1.  Field of the Invention
The present invention relates to a printing apparatus and a conveyance control method.  Particularly, the present invention relates to a printing apparatus and a conveyance control method which perform accurate conveyance control even when, e.g., the leading edge or trailing edge of a printing medium enters between or passes through conveyance rollers. 
2.  Description of the Related Art
Recent printing apparatuses such as printers use not only plain paper but also printing media such as photo special paper to print photo images in many occasions.  In particular, an inkjet printer which uses smaller ink droplets for printing can obtain an image quality equal to or higher than a film photo. 
Accordingly, conveyance of printing media is also required to be more accurate.  Conveyance rollers use high precision rollers with, e.g., a grindstone coating on a metal shaft.  A DC motor used to drive the conveyance rollers is controlled by a cord wheel and an encoder sensor provided coaxially, thereby simultaneously ensuring high accuracy and high-speed conveyance. 
Only one pair of conveyance rollers does not suffice for accurately printing an image up to the trailing edge of a printing medium.  To implement, e.g., marginless print, some proposed arrangements have another pair of conveyance rollers downstream in the printing medium conveyance direction.  In such an arrangement, however, when the trailing edge of a printing medium passes through a conveyance roller pair upstream in the conveyance direction, the conveyance amount may change, resulting in density unevenness in the image.  To ensure a conveyance accuracy up to the trailing edge of a printing medium, the nozzles of the printhead to be used for printing on the trailing edge part of a printing medium are restricted, thereby reducing the conveyance amount.  In addition to the restriction on the nozzle use of the printhead, conveyance of the trailing edge part of the printing medium is controlled to maintain the printing quality (Japanese Patent Laid-Open No.  2002-225370).  The mechanical accuracy of the conveyance roller pair downstream in the conveyance direction is also increased to ensure the conveyance accuracy. 
In recent years, the need for further improving the printed image quality and the printing speed has risen more and more.  To meet these requirements, the print width of a printhead increases, the number of passes of multipass printing decreases, and the printing medium conveyance length of each pass printing increases.  To attain a higher image quality, ink droplets to be used in printing become smaller.  This also indicates that it is necessary to more accurately convey a printing medium. 
In the above-described prior art, however, printing on the trailing edge part of a printing medium is performed without sufficiently exploiting the performance of the printhead, creating a bottleneck for high-speed printing of a market demand. 
More specifically, in a printer having another conveyance roller pair downstream in the conveyance direction of a printing medium to cope with, e.g., marginless printing, when the trailing edge of a printing medium passes through the conveyance rollers on the upstream side, and only the conveyance rollers on the downstream side convey the printing medium, it is affected by, e.g., idler gear driving.  This makes it difficult to ensure conveyance accuracy.  To ensure the accuracy, the number of nozzles in use by the printhead must be restricted.  This is a great obstacle in speeding up printing.