JPH0771177B2 - Halftone recording device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a halftone recording apparatus for thermal transfer recording or thermal recording, and particularly to an apparatus for obtaining a high quality halftone image without density unevenness. is there.

[Conventional technology]

Since a thermal recording device and a thermal transfer recording device which are generally used have relatively simple configurations, a printer,
It is widely applied to various types of recording such as copying machines and facsimile machines. In such various recording apparatuses, in order to perform halftone recording, for example, a thermal transfer recording method using a sublimation type ink sheet is used. This thermal transfer recording method involves sublimating a dye ink in an amount corresponding to the amount of heat generated by the electric energy applied to a plurality of heating resistors forming a thermal head for recording, and then performing the predetermined recording with this dye ink. The required recording is performed by transferring it to paper. The heating amount by the heating resistor is controlled by the number of electric pulses applied to the heating resistor and the connection time width.

This method of thermal transfer recording is easy to control and can achieve relatively good halftone recording.

Such a conventional halftone recording system is disclosed in, for example, Japanese Patent Laid-Open No. 60-
FIG. 6 is a waveform diagram of the energizing pulse applied to each heating resistor constituting the thermal head in the conventional halftone recording method, and t _W is the pulse width of the energizing pulse. , T _P is the repetition period of the energizing pulse, and M is the number of energizing pulses (here, three). The number of pulses of this energizing pulse is selected and set in advance corresponding to the density for each gradation level. In this way, by applying the energizing pulse of the number of pulses corresponding to each gradation to each heating resistor, the ink for the energy corresponding to the number of pulses is sublimated, and the halftone recording of each density is performed. Then, the energizing pulse corresponding to each is normally applied to each heating resistor provided in line on the thermal head for one line to perform recording for one line, and the recording paper is fed in the sub-scan at a constant speed. On the other hand, the recording is performed for each line sequentially to perform planar recording.

[Problems to be Solved by the Invention]

In the conventional halftone recording method as described above, for example, the number of recording dots of black or gray pixels to which a conduction pulse is applied to the heating resistor and the number of non-recording dots of white pixels to which a conduction pulse is not applied are 1 Since the ratio in the line or the gradation level of the recording dot is different for each line, the applied energy collectively supplied for the recording, that is, the amount of current is also different for each line. The voltage drop due to the wiring resistance from the power supply to the thermal head or the voltage drop in the thermal head changes for each line, and this voltage change affects the energizing pulse to the heating resistor and the same gradation level is obtained. However, there is a problem in that the heat generation amount of the heating resistor is different, the recording density is different in the same gradation, and the recording unevenness occurs. That is,
At the same time, since the total amount of applied energy supplied to the heating resistor for recording is different in each line, there is a problem that the recording density at the same gradation level varies.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a halftone recording apparatus in which the recording density of the same gradation level becomes constant regardless of the magnitude of the energy applied simultaneously. To aim.

[Means for Solving the Problems]

A halftone recording apparatus according to the present invention is a thermal head including a plurality of heating resistors, and a predetermined number of energizing pulses or energizing pulse widths according to a gradation level signal input corresponding to each of the heating resistors. Supply means for supplying each of the heating resistors collectively, dividing the gradation level signal into a plurality of groups,
A first table in which the indexes corresponding to the respective groups are stored in advance, a value obtained by accumulating the minimum value and the maximum value of the indexes in the first table for the number of heating resistors is obtained, and the minimum cumulative value and the maximum cumulative value are obtained. The values are divided into a plurality of cumulative index groups, and the correspondence between each of the divided cumulative index groups and the gradation level signal and the energizing pulse number or energizing pulse width is stored in advance for the number of divisions of the cumulative index group. No. 2 table, storage means for inputting and storing the gradation level signal, and the number of heating resistors for driving the index in the first table corresponding to the gradation level signal stored in the storage means. About
Based on the cumulative value and the gradation level signal stored in the storage means, the cumulative index group and the gradation level signal in the second table are referred to so that the recording density at the same gradation level becomes constant. It is provided with a deciding means for deciding the energizing pulse number or energizing pulse width for each heating resistor supplied from the supplying means.

