1. Field of the Invention
The present invention relates to a printing system with a print head that carries out scans on a printing medium to implement printing. More specifically the present invention pertains to a technique of improving the picture quality of a resulting recorded image in such a printing system.
2. Description of the Related Art
The xe2x80x98interlace printing methodxe2x80x99 disclosed in U.S. Pat. No. 4,198,642 is a proposed technique that improves the picture quality of a resulting printed image in a printer, such as a serial ink jet printer, in which a print head carries out main scans and sub-scans on a printing medium to implement printing. FIG. 23 illustrates an exemplified process of interlace printing. A variety of parameters are used in the following description. In the example of FIG. 23, it is assumed that a number of nozzles N that are actually used for formation of dots is equal to three. A nozzle pitch k [dots] represents an interval between centers of adjoining nozzles on the print head, which is expressed by a pitch of pixels in a recorded image (dot pitch w) as the unit. In the example of FIG. 23, k is equal to two. L denotes a feeding amount of the sheet of paper in sub-scan and is set equal to three raster lines in the example of FIG. 23.
In the drawing of FIG. 23, circles, each including a number of two figures, represent the recording positions of the respective dots. In the encircled number of two figures, the left-side figure represents the nozzle number and the right-side figure represents the recording sequence (which time of main scan the dot is recorded).
In the process of interlace printing shown in FIG. 23, the first main scan causes a second nozzle and a third nozzle to record dots on the respective raster lines, whereas no dots are formed by a first nozzle. After the sheet of paper is fed by the amount corresponding to three raster lines as shown in FIG. 23, the second main scan is carried out to form raster lines with the first through the third nozzles. The step of feeding the sheet of paper by the amount corresponding to three raster lines and the step of carrying out a main scan to form raster lines are repeated to record an image. The first main scan does not cause the first nozzle to form any raster line, since the second and the subsequent main scans do not form an adjoining raster line immediately below the imaginary raster line formed by the first nozzle in the first main scan.
The interlace printing forms raster lines intermittently in the sub-scanning direction in the above manner to record an image. Although FIG. 23 refers to the specific nozzle pitch, a variety of values may be set to the feeding amount L according to the nozzle pitch k and the number of nozzles N to realize the interlace printing. In general, the interlace printing process is characterized by the arrangement of a nozzle array on the print head and the method of sub-scans. The nozzle array includes N nozzles aligned in the sub-scanning direction. When the interval between the centers of adjoining nozzles is set equal to k times the pitch of pixels, N and k should be integers that are prime to each other. The feeding amount L in sub-scan carried out after each main scan is set equal to N times the pitch of pixels.
The interlace printing has an advantage of dispersing a deviation of dot-forming positions of dots due to the distributions of the positional accuracy of nozzles and the ink-spouting characteristics on a resulting recorded image. The interlace printing method accordingly reduces the effects of the distributions of the nozzle pitch and the ink-spouting characteristics and improves the picture quality of the resulting recorded image.
In the prior art technique of interlace printing, adjacent dots in the sub-scanning direction are recorded by different nozzles, whereas adjacent dots in the main scanning direction are recorded by an identical nozzle. There may be a deviation of dot-forming positions in the whole raster due to a distribution of ink-spouting characteristics of the individual nozzles. This may result in deteriorating the picture quality. A similar problem arises in the case of non-interlace printing.
In order to cancel the deviation of dot-forming positions in the whole raster and improve the picture quality of the resulting recorded image, the overlap printing technique may be adopted (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 3-207665 and JAPANESE PATENT PUBLICATION GAZETTE No. 4-19030). The overlap printing process causes each raster line to be formed by different nozzles in two or more main scans. By way of example, when each raster line is formed by two main scans, the first main scan records dots only in pixels of odd ordinal numbers, which are allocated from one end of the raster line, by one nozzle. After a sub-scan, the second main scan records dots in pixels of even ordinal numbers on the raster line by a different nozzle.
Application of the overlap printing technique prevents a deviation of dot-forming positions in the whole raster and improves the picture quality of the resulting recorded image. The overlap printing process, however, requires two or more main scans to form each raster line and accordingly causes another problem, that is, lowering the printing speed.
A printer recently proposed records dots in each pixel in an overlapping manner and thereby increases the number of expressible tones in each pixel. In this printer, two or more main scans are also required to record dots in an overlapping manner and complete each raster line. This also causes a decrease in printing speed.
One object of the present invention is thus to provide a technique of preventing a deviation of dot-forming positions on each raster line without lowering the printing speed, so as to improve the picture quality of a resulting recorded image.
Another object of the present invention is to provide a technique of improving the printing speed in a printing system that records dots in an overlapping manner.
At least part of the above and the other related objects is realized by a printing system that carries out a main scan, which reciprocates a print head relative to a printing medium, and forms a raster line, which is a row of dots aligned in a main scanning direction, so as to print an image on the printing medium, wherein the print head has a plurality of nozzles which form an identical type of dots on a raster line in one main scan.
In the printing system of the present invention, dots are recorded by different nozzles on each of at least part of raster lines in the course of one main scan. This structure enables a deviation of dot-forming positions due to a distribution of the characteristics of the individual nozzles to be effectively dispersed on each raster line and thereby improves the picture quality of a resulting recorded image. The structure attains this effect without increasing the number of main scans required to form each raster line. The printing system of the present invention thus improves the picture quality of the resulting recorded image without lowering the printing speed.
In accordance with one application of the printing system, a plurality of dots may be recorded in each pixel on a raster line by a plurality of nozzles in the course of one main scan. Application of the above structure to the printing system that records dots in each pixel in an overlapping manner improves the printing speed while increasing the number of expressible tones in each pixel to enhance the picture quality.
