JPH0825297B2 - Halftone recording method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a halftone recording method in a thermal transfer recording apparatus or a thermal recording apparatus, and more particularly to control of recording density of a thermal head.

[Prior Art] A thermal transfer recording device or a thermal recording device is widely used as a recording means for a word processor, a copying machine, a facsimile or the like because of its relatively simple structure.

There is a method using a sublimation type ink sheet, for example, in order to record a halftone in a thermal transfer recording apparatus. In this method, the dye ink is sublimated according to the amount of heat generated by the heating resistor that constitutes the thermal head, and the recorded image is transferred and recorded on the recording paper. The amount of heat generation is controlled by the number or pulse width.

In the thermal transfer recording apparatus using the sublimation type ink sheet, good halftone recording is performed by the simple control method as described above. However, since the main factor that determines the recording density of each gradation level is the heating temperature of the heating resistor of the thermal head, fluctuations in the heating temperature due to heat storage and changes in the environmental temperature have a large effect on the recording density. Therefore, there is a problem that the recording density of each gradation level is not faithfully recorded. Therefore, many correction methods have been conventionally proposed.

FIGS. 4 and 5 are waveform diagrams showing the waveforms of pulse signals applied to the thermal head in the conventional halftone recording method disclosed in, for example, Japanese Patent Laid-Open No. 60-9271, and the heating temperature of the heating resistor. FIG. 6 is a characteristic diagram showing the relationship between the pulse width of the pulse signal and the pulse width of the pulse signal.

In FIG. 4, Tw is the pulse width of the pulse signal applied to the thermal head, Tp is the pulse signal repetition period, and N is the pulse signal repetition period.
Indicates the number of pulses.

In FIG. 5, the horizontal axis represents the heat generation temperature of the heating resistor, and the vertical axis represents the pulse width Tw of the pulse signal.

Next, the operation of the above-mentioned conventional example will be described with reference to FIGS. 4 and 5.

First, the pulse number N is set in advance corresponding to the recording density of each gradation level. For example, the pulse signal shown in FIG. 4 has three pulses (N = N) corresponding to a predetermined gradation level.
This is the case when 3) is set.

However, in order to obtain an arbitrary gradation level, the number of pulses N
Even if is constant, the recording density fluctuates due to the heat generation temperature of the heating resistor, etc., so that the recording density at the same gradation level also fluctuates.

Therefore, with respect to fluctuations in the heat generation temperature of the heat generation resistor, a temperature detection element such as a thermistor is used to refer to the heat generation temperature of the heat generation resistor for each line, and pulse characteristics are obtained as shown in FIG. The pulse width Tw of the signal was controlled and corrected so that the same recording density was obtained with the same number of pulses N.

[Problems to be Solved by the Invention] The conventional halftone recording method as described above has the following problems.

(A). Even if the temperature signal is detected by a thermistor and the pulse signal is corrected, the time constant of the thermistor is about several seconds, and the heating temperature of the heating resistor that changes with the time constant of several tens of μS to several mS must be accurately controlled. Since it is impossible, a sufficient effect was not obtained.

(I). When there are many pixels with a high gradation level in one recorded image, the effect of the heat storage phenomenon of the heating resistor that constitutes the thermal head is great, and there are several tens of gradation levels at the start and end of recording. Has also changed.

(C). In the case of high speed recording, the thermal effect (heat storage) in the past recording cannot be ignored and the recording density for each gradation level could not be faithfully reproduced.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to prevent variations in the recording density at the same gradation level due to the thermal storage phenomenon of the thermal head, and to obtain a uniform recording density. The purpose is to obtain a halftone recording method.

[Means for Solving the Problems] In the halftone recording method according to the present invention, gradation levels are classified into a plurality of groups each having a gradation level or less, and a heat storage index assigned to each group is input and input. The gradation level determining means for judging the gradation level of the gradation level signal and outputting the heat storage index of the group in which the gradation level is classified, and the counter for the heat generating resistors constituting the thermal head are provided. A heat storage index accumulating means for accumulating the heat storage index for a predetermined recording period for each resistor and outputting a cumulative heat storage index,
The cumulative heat storage index is divided into a plurality of groups, each group has a pulse number assigned for each gradation level,
The thermal head includes pulse generating means for generating a pulse signal of a pulse number selected based on the gradation level of the input gradation level signal and the cumulative heat storage index output from the heat storage index accumulating means. The uniform recording density is obtained by preventing the fluctuation of the recording density at the same gradation level due to the heat storage phenomenon.

[Operation] According to the present invention, the gradation levels of the input gradation level signal are classified by the gradation level determination means that classifies the gradation levels into a plurality of groups of the gradation levels or less and has a heat storage index assigned to each group. The heat storage index of the group in which the gradation level is determined and the gradation level is classified is output. Further, the heat storage index accumulating means having a counter for the heat generating resistors forming the thermal head accumulates the heat storage index for a predetermined recording period for each of the heat generating resistors and outputs a cumulative heat storage index. Furthermore, the cumulative heat storage index is classified into a plurality of groups, by a pulse generating means having a number of pulses assigned for each group and for each gradation level,
A pulse signal having a number of pulses selected based on the gradation level of the input gradation level signal and the cumulative heat storage index output from the heat storage index accumulating unit is generated.