[Action]

In the present invention, the gradation level signal input corresponding to each heating resistor is stored in the storage means by the determining means, and the gradation level signal in the first table corresponding to the gradation level signal stored in this storage means is stored. The indexes are accumulated for the number of driven heating resistors, and the cumulative index group and the gradation level signals in the second table are referred to based on the accumulated value and the gradation level signal stored in the storage means. The number of energizing pulses or energizing pulse width is determined for each heating resistor. The determined energization pulse number or energization pulse width is collectively supplied by the supply means for each predetermined number of heating resistors, and the recording density for the same gradation becomes constant.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the present invention.
For example, (1) is an input terminal to which the gradation level signal S of 6-bit configuration is input, and (2) is each of the thermal head (3) which is based on the gradation level signal S from this input terminal (1). A pulse generation means having a function as a determining means for obtaining the amount of applied energy supplied to the heating resistors, for example, the number of energizing pulses, and a function as a supplying means for supplying the obtained number of pulses to each heating resistor, Reference numeral (4) is a gradation level signal judging means for judging the gradation level signal S from the input terminal (1) and outputting a voltage drop index corresponding thereto. Here, the voltage drop index is as follows. That is, the total amount of applied energy supplied to the thermal head (3) at different times varies from line to line depending on the number of recording pixels (recording dots) on each recording line and the gradation level of each recording dot. As the load current of the power source changes, the voltage drop of each part changes and the voltage of the energizing pulse also changes. The amount of change, that is, the value for estimating the amount of voltage drop is the voltage drop index. This voltage drop index can be calculated by obtaining the resistance value of the thermal head or each wiring, or can be easily obtained by a recording experiment or the like. (5) is a correction coefficient corresponding to the cumulative voltage drop index, which is obtained by accumulating the voltage drop index from the gradation level signal determining means (4) for a predetermined number of heating resistors that simultaneously record, for example, one line. Is a voltage drop index counting means for outputting to the pulse generating means (2). The pulse generating means (2) uses the correction coefficient from the voltage drop index counting means (5) to determine the amount of applied energy obtained from the gradation level signal S, for example, the number of energizing pulses, so that the recording density of the same gradation is constant. And supply to the thermal head.

Here, in FIG. 2, one line consists of 1024 dots,
Further, when the gradation level is 64 levels, it is a characteristic diagram showing the change of the recording density with respect to the number of simultaneous recording dots when no correction is made, and each curve (21) to (24) shows the following cases. .

Curve (21): When the gradation level of the recording dot is 8 Curve (22): When the gradation level of the recording dot is 16 Curve (23): When the gradation level of the recording dot is 32 Curve ( 24): When the gradation level of the recording dot is 64. As can be seen from FIG. 2, first, when the gradation level is 8, the number of simultaneous recording dots is as shown in the curve (21). The recording density does not change even if it is increased. What is shown here is that the load current is small due to the low gradation level, and the voltage drop amount of the energizing pulse is almost equal to zero.

Next, when the gradation level is 16, as shown by the curve (22), the recording density slightly changes as the number of simultaneous recording dots increases.

Further, when the gradation level is 32, as shown by the curve (23), the recording density changes considerably as the number of simultaneous recording dots increases. This is because the voltage drop amount of the energizing pulse is considerably large.

Furthermore, when full-scale printing is performed when the gradation level is 64, as shown by the curve (24), as the number of simultaneous recording dots increases, the degree of increase in the voltage drop amount of the energization pulse increases. The recording density is further reduced, and as a result, the recording density is further reduced.

As described above, when the number of simultaneous recording dots is large, the voltage of the energizing pulse is lowered and the energy supplied by the energizing pulse is reduced, so that the recording density is lowered. Based on such characteristics, the energizing pulse may be determined so that the recording density becomes constant regardless of the number of simultaneous recording dots. That is, since the method of changing the recording density differs depending on the gradation level of the recording dot, the voltage drop index corresponding to the gradation level of each dot is first obtained by the gradation level signal determining means (4), and the voltage thereof is determined. The voltage drop index counting means (5) calculates the cumulative value of the simultaneous recording dots of the drop index, and the pulse generation means (2) determines the energizing pulse based on the cumulative value.

