according to the present invention, the characteristic of image density with respect to the number of recorded dots for tone expression of a thermal printer can be approximated to an ideal curve.
It is now possible to bring the density of recorded images closer to the natural state,
The gradation discontinuity that was previously noticeable in low and high density areas (
By removing false contours), the quality of recorded images can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the principle of the present invention, FIG. 2 is a diagram showing the configuration of an embodiment of the present invention, FIG. 3 is a diagram showing image density expressed with respect to the number of conventional recording dots, and FIG. The figure shows the ideal characteristics of image density with respect to the number of recorded dots, Figure 5 shows the block configuration of divided drive, Figure 6 shows the timing of divided drive, and Figure 7 shows the application of auxiliary pulses. Figure 8 shows the increase in recording dot area due to the presence of the auxiliary pulse. Figure 9 shows the relationship between recording pulse width and recording dot area ratio. Figure 10 shows the relationship between the recording pulse width and the recording dot area ratio. Diagram showing the relationship between width and recording dot area, 1st
Figure 1 is a diagram showing the process of optimizing image density characteristics with respect to the number of recorded dots, Figure 12 is a cross-sectional view perpendicular to the printing line of a thermal transfer printer, and Figure 13 is a diagram showing the pixel matrix and recorded dots. be. 10... Thermal printer 11... Pixel matrix 12... Gradation level holding means 14... Thermal head 15... Pulse applying means

Claims (1)

[Scope of Claims] A thermal printer (10) that expresses the gradation of an image by controlling the number of recording dots in a pixel matrix (11) and prints by driving linearly arranged heating elements. A gradation level holding means (12) for holding the number of recording dots (n) representing the gradation level, and a thermal head corresponding to the number of recording dots (n) output from the gradation level holding means (12). A recording pulse is applied to the heating element provided in (14), and an auxiliary pulse is applied to the heating element adjacent to the heating element according to the number of recording dots in the pixel matrix within the application time of the recording pulse. A thermal head driving method having a pulse applying means (15) for applying pulses.