Oce 3121 Technical & Service Manual page 280

A-D Conversion Circuit
The A-D conversion circuit converts Analogue signals from the CCD to 8-bit
digital image signals.
In general, the A-D conversion signal performs A-D conversion based on the
reference voltage. As such, increasing or decreasing the reference voltage will
increase or decrease the dynamic range (difference between maximum output
level and minimum output level). Thus affecting the signal level after A-D con-
version. (This means faithful digital conversion of Analogue inputs is not pos-
sible.)
The ABC circuit takes advantage of this fact. The reference voltage is varied
according to the A-D conversion circuit output signal level to vary the dynamic
range. Also, cancelling the background density of documents. (This processing
is executed for every main scanning line.)
ABC (automatic background control)
If 'priority on speed' is selected, the ABC circuit operates as part of AE (auto-
matic exposure control settings. Image signals after A-D conversion are re-
turned to the ABC circuit for the following processing.
Identifies the A-D conversion circuit output signal level.
I
Varies the reference voltage, based on the results of identification.
I
The reference voltage obtained here is sent to the A-D conversion circuit. The
"priority on speed" function can be adjusted in service mode (*3*).
504 TSM Océ 3121 Functional description