text: The present invention relates to a printing apparatus and printing method, and more particularly, to a printing apparatus for performing printing by discharging an ink droplet on a printing medium.
  Currently, printing apparatuses that require high-speed and high-precision printing mainly employ an ink-jet printing method. A printhead used in such printing apparatuses generally has an array of plural nozzles for discharging ink. Ink discharging techniques include a method of applying foaming energy generated when heaters in the nozzles are driven, or a method of applying contraction of piezoelectric devices provided in the nozzles, and so forth. In either of these methods, disadvantages are brought about when all nozzles are simultaneously driven: for instance, deteriorated printing quality caused by an influence of crosstalk between nozzles, or a large-capacity power source required for temporarily supplying a large electric current. Therefore, many printing apparatuses employ a method (time-divisional driving method) in which all nozzles are divided into blocks, each having a number of nozzles, and the blocks of nozzles are driven while sequentially shifting the block to be driven.
  The amount of ink discharged by nozzles of a printhead varies because of an uneven formation of nozzles. If printing is performed with such nozzles, some degree of density unevenness is generated in an image. Therefore, to realize high-precision printing, as disclosed in Japanese Patent Application Laid-Open No. 61-120578, each image area is overlappingly printed by plural scans (multi-pass printing) instead of printing an image by a single scan, and ink is randomly discharged from various parts of the nozzle array to form an image, thereby reducing the density unevenness.
  However, to satisfy the recent demand for high printing speed, i.e., improved throughput, the driving cycle of the block in the time-divisional method has conventionally been reduced. However, to assure the minimum time necessary for driving discharge heaters and generating enough foaming energy, the driving cycle of the block can no longer be reduced. In addition, the so-called multi-pass printing, in which one area is printed complementarily by a plurality of scanning operations to realize high-precision printing, requires as much time as the number of scans (the number of passes). As a result, the printing speed is further deteriorated.
  The present invention has been proposed to solve the conventional problems, and has as its object to provide a printing apparatus and printing method employing a conventional printhead, which can improve printing speed while maintaining high printing quality without requiring a large-capacity power source.
  Another object of the present invention is to provide a printing apparatus and printing method which can achieve improved printing speed when color printing or high-precision printing is performed by multi-pass printing.
  A further object of the present invention is to provide a printing apparatus and printing method which realize high-speed printing while maintaining high printing quality when multi-pass printing is performed with a plurality of printheads.
  In order to attain the above objects, the present invention provides a printing apparatus for printing an image on a print medium by scanning a carriage, having at least one printhead including a plurality of printing elements, and performing multi-pass printing in which one area is printed complementarily by a plurality of scans, said printing apparatus comprising: division means for dividing the plurality of printing elements into a plurality of blocks; selection means; and control means for controlling moving speed of the carriage in accordance with a number of scans in the multi-pass printing.
  Furthermore, in order to attain the above objects, the present invention also provides a printing method for printing an image on a print medium by scanning a carriage, having at least one printhead including a plurality of printing elements, and performing multi-pass printing in which one area is printed complementarily by a plurality of scans, said printing method comprising the steps of: dividing the plurality of printing elements into a plurality of blocks; selecting a different block from the plurality of blocks for each scan of the multi-pass printing; time-divisionally driving printing elements included in the selected block; and controlling moving speed of the carriage in accordance with a number of scans in the multi-pass printing.
  More specifically, a plurality of printing elements in a printhead are divided into a plurality of blocks, different blocks are selected from the plurality of blocks for each pass of multi-pass printing, the printing elements of the selected blocks are time-divisionally driven, and moving speed of a carriage is controlled in accordance with the number of passes in multi-pass printing.
  By virtue of this, for instance, in a case where the number of selected blocks for printing each pass of multi-pass printing is one half of the total number of blocks, the carriage moving speed is doubled to maintain high printing speed while achieving high-quality image printing. In addition, since the number of printing elements driven at once is controlled to a small number, it is possible to use a conventional printhead, and moreover, it is possible to prevent an increased cost and weight of a printing apparatus due to an increased capacity of a power source.
  In this case, by transferring image data in correspondence with carriage movement, a storage device, e.g., mask ROM or the like, for obtaining sampling data for each block to be driven becomes unnecessary. Accordingly, the overall cost of the printing apparatus can be cut down.
  Furthermore, since block selection is controlled such that all of a plurality of blocks are driven each time printing is performed by scanning the carriage the same number of times as the number of passes in multi-pass printing, all printing elements are driven the same number of times in the same cycle. Therefore, when performing high-speed printing, the usage frequency of each printing element can be kept equal, thus preventing a shortened life cycle of the printhead and reduced printing quality due to uneven performance of the printing elements.
  Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.