JPH0117886Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a thermal printer that reduces printing time.

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 using a heating head 4. is attached to the recording paper 2. When drawing images with shading using this thermal printer, use the second
As shown in the figure, this is carried out by scanning a screen 5 subdivided into, for example, 1024×512 pixels in the direction of arrow H with a heating head 4 having 512 heating elements 6. In this case, scanning is performed by moving the head 4 intermittently 1024 times. At each stop of this intermittent movement, each heating element 6 energizes each pixel for a time corresponding to the density of the image to heat it, thereby drawing a line in the direction of arrow V (hereinafter referred to as V line). In the case of the printer shown in FIG. 1, the head 4 is fixed, and the scanning is performed by intermittently rotating the drum 1. Further, the above-mentioned stop time for one time is, for example, 16 to 18 msec, and the energization time of each heating element 6 is controlled with this time as the maximum.

As shown in the figure, the head 4 is divided into, for example, two blocks 4 ₁ and 4 ₂ and has a configuration in which 256 heating elements 6 are arranged in each block, and each block is operated in sequence. has been completed.

The reason for dividing the head 4 into a plurality of blocks in this way is due to reasons such as the structure of the head 4 and restrictions on current capacity. Note that 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.

FIG. 3 shows a conventional example of a head control circuit.

In this embodiment, the energization time of each heating element 6 of the head 4 is controlled according to the density of the image, but 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.

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 the case of FIG. 2, the above sampling is performed for each 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 data written in the memory 14 in the order described below. The gradation level generator 18 generates density reference level data step by step every time the carry pulse _P1 of the frequency division counter 17 is applied. In this example, the concentration is
It shall be expressed in 32 gradations, and the above reference level D ₁
~D ₃₂ are sequentially generated and applied to the comparator 19.
First, when the reference level D ₁ is indicated, data 256 of the first half of the first V line is stored from the memory 14.
are sequentially read out and sequentially compared with level _D1 . A comparison result of "1" (high level) when the data is higher than the level _D1 and "0" (low level) when it is lower than the level D1 is output and held at the corresponding address of the latch circuit 20. When the comparison with respect to level _D1 is completed, the comparison result of the latch circuit 20 is applied to the heating element 6 of the block ₄₁ of the head 4 via the head drive circuit 21. and “1”
The heating element 6 to which the comparison result 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 256 pieces of data as above are sequentially compared. After this comparison result is held in the latch circuit 20, the head drive circuit 21
The heating element 6 whose comparison result is " ₁ " is energized. A comparison is similarly made with reference levels _D3 to _D32 , and the heating element 6 of block ₄₁ is energized based on the comparison result. Each heating element 6 is continuously energized when the sequentially sent comparison result is "1", and is stopped energized when it is "0". Therefore, a PWM signal having a pulse width corresponding to the density of the corresponding pixel is applied to each heating element 6.

When printing of the first half of the V line is completed in this way, the level generator 18 converts the pulse P ₁ to 1/3
It is reset by the carry pulse _P3 of the counter 23, which divides the frequency by two.

Next, 256 pieces of data in the latter half of the same V line are sequentially read out from the memory 14 and sequentially compared with the reference levels D ₁ to D ₃₂ in the same manner as described above. These comparison results are held at the corresponding addresses in the latch circuit 20, and then applied to the heating element 6 of the block ₄₂ via the head drive circuit 21 to energize it.

With the above, all pixels of the first V line
When printing of 512 pieces is completed, the drum 1 shown in FIG. 1 rotates by one pixel pitch and then stops. Soshi 2
The data of the th V line is changed in the same way as above.
Each signal is converted to a PWM signal and printing is performed. After printing is completed, the drum 1 rotates again by one pixel pitch. 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.

In the thermal printer described above, a density gradient occurs on the screen between the start and end of printing. That is, at the start of printing, the heating element 6 of the head 4 is cold, so the pixels are printed thinly, but at the end of printing, the heating element 6 is warmed, so the pixels are printed darkly even for the same heating time. Therefore, in the screen 5 of FIG. 2, the left side is light and the screen gradually becomes darker toward the right.

If the screen to be printed has uniform density,
It is sufficient to simply reduce the applied power or energization time to the heating element 6 in accordance with the density gradient from the start to the end of printing, but normally the screen is rarely of uniform density and depends on the image being drawn. The density of pixels changes in a complex manner. Conventionally, as a method of correcting such density distribution, the printing history of each heating element 6 from the start of printing to the current printing time is stored in memory, and the applied power is adjusted according to the history. Alternatively, a method of controlling the energization time has been proposed. However, this method is difficult to implement because it requires a large amount of memory and control circuitry, resulting in a very complicated circuit.

This invention is intended to solve the above problems.
Embodiments of the present invention will be described below with reference to the drawings.

Assume that the input image signal S _A in FIG. 3 mentioned above is a television video signal, and the 512 scanning lines in the H direction scanned by the head 4 in FIG. 2 correspond to the scanning lines of the television screen. Then, the level change of the signal S _A during 1H (H: horizontal scanning period) directly represents the printing history of one heating element 6 from the start to the end of printing. Therefore, density correction can be performed by creating a correction signal indicating the change in temperature rise of the heating element 6 based on this printing history and correcting the original signal _SA with this correction signal.

