JPH0370632B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention draws an image by scanning the screen while intermittently moving a head composed of a plurality of heating elements arranged in a line relative to the screen. When the intermittent movement is stopped, first, the odd-numbered (or even-numbered) heating elements are energized for a time corresponding to the gradation of image density of the screen printed by these heating elements, Next, image density of the screen printed by these heating elements is applied to each of the even (or odd) heating elements arranged adjacent to each of the odd (or even) heating elements. The purpose of this invention is to perform line density correction with respect to a thermal printer configured to print on the screen by energizing for a time corresponding to the gradation of the image.

As shown in FIG. 1, a thermal printer stacks a thermal ink paper 3 on a recording paper 2 wrapped around a rotating drum 1, and heats the ink paper 3 locally by applying heat from above with a heating head 4. The ink is made to adhere to the recording paper 2. When drawing images with shading using this thermal printer, as shown in FIG. This is done by scanning in the H direction. In this case, scanning is performed by moving the head 4 intermittently 1024 times, and when the head 4 stops moving intermittently, each heating element 6 is energized to each pixel for a time corresponding to the gradation of image density. This generates heat. In the case of the printer shown in FIG. 1, the head 4 is fixed and the scanning is performed by the drum 1 rotating intermittently.

As shown in the figure, the head 4 is divided into, for example, eight blocks 4 ₁ to 4 ₈ and has a configuration in which 64 heating elements 6 are arranged in each block, and each block is operated in sequence. It is done like this. The reason why the head 4 is divided into a plurality of blocks in this manner is due to reasons such as the structure of the head 4 and restrictions on current capacity. In addition, when drawing a color image, ink paper of four colors, such as magenta, yellow, cyan, and black, is used, and the above-mentioned scanning is performed for each color.

Head 4 divided into multiple blocks in this way
If the blocks are operated sequentially using For example, when the block 4 ₁ is operated first and its 64 heating elements 6 are energized to generate heat, the heat of the 64th heating element 6 causes the adjacent block 4 ₂ to operate.
The first heating element 6 of is heated. When block ₄₂ is operated next, since the first heating element 6 has been warmed in advance, the pixel printed by this heating element 6 will be darker than other pixels. This phenomenon also occurs for other blocks ₄₃ to ₄₈ . Therefore, when scanning is completed, streaks will occur at the boundaries between the blocks.

In order to prevent this phenomenon, conventionally each block is further divided into odd-numbered heating elements and even-numbered heating elements, and blocks ₄₁ to ₄₈ are first placed in odd-numbered positions. The heating elements placed in the blocks 4 ₁ to 4 ₈ are then activated. According to this method, even if the heating elements placed at both ends of each block are preheated by the heating elements placed at both ends of other blocks, the time from the time of preheating until the start of printing is becomes longer,
During this time, this heating element is cooled down. Therefore, it is not affected by preheating, and the above-mentioned streaks can be eliminated.

On the other hand, in this type of printer, improvements are being made to shorten the overall printing time by reducing the number of blocks in the head as much as possible or by shortening the time during which electricity is applied to the heating element as much as possible. If the above method is applied to a head with a small number of blocks or a short energized time, the time it takes for the predicted heating element to start printing will be shortened, so the heating element will not be cooled down sufficiently. Electricity will be supplied to the house. Therefore, the even lines on the completed screen will be darker than the odd lines.

The present invention was invented in order to solve the above-mentioned problem, and in the thermal printer mentioned at the beginning, the heating element is placed in the even (or odd) number and the heating element is placed in the odd (or even) number. Even when printing with the same image density with the heating elements arranged in This is to compensate for the preheating of the heating element by making the time shorter than the energization time. In other words, the line density is corrected by actively utilizing the initial preheating.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 shows an embodiment of a head control circuit according to the invention to which the above principle is applied.

The energization time to each heating element 6 of the head 4 is controlled according to the density of the image in this embodiment, and this control signal has a pulse width corresponding to the density of each pixel.
A PWM signal (pulse width modulation signal) is used.

Further, in this embodiment, a head 4 in which 512 heating elements 6 are divided into two blocks 4 ₁ and 4 ₂ is used.
The 256 heating elements of each block 4 ₁ , 4 ₂ are further
Each of them is divided into odd-numbered heating elements (hereinafter referred to as "odd-numbered group") and even-numbered heating elements (hereinafter referred to as "even-numbered group").