Therefore, the pulse signal is corrected by the pulse generating means on the basis of the accumulated heat storage index, the variation of the recording density at the same gradation level is prevented from occurring during the halftone recording, and the uniform recording density is achieved.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, (1) is an input terminal, and (2) is a gradation level determination means, which is a gradation level signal determination circuit connected to the input terminal (1) in this embodiment.

(3), (4) and (5) constitute a heat storage index accumulating means. In this embodiment, (3) is the first selector connected to the gradation level signal judging circuit (2), and (4) is A heat storage index counting circuit connected to the first selector (3) and a second selector connected to the heat storage index counting circuit (4).

(6) is a pulse generator, and in this embodiment, a pulse generator connected to the input terminal (1) and the second selector (5), and (7) is connected to this pulse generator (6). , A thermal head composed of, for example, 1024 heating resistors. The heat storage index counting circuit (4) has 10
It consists of 24 counters.

2 and 3 are characteristic diagrams showing the temperature characteristic and the recording pattern of the heating resistor. 2 and 3, P _A , P _B , P _C , P, _D and P _E indicate recording patterns,
The numbers in the circles indicate the gradation levels. The horizontal axis of the figure shows the number of recording lines, and the vertical axis shows the heat generation temperature of the heat generating resistor. A, B, C, D and E are recording patterns
The curves of the temperature characteristics when the heating resistors are heated by P _A , P _B , P _C , P _D and P _E are shown.

First, the principle of the present invention will be described with reference to FIGS. 2 and 3.

In FIG. 2, paying attention to the third line from the left of the recording patterns P _A and P _B , although the heating resistor is made to generate heat at the same gradation level 64, the past recording patterns are different, and therefore the temperature characteristic curve As shown in A and B, the heating temperatures of the heating resistors do not match. Therefore, this is the cause of uneven recording density.

In FIG. 3, as indicated by curves C, D, and E of the temperature characteristic, the larger the gradation level, the more heat accumulated in the past, so the change in the temperature characteristic is large. Also, the temperature characteristic curve E
As shown in, when the heating resistor is made to generate heat at gradation level 10, there is little heat storage. Therefore, in such a case, it is shown that it is not necessary to consider the heat storage.

In this way, if the characteristics of the number of recording lines and the heat generation temperature are obtained using the gradation level as a parameter, the curve of the temperature characteristics corresponding to the number of gradation levels is obtained strictly, but the temperature may be regarded as the same characteristics. There are characteristic curves, which can be divided into several groups.

The present invention focuses on the phenomenon as described above. The gradation level signal L is classified into a plurality of groups having a gradation level number or less, and a preset heat storage index S is assigned to each group.

Here, the heat storage index S is a value representing an unnecessary heat storage amount remaining in the heating resistor when one dot is recorded, and the heat storage amount of the thermal head (7) is expressed by accumulating the heat storage index S. it can.

Then, the heat storage index S is accumulated (added) according to the recording progress (recording pattern), and the pulse signal is corrected based on the accumulated result. The value of the heat storage index S described above can be easily obtained by thermal analysis calculation or experiment.

Next, the operation of the above-described embodiment will be described with reference to Tables 1 and 2.

Table 1 shows the relationship between the gradation level and the heat storage index S, and Table 2 shows the relationship between the optimum cumulative heat storage index and the pulse number.

First, the gradation level signal L is input from the input terminal (1). The gradation level signal L is composed of, for example, 6 bits and has gradations of 1 to 64 levels.

In the gradation level signal determination circuit (2), the gradation level signal L is converted into the heat storage index S. The gradation level signal determination circuit (2) determines the heat storage amount when one dot is recorded. That is, the gradation level signal determination circuit (2) determines the gradation level of the gradation level signal L and outputs the heat storage index S corresponding to this gradation level to the first selector (3).

The gradation levels of the gradation level signal L are 1 to 10, 11 to 20,
The heat storage indexes S0.0, 0.4, 0.8 and 1. are classified into four groups of 21 to 32 and 33 to 64 and preset for each group.
0 is assigned.

Then, in the first selector (3), the heat storage index S
Is transmitted to one of the counters of the heat storage index counting circuit (4). The first selector (3) sequentially selects and connects the gradation level signal determination circuit (2) and one of the counters of the heat storage index counting circuit (4).

Further, the heat storage index S is accumulated (added) in the heat storage index counting circuit (4). That is, the heat storage index S is accumulated for each heating resistor from the start of recording to the recording line by each counter, and the accumulated heat storage index Q is output to the second selector (5).

