The present invention generally relates to image forming systems, such as printers, copiers and facsimiles, and in particular to both side image forming systems capable of printing images on both the sides of paper or the like.
In a printer employing electrophotography printing, such as a laser printer, video data is formed per page on a memory region by using source data (font data of a character, image data and so on) and page data (data formed per page on the basis of data supplied from a host computer system). Then the video data thus formed is read out from the memory and a corresponding image is printed on a sheet of paper.
Currently, a printer having a both side printing function is available. In a both side printing process which achieves the both side printing function, a front side of paper is subjected to the printing operation and the paper is passed through a path for turning the paper over. Then, a rear side of the paper is subjected to the printing of the next page. In the conventional printer, the path for turning the paper over (paper inversion path) is considerably long. Therefore, it becomes possible to print another paper while the preceding paper is being turned over in the paper inversion path. With this printing process, an increased printing speed can be obtained.
However, in order to attain the above both side printing operation, it is necessary to intermittently skip a page to be printed. It is therefore required to form video data amounting to a few of pages in the memory in advance. For this requirement, a memory having an extremely large capacity must be provided in the printer. In particular, a printer capable of handling a sheet of paper of a large size such as A3 becomes expensive.
The above fact holds true for facsimiles and copiers employing the electronic photograph printing method.