In FIG. 3, an analog image signal S _A for one screen is applied to the input terminal 10 . This signal S _A
may be a one frame or one field television signal, for example reproduced from a magnetic sheet or the like. After this signal S _A is sampled and held in the sampling and hold circuit 11, the A/D
The converter 13 converts it into a digital image signal S _D. In addition, in the case of Figure 2, the above sampling is
Vertical line indicated by arrow V (hereinafter referred to as “V line”)
).
The A/D converted signal S _D is written into a memory 14 consisting of a RAM.

On the other hand, the VCO (voltage controlled oscillator) 15 oscillates at a predetermined frequency, and its oscillation output drives the control circuit 12 and the address counter 1.
6 and the frequency division counter 17 for counting. The counter 16 is controlled by the control circuit 12 so as to read out the data written in the memory 14 in the order described below. The gradation level generator 18 is a carrier of the frequency division counter 17.
Each time CA is added, concentration reference level data is generated step by step. In this example, the density is expressed in 32 gradations, and the above reference levels D ₁ to _{D 32}
are sequentially generated and applied to the comparator 19. First, when reference level D ₁ is indicated, memory 1
128 pieces of data of odd groups of blocks ₄₁ of the 4th to 1st V lines are sequentially read out and sequentially compared with level _D1 . And when the data is lower than level D ₁ , for example "1" (high level),
When it is high, a comparison result of "0" (low level) is output and held at the corresponding address of the latch circuit 20.

When the comparison with level _D1 is completed, the comparison result of the latch circuit 20 is transferred to the head drive circuit 22.
It is applied to the odd groups of heating elements 6 of the block ₄₁ of the head 4 through. Then, the heating element 6 to which the comparison result of "1" has been added is energized and printing is performed. While this printing is being performed, the next reference level _D2 is displayed, and the same 128 pieces of data as above are sequentially compared. After this comparison result is held in the latch circuit 20, it is added to the odd numbered group of the block ₄₁ via the head drive circuit 22, and the heating element 6 whose comparison result is "1" is energized. A comparison is similarly made with respect to the reference levels _D3 to _D32 , and based on the comparison result, the odd-numbered heating elements 6 of the block ₄₁ are energized. Each heating element 6 is continuously energized when the sequentially sent comparison result is "1", and is stopped energized when it is "0". Therefore, 128 PWM signals having 32 levels of pulse width depending on the density of the pixel corresponding to each data are applied to each heating element. As a result, each heating element 6 is energized for a time corresponding to the pulse width. In this case, the maximum heating time is set to about 16 to 18 ms, for example.
The energization time is controlled within this time.

When the pixels of the odd group of block ₄₁ are printed in this way, the level generator 18 is set by the counter 23 which divides the carry CA by 1/32. Next, 128 pieces of data of odd groups of blocks ₄₂ on the same V line are sequentially read out from the memory 14, and the reference level is leveled in the same manner as above.
D ₁ to D ₃₂ are compared respectively. These comparison results are held at the corresponding addresses in the latch circuit 20 and then sent to the block 4 via the head drive circuit 22.
Electricity is applied to the heating elements 6 of the _two odd-numbered groups.

After the pixels of the odd group on the first V line are printed in this way, 128 pieces of data of the even group of block ₄₁ on the same V line are read out from the memory 14. The read data is processed in the same way as described above, converted into a PWM signal, and applied to the even-numbered heating elements 6 of the block ₄₁ . Next, the 128 pieces of data in the even group of block ₄₂ are read out, the same processing is performed, and the PWM signal is applied to the heating elements 6 in the even group of block ₄₂ . As described above, all 512 pixels of the first V line are printed.

When printing of the first V line is completed, the drum 2 shown in FIG. 1 rotates by one pixel pitch and then stops. Then, the data of the next V line is converted into a PWM signal and printed in the same manner as described above, and the drum 1 is rotated by one pixel pitch again.
Note that the output of the counter 23 is divided by 1/4 by the counter 27.
The drum 1 is rotated by using the frequency-divided pulse as a drum feed pulse. After that, the same operation is repeated a total of 1024 times, whereby the scanning in the H direction is completed and an image is drawn.

As described above, this embodiment is intended to shorten the energization time for the even-numbered groups, and this shortening of the energization time is achieved by making the frequency of the VCO 15 higher than that for the odd-numbered groups. In addition, if the frequency of VCO 15 is increased, the heating element 6
The pulse width of the PWM signal that controls this becomes shorter.

