Patent ID: 7940421

Claim:
A tandem type color image forming apparatus, which continuously forms color images on both faces of a recording sheet, each of the color images containing two or more colors, the color image forming apparatus comprising: a pseudo index creating circuit which creates a plurality of pseudo index signals each having a prescribed cycle period; a plurality of signal creating sections each of which being provided for each color, wherein a signal creating section for each color comprises: two or more image area setting counters, exclusively provided to the signal creating section for each color and configured to be activated in parallel with each other, for generating an image valid area signal to set a starting position and a width of an image formation area in a sub-scanning direction; and two or more selectors exclusively provided to the signal creating section for each color, each of the selectors corresponding to each of the two or more image area setting counters and configured to select one of the plurality of pseudo index signals in parallel with each other for scanning in a main scanning direction; and a control section for executing color image formation control on a front face and a rear face of the recording sheet, based on an image top signal for controlling writing of an image on an image carrier and the image valid area signal for each color created by the signal creating section, wherein the control section is configured to control the signal creating section for each color in such a way: in a case of image formation for each color on the front face of the recording sheet, to sequentially select a first counter out of the two or more image area setting counters to output signals for a front face image formation, to select a first pseudo index signal for the front face image formation out of the plurality of pseudo index signals by a selector corresponding to the first counter, and to create the image valid area signal for the front image formation by activating the first counter; and in a case of image formation for each color on the rear face of the recording sheet, to sequentially select a second counter out of the two or more image area setting counters to output signals for a rear face image formation, to select a second pseudo index signal for the rear face image formation out of the plurality of pseudo index signals by another selector of the two or more selectors corresponding to the second counter, and to create the image valid area signal for the rear image formation by activating the second counter, wherein the control section controls the image formation for each color based on the created image valid area signal.