Here, as can be seen from the above, when the characteristic curve showing the relationship between the recording density and the number of simultaneous printing dots is obtained using the gradation level as a parameter, strictly speaking,
As many characteristic curves as there are gradation levels can be obtained.
However, some of them may be regarded as exhibiting almost the same characteristics, and by combining them, they can be divided into a certain number of groups. That is, the gradation level signal of N stages is divided into an appropriate number of n groups (where N ≧ n), and the voltage drop index is previously stored in the gradation level signal judging means (4) for each group. Set as a table. Fig. 3 shows an example of this, where the levels of 64 gradations are divided into four groups,
A voltage drop index is set for each.

Then, the voltage drop index for each dot is accumulated by the voltage drop index counting means (5) for the simultaneous recording dot, for example, for one line, and the correction coefficient corresponding to the accumulated value is obtained. It is divided into groups, and the correction coefficient is set in the voltage drop index counting means (5) for each group. Fig. 4 shows an example of this.
Minimum value 0 of the voltage drop index in the first table shown in the figure
The cumulative voltage drop index obtained by accumulating the maximum value 1 and the maximum value 1 for the number of 1024 dots is divided into 16 groups (cumulative index group) between the obtained minimum cumulative voltage drop index 0 and the maximum cumulative voltage drop index 1024. The correction coefficient is set for each.

Further, the pulse generating means (2) obtains the number of energizing pulses for obtaining the same recording density for the same gradation from the correction coefficient output for each line and the gradation level signal S, and the energization is performed. The pulse is applied to the thermal head (3). FIG. 5 shows a part of a table showing the correspondence between the gradation level signal and the correction coefficient and the pulse number of the energizing pulse, and such a table is set in the pulse generating means (2). In this embodiment, the tables shown in FIGS. 4 and 5 constitute the second table.

Next, the operation of the above embodiment will be described. Now, it is assumed that gradation level signals corresponding to any one of the gradation levels 1 to 64 of each dot are sequentially input to the input terminal (1). This signal is first inputted to the gradation level signal judging means (4), and the voltage drop index showing the degree of the voltage drop amount when recording the corresponding one dot is obtained. That is, referring to the table of FIG. 3, for example, when the input gradation level signal is included in the gradation levels 1 to 8, the voltage drop index 0 corresponding to this is given as the gradation level signal judging means (4). Is output from. Similarly, for example, when the input gradation level signal is included in the gradation levels 33 to 64, the voltage drop index 1 corresponding to this is output from the gradation level signal determination means (4). become.

In the voltage drop index counting means (5), the voltage drop index output from the gradation level signal determining means (4) at the preceding stage is accumulated, for example, by an amount corresponding to one line and simultaneous recording dots. A correction coefficient corresponding to the accumulated value is obtained from the table of FIG. 4 and output to the pulse generating means (2). For example, if the cumulative voltage drop index is 185, the correction coefficient corresponding to this is set to 1.02 from the table of FIG. 4 and output to the pulse generating means (2).

Next, the pulse generation means (2) stores the input gradation level signal in the storage means in the pulse generation means (2) for each recording dot, and the gradation level and voltage drop index of each recording dot. From the correction coefficient output from the counting means (5), the optimum number of energizing pulses corresponding to the gradation level of each recording dot is obtained by referring to the table of FIG. Here, even with the same gradation level signal, if the correction coefficient is different, the number of different pulses is determined so that the fluctuation of the voltage drop amount of the energizing pulse is absorbed and the same density is obtained. Then, the obtained number of energizing pulses is supplied to each heating resistor of the thermal head (3) to perform recording. For example, when the correction coefficient is 1.02 as described above at the gradation level 6, the number of energizing pulses is set to 33.

As described above, by appropriately performing the correction in accordance with the voltage drop amount corresponding to the gradation level of the simultaneous recording dot, the recording density at the same gradation level can be always made constant. In this embodiment, the gradation level signal determining means (4), the voltage drop index counting means (5), and the pulse generating means (2) constitute the determining means.