In accordance with one preferable application of the present invention, the plurality of nozzles are arranged at a predetermined interval k1 in the main scanning direction, the predetermined interval being an integral multiple of a pitch of pixels in the main scanning direction.
In the printing system of this preferable structure, all the nozzles can record dots in the respective pixels at an identical drive timing. The structure of setting an identical drive timing to all the nozzles and controlling the on-off state of each nozzle enables dots to be recorded in arbitrary pixels. This structure simplifies a driving circuit or a driving mechanism for driving the respective nozzles.
In accordance with another preferable application of the present invention, the printing system further includes a drive unit which controls actuation of the plurality of nozzles and causes the plurality of nozzles to record dots at different positions on the raster line.
In the printing system of this preferable structure, dots can be recorded at different positions on each raster line by the plurality of nozzles. This structure enables a deviation of dot-forming positions to be effectively dispersed on each raster line and thereby improves the picture quality of the resulting recorded image. The driving unit may drive the respective nozzles at different timings or alternatively at an identical timing.
In the printing system of this structure, it is preferable that the plurality of nozzles are arranged to satisfy a relationship in which the predetermined interval k1 of the plurality of nozzles in the main scanning direction expressed as the pitch of pixels in the main scanning direction is prime to a number of the plurality of nozzles N1 aligned in the main scanning direction.
In this structure, it is further preferable that the drive unit drives the plurality of nozzles simultaneously at a specific timing to record dots in every N1-th pixel in the main scanning direction.
In the printing system of this structure, the respective nozzles are set to the ON state simultaneously at predetermined intervals, that is, at the intervals of recording dots by every shift of N1 dots in the main scanning direction. This structure enables dots to be recorded in different pixels. This structure facilitates the control for dot formation and simplifies the structure of the driving circuit for driving the nozzles.
This effect will be described more concretely. FIG. 11 illustrates the process of recording dots with the print head having two nozzle groups, that is, nozzle groups A and B. Since the print head has two nozzle groups, N1 is equal to two. The interval between the nozzle groups A and B in the main scanning direction corresponds to three dots; that is, k1=3. In this example, N1 and k1 are prime to each other. The dots formed by the print head are shown by the open circles and the open squares in FIG. 11. The open circles represent dots formed by the nozzle group A, whereas the open squares represent dots formed by the nozzle group B.
FIG. 12 illustrates the process of recording dots at the time point when the print head shifts in the main scanning direction by N1 dots (two dots in this example). As shown in FIG. 12, the structure of the printing system enables dots to be recorded in new pixels where no dots have been recorded previously. FIGS. 13 and 14 illustrate the process of recording dots at the time points when the print head further shifts in the main scanning direction. As shown in FIGS. 13 and 14, the structure enables dots to be recorded in new pixels where no dots have been recorded previously.
The above example regards the case of N1=2 and k1=3. The similar effects can be exerted as long as the number of nozzles N1 and the interval k1 between the adjoining nozzles in the main scanning direction are prime to each other.
In accordance with still another preferable application of the present invention, the print head has plural sets of the plurality of nozzles, the plural sets being arranged at a given interval in a predetermined direction crossing the main scanning direction, the given interval being an integral multiple k2 of a pitch of pixels in the predetermined direction.
The printing system of this preferable structure enables a plurality of raster lines to be formed simultaneously, thereby improving the printing speed. In another aspect, this preferable structure implies a plurality of nozzle groups aligned in the main scanning direction, each group including the plurality of nozzles arranged at a given interval in the predetermined direction crossing the main scanning direction. The given interval is an integral multiple of the pitch of pixels in the predetermined direction. By way of example, the print head may have a plurality of nozzle groups A and B arranged in a main-scanning direction as shown in FIG. 11. In accordance with one modification, part of the nozzle groups may consist of only one nozzle. The printing system described above includes both type of system that carries out the other scanning and not.
In the printing system of this preferable structure, it is preferable that the print head has the plural sets of the plurality of nozzles that are arranged to satisfy a relationship in which the integral multiple k2 and a number of the plural sets N2 are prime to each other.
In accordance with one application of this structure, the printing system further includes a sub-scanning unit that carries out a sub-scan, which shifts the printing medium relative to the print head by a predetermined amount of feed in a sub-scanning direction or in the predetermined direction crossing the main scanning direction every time when one raster line is formed, the predetermined amount of feed is N2 times the pitch of pixels in the sub-scanning direction.
The printing system of this structure realizes interlace printing. The interlace printing enables a deviation of dot-forming positions due to a distribution of the characteristics of the individual nozzles to be effectively dispersed on each raster line as well as among different raster lines, thereby further improving the picture quality of the resulting recorded image.
The present invention is also directed to a method of carrying out a main scan, which reciprocates a print head relative to a printing medium, and forming a raster line, which is a row of dots aligned in a main scanning direction, so as to print an image on the printing medium, wherein the print head has a plurality of nozzles which form an identical type of dots on a raster line in one main scan and are arranged to satisfy a relationship in which a predetermined interval k1 of the plurality of nozzles in the main scanning direction expressed as a pitch of pixels in the main scanning direction is prime to a number of the plurality of nozzles N1 aligned in the main scanning direction, the method including the steps of:
(a) driving the print head in the main scanning direction; and
(b) driving the plurality of nozzles simultaneously at a specific timing to record dots in every N1-th pixel in the main scanning direction.
As discussed previously in the printing system of the present invention, this method enables the respective raster lines to be formed by different nozzles, thereby improving the picture quality of a resulting recorded image. This method also enables the control for that purpose to be readily realized.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.