HP 8550 Service Manual - Page 206

ABC (auto background control) circuit AE Mode The copy module's AE mode (in black-and-white text mode) may be set to either of the following: l Priority on speed (factory default) l Priority on image quality In general, "priority on speed" is best for mostly text originals, and turns out first prints in a short time because of the absence of pre-scanning. "Priority on image quality" is best for originals consisting mostly of images. In "priority on image quality" mode, the copy module uses different density correction curves according to types of originals, and performs pre-scanning over the entire face of the original to take measurements of the density. ABC circuit The A/D conversion circuit converts BGR analog image signals representing each single line in main scanning direction from the CCD into 8-bit digital image signals for B, G, and R. The circuit operates in reference to reference voltage. The range (the difference between maximum output level and minimum output level) increases and decreases in response to increases and decreases in the reference voltage, affecting the level of signals after A/D conversion. In auto background control (ABC), the range of the reference voltage is varied according to the A/D conversion circuit output signal level to vary the background level of an original to enable adjustment of the density of the background. 204 Chapter 5 - Theory of operation EN