In the above embodiment, the number of groups, the correction coefficient, etc. when dividing the gradation level signal or the cumulative voltage drop index as shown in FIGS. 3 and 4 are various depending on the thermal head (3) used. It may be decided based on the characteristics.

Further, in the above embodiment, the case where the recording density change due to the change in the voltage drop amount is corrected by changing the applied energy amount by changing the number of energizing pulses is not limited to this. A similar effect can be obtained even when the applied energy is changed by changing the pulse width to correct the density change.

Furthermore, by adding a thermal head heat storage correction function relating to changes in the environmental temperature, it is possible to improve the accuracy of constant recording density for the same gradation.

Further, in the above-mentioned embodiment, the case where the accumulation of the voltage drop index is recorded at the same time is described. However, this is not the case where the simultaneously recorded dot is one line, but one line is divided into a plurality of sections. In the case of recording, the same effect can be obtained by accumulating the voltage drop index in units of this section and determining the energizing pulse.

〔The invention's effect〕

As described above, according to the present invention, the gradation level signal input corresponding to each heating resistor is divided into a plurality of groups, and the index corresponding to each group is stored in advance.
Table and the value obtained by accumulating the minimum value and the maximum value of the index in the first table for the number of the heating resistors, and dividing the minimum cumulative value and the maximum cumulative value into a plurality of cumulative index groups,
A second table in which correspondences between each of the divided cumulative index groups and the gradation level signals and the energization pulse number or energization pulse width are stored in advance for the number of divisions of the cumulative index group;
The gradation level signal is input and stored, the exponent in the first table corresponding to the stored gradation level signal is accumulated for the number of driving heating resistors, and the accumulated value and the storage are stored. Based on the gradation level signal, the cumulative index group and the gradation level signal in the second table are referred to determine the number of energizing pulses or the energizing pulse width supplied to each driving heating resistor. Therefore, the applied energy pulse to be supplied to each of a predetermined number of heating resistors simultaneously performing recording is determined based on the size of the gradation level and the number of driving heating resistors, and the recording density of the same gradation is determined. Is always constant and the recorded image quality is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram showing a change in recording density with respect to the number of simultaneous recording dots when no correction is made, and FIG. 3 is a gradation level and a voltage drop index. FIG. 4 is a table diagram showing an example of the correspondence of FIG. 4, FIG. 4 is a table diagram showing an example of the correspondence between the cumulative voltage drop index and the correction coefficient,
FIG. 5 is a table diagram showing an example of the correspondence between the combination of gradation level and correction coefficient and the number of energizing pulses.
The figure is a waveform diagram of energizing pulses for explaining the operation of the halftone recording method. In the figure, (1) is an input terminal, (2) is a pulse generating means, (3) is a thermal head, (4) is a gradation level signal determining means, and (5) is a voltage drop index counting means. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A thermal head comprising a plurality of heating resistors, wherein the number of energizing pulses or energizing pulse widths corresponding to a gradation level signal input corresponding to each of the heating resistors is set for a predetermined number of heating resistors. A first table in which the gradation level signals are divided into a plurality of groups and the exponents corresponding to the respective groups are stored in advance; the minimum and maximum values of the exponents in the first table Is calculated for the number of the heat generating resistors, and the value between the minimum cumulative value and the maximum cumulative value is divided into a plurality of cumulative index groups, each of the divided cumulative index groups, the gradation level signal, and the energizing pulse number. Or a second table in which the correspondence with the energization pulse width is stored in advance for the number of divisions of the cumulative index group, and storage means for inputting and storing the gradation level signal,
The indexes in the first table corresponding to the gradation level signals stored in the storage means are accumulated for the number of driving heating resistors, and the accumulated value and the gradation level signal stored in the storage means are accumulated. Based on the cumulative index group and the gradation level signal in the second table based on the above, the energizing pulse for each heating resistor supplied from the supply means so that the recording density at the same gradation level becomes constant. A halftone recording apparatus comprising a determining means for determining the number or energizing pulse width.