Further, in the second selector (5), the cumulative heat storage index Q is transmitted to the pulse generator (6) from one of the counters of the heat storage index counting circuit (4). The second selector (5) sequentially selects and connects one pulse generator (6) of the counter of the heat storage index counting circuit (4).

Then, in the pulse generator (6), a pulse signal P is generated based on the gradation level of the gradation level signal L and the cumulative heat storage index Q and applied to the thermal head (7).

The cumulative heat storage index Q is not entirely shown in Table 2,
The pulse number N of the pulse signal P is assigned to each group and is preset for each gradation level (1 to 64).

For example, when 40 recording dots with a gradation level of 20 are continuously recorded, the cumulative heat storage index Q is as shown in Table 1.
When the gradation level is 20, the heat storage index S is 0.4, so 0.4 ×
40 = 16 (round the number after the decimal point). Therefore, when a recording dot having a gradation level of 30 is recorded thereafter, the pulse number N of the pulse signal P becomes "98" according to Table 2. This "98" is reduced by "2" from the pulse number N "100" when the cumulative heat storage index Q is almost zero.

Thus, the pulse signal P corresponding to the gradation level signal L
Is corrected in the paper-laying direction to optimize the amount of heat generated by each heating resistor forming the thermal head (7).

The heat storage index S described above can be easily obtained from recording experiments and heat calculations. However, the heat storage index S differs depending on the thermal response characteristics of the thermal head (7).

Further, in the above embodiment, the group classification number and the heat storage index S for the gradation level, and the group classification number and the pulse number N for the cumulative heat storage index Q are set as shown in Table 1 and Table 2, respectively. The value of varies depending on the characteristics of the thermal head (7) and is not limited to the value set above.

Further, in the above embodiment, the number N of pulses of the pulse signal P is corrected, but it goes without saying that the intended purpose can be achieved even if the pulse width is corrected.

Further, in the above embodiment, when the heat storage index S is accumulated, it is accumulated for each heat generating resistor, but it may be accumulated for each of a plurality of heat generating resistors, and the same operation as that of the above embodiment is expected. it can.

Furthermore, in the above embodiment, the heat storage index S was accumulated from the start of recording to the recording line, but it goes without saying that the desired purpose can be achieved even if accumulated up to a plurality of lines before the recording line.

Furthermore, in addition to the heat storage correction, when the correction of the environmental temperature is added, a higher accuracy can be obtained.

[Effects of the Invention] As described above, the present invention classifies the gradation levels into a plurality of groups of the gradation levels or less and has a heat storage index assigned to each group, and the input gradation level signal It has a gradation level judging means for judging a gradation level and outputting a heat storage index of a group in which the gradation level is classified, and a counter for a heating resistor forming a thermal head, and a predetermined value for each heating resistor. A recording period of the heat storage index accumulating means for accumulating the heat storage index and outputting a cumulative heat storage index,
The cumulative heat storage index is divided into a plurality of groups, each group has a pulse number assigned for each gradation level,
Since the pulse generation means for generating the pulse signal of the pulse number selected based on the gradation level of the input gradation level signal and the accumulated heat storage index output from the heat storage index accumulating means is provided, the thermal Prevents the occurrence of fluctuations in recording density at the same gradation level due to the heat storage phenomenon of the head,
There is an effect that a uniform recording density can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are characteristic diagrams showing temperature characteristics and a recording pattern of a heating resistor, and FIG. 4 is a thermal head in a conventional halftone recording system. Waveform diagram showing the waveform of the pulse signal applied to
FIG. 5 is a characteristic diagram showing the relationship between the heating temperature of the heating resistor and the pulse width of the pulse signal in the conventional halftone recording method. In the figure, (2) ... gradation level signal determination circuit, (3) ... first selector, (4) ... heat storage index counting circuit, (5) ... second selector, (6) ... Pulse generator, (7) ... Thermal head.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B41J 3/20 114 C (56) References JP-A-58-104775 (JP, A) JP-A-SHO 59-229365 (JP, A) JP 60-30265 (JP, A) JP 62-278062 (JP, A) JP 57-25977 (JP, A) JP 57-116669 (JP, A)

Claims (1)

[Claims] 1. A gradation level is classified into a plurality of groups of a gradation level or less, and a heat storage index is assigned to each group, and the gradation level of an input gradation level signal is judged to determine the gradation level. It has a gradation level determination means for outputting the heat storage index of the group into which the levels are classified, and a counter for the heat generating resistors that form the thermal head, and accumulates the heat storage index for a predetermined recording period for each of the heat generating resistors. A heat storage index accumulating means for outputting a cumulative heat storage index, and having a pulse number assigned to each group and each gradation level by classifying the cumulative heat storage index into a plurality of groups,
A pulse generating means for generating a pulse signal of a pulse number selected on the basis of the gradation level of the input gradation level signal and the accumulated heat storage index output from the heat storage index accumulating means; The halftone recording method is characterized by obtaining uniform recording density by preventing fluctuations in recording density at the same gradation level due to the heat storage phenomenon of.