Figure 4 shows the principle circuit of the present invention based on the above concept, where the signal S _A is connected to the clamp circuit 25.
After the DC level is clamped at , it is applied to the correction time constant circuit 26 and integrated or differentiated with a time constant to be described later to become the correction signal S _c . This S _c is the calculation circuit 2
7, it is subtracted from the original signal S _A , and this subtracted output is applied to the sampling and holding circuit 11. Note that 28 is a variable resistor for adjusting the amount of correction. Further, the correction time constant circuit 26 is reset by the horizontal blanking signal H.BLK every 1H. If you convert the sampled signal into a PWM signal as described above, for example, in the case of a screen with uniform density,
The pulse width of the PWM signal is longest at the start of printing.
It becomes gradually shorter as it is printed.

Next, a concrete example based on the above principle will be described.

FIG. 5 shows a first embodiment, in which the input video signal
S _A is applied to the transistor Q ₁ constituting the clamp circuit 25, and is applied to the transistor Q ₂ through the capacitor C ₁ . In this case, the DC level is clamped by transistor _Q3 , which operates with clamp pulse _P1 . The signal taken out from the transistor Q ₂ is differentiated by a variable resistor R ₁ and a capacitor C ₂ that constitute the correction time constant circuit 26,
This differential waveform and the signal obtained through variable resistor _R2 are added. In addition, switch 29 is set to H.
It is closed by the BLK signal and resets by discharging capacitor _C2 .

This circuit has a short time constant of, for example, several microseconds, which corresponds to several seconds in terms of printing time.
Therefore, this circuit is suitable for use in a head 4 where the temperature rise is saturated in a short period of time. Furthermore, the signal
The H/BLK period of S _A needs to be converted to a white level in advance.

FIG. 6 shows a second embodiment, and parts corresponding to those in FIG. 5 are given the same reference numerals.

The signal S _A is a transistor as in the case of Figure 5.
It is applied to Q ₁ , its level is lowered by variable resistor R ₃ and applied to transistor Q ₄ , and further applied to transistor Q ₅ through capacitor C ₃ . At this time, it is clamped by the transistor _Q6 operated by the clamp pulse _P1 . The clamp level of the original signal S′ _A applied to the transistor Q ₂ is determined by the voltage V _c1 , and the clamp level of the signal S _A0 applied to the transistor Q ₅ is determined by the variable voltage V _c2 . Therefore, the signal S _A0 has a higher DC level and a lower peak value than the original signal S' _A.
Also the base of transistor Q ₂ and the base of transistor Q ₅
is connected to the emitter of the transistor through the variable resistor _R4 , so a current corresponding to the level difference between the signals S′ _A and S _A0 flows through the variable resistor _R4 to the transistor.
It flows into the resistor _R5 on the emitter side of _Q5 .

According to this circuit, the target values for correction can be changed for the white level and the black level, respectively. Further, substantially the same effect can be obtained as when the correction time constant is set to, for example, about 20 μsec (several tens of seconds in terms of printing time) and is integrated. Therefore, this circuit is suitable for use in the head 4 where the temperature rise rate is moderate.

FIG. 7 shows a third embodiment, in which a very long time constant (several 100 μsec, several minutes in printing time) can be corrected by R _{5 C1} . In this circuit, the corrected final values of the black level and white level are the same level. Note that _R6 is a variable resistor for adjusting the white level. This circuit is suitable for use in the head 4, which requires a long period of time until the temperature rise reaches saturation. FIG. 8 shows an example of the schematic structure of the head 4.
A rubber member 3 is attached to a member 30 made of glass epoxy or the like.
A glass member 32 is attached through 1, and a heating element 6 is provided on this glass member 32. This heating element 6 is covered with a wear-resistant protective film 33.

The temperature rise of the protective film 33 is rapid and saturates in a short time. Therefore, the circuit shown in FIG. 5 is suitable for correcting the concentration change caused by the temperature rise in this part. Further, the circuit shown in FIG. 6 is suitable for density correction for temperature rise of the glass 32. Furthermore, it takes a long time for the overall temperature rise in head 4 to reach saturation.
The circuit shown in FIG. 7 is suitable for density correction for this purpose.

As described above, the present invention provides, in this type of thermal printer, a correction signal that changes with a time constant according to the temperature rise of the heating head, based on video information in a predetermined period (1H) of the input video signal, and This correction signal is used to correct the video signal.

Therefore, according to the present invention, since the video signal is corrected before being applied to the head control circuit and then stored in the memory, density changes caused by the temperature characteristics of the heating head can be corrected using a simple circuit. be able to.

[Brief explanation of the drawing]

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 example of a head control circuit. Fig. 4 is a principle circuit block diagram showing an embodiment of the present invention. Figs. 5 to 7 are circuit diagrams showing the first to third embodiments of the invention. Fig. 8 is a cross-sectional side view showing an example of the structure of the head. It is a diagram. The symbols used in the drawings are 4...head, 6...heating element, 26...correction signal generation circuit, 27...computer, S _A ...video signal.

Claims (1)

[Scope of Claim for Utility Model Registration] A thermal printer that draws an image by scanning the screen with a heating head having a plurality of head elements, an input terminal for inputting a video signal, and an input terminal for inputting the video signal. The correction means includes a correction means for correcting each horizontal scanning line of an image of one screen, and a storage means for storing the video signal output from the correction means for one screen as a digital signal, and the correction means corrects the image for each horizontal scanning line. The video information for each horizontal scanning line of the signal is made to correspond to each of the plurality of head elements, and the video information for each horizontal scanning line is set at a time constant according to the temperature rise of the head element to which it corresponds. The correction is performed using a changing correction signal, the corrected video signal is stored for one screen in the storage means, and the heating head is driven by the signal output from the storage means. A thermal printer with special features.