For this reason, the frequency control terminal CONT of VCO15
The movable terminal of the variable resistor 24 is connected to the frequency range control terminal RANGE, and the movable terminal of the variable resistor 25 is connected to the frequency range control terminal RANGE.
A voltage +B of, for example, +5V is applied to one end of the control terminal 26, and an odd/even switching signal O/D is applied to the control terminal 26. The signal O/D is a signal that is at a high level when an odd number group is operating, and is at a low level when an even number group is operating.

By doing this, when the signal O/D is high level, the RANGE terminal of VCO15 is +5V,
When the level is low, it can be set to any value between 0 and 5V. First, set the RANGE terminal to +5V, adjust the CONT terminal voltage with the variable resistor 24,
Set the energization time for odd-numbered groups. next
With the RANGE terminal voltage in a variable state, the variable resistor 25 adjusts the energization time to even-numbered groups so that all lines have the same concentration. In addition,
The configuration may be such that the voltage at the CONT terminal is made variable.

Although this embodiment uses a head 4 with two blocks, the present invention can also be applied to a case where a head with a smaller or larger number of blocks is used.

In addition, according to the experimental results, if the energization time at the maximum concentration for each odd group is 16 to 18 ms, the frequency of the VCO 15 is increased by about 20% during the next operation of the even group, and the above maximum concentration is reached. It was found that if the current application time was shortened to about 13 to 15 ms, the concentration could be corrected to be more uniform.

As described above, the present invention is configured to draw an image by scanning the screen while intermittently moving a head in which a plurality of heating elements are arranged in a row relative to the screen. When the intermittent movement stops, first, the odd-numbered (or even-numbered) heating elements are energized for a time corresponding to the image density gradation of the screen printed by these heating elements, and then the The even numbered (or odd) numbered heating elements arranged adjacent to each of the odd numbered (or even numbered) numbered heating elements have a gradation of image density of the screen printed by these heating elements. In a thermal printer configured to print on the screen by turning on electricity for a corresponding time,
Even when printing with the same image density with the even (or odd) heating element and the odd (or even) heating element, the even (or odd) heating element The time for energizing the heating element was made shorter than the time for energizing the odd-numbered (or even-numbered) heating elements.

Therefore, even if by energizing the odd (or even) heating element, the next even (or odd) heating element to be energized is preheated, both prints with the same image density. When printing, the density of the print made by the even (or odd) numbered heating element is different from the density of the print made by the odd (or even) numbered heating element. do not have. Therefore, unnecessary variations in print density do not occur, making it possible to print with uniform density.

Also, by utilizing the fact that the odd (or even) heating element has preheated the even (or odd) heating element that will be energized next, both print with the same image density. Even in this case, the time for energizing the even-numbered (or odd-numbered) heating elements is shorter than the energization time for the odd-numbered (or even-numbered) heating elements. Therefore, there is no need to provide a period in which no current is applied until the temperature of the preheated even-numbered (or odd-numbered) heating element returns to the temperature before preheating, so time loss in control can be eliminated. , Therefore, printing speed can be improved.

In addition, since the odd (or even) numbered heating elements and the even (or odd) numbered heating elements are energized alternately, even when the head is divided into multiple blocks, There are no streaks at the boundaries between blocks.

Furthermore, since each heating element is energized for a time corresponding to the gradation of image density on the screen printed by the heating element, an image having desired shading can be drawn.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing an example of the basic configuration of a thermal printer, Fig. 2 is a schematic diagram of a screen for drawing an image and a heating head, and Fig. 3 is a circuit block diagram showing an embodiment of the present invention. . In addition, in the symbols used in the drawings, 4...heating head, ₄₁ to ₄₈ ...block, 5...screen, 6
... Heating element, 15 ... VCO.

Claims (1)

[Scope of Claims] 1. A head configured by arranging a plurality of heating elements in a line is configured to draw an image by scanning the screen while intermittently moving it relative to the screen; When the movement is stopped, first, the odd-numbered (or even-numbered) heating elements are energized for a time corresponding to the image density gradation of the screen printed by these heating elements, and then the odd-numbered According to the gradation of the image density of the screen printed by the even (or odd) heating elements arranged adjacent to each of the (or even) heating elements, In a thermal printer configured to print on the screen by being energized for only a certain amount of time, the heating element placed in the even (or odd) number and the heating element placed in the odd (or even) number. Even when printing with the same image density in A thermal printer characterized by its short length.