Horizontal Synchronization Control - Canon LBP-1120 Service Manual

2. Laser Diode Automatic Power Control
The laser diode automatic power control (APC) is performed by the IC801. The APC makes the laser diode emit laser
beams at a constant intensity. It is performed prior to the image formation (Initial APC) and during the between-line
(between line APC).
When the CNT0 and CNT1 signals, input to the IC801 from the CPU, are "L" and "H" respectively, the IC801
makes the sample hold circuit enter the sample mode (mode in which the APC is performed). At the same time, the
IC801 allows the laser diode (LD) to emit laser beams.
The intensity of the laser beam is detected by the photo diode (PD), converted into voltage, and then fed back to the
IC801.
To control the laser intensity, the IC801 maintains the fed-back voltage equal to the reference voltage by increasing
or decreasing the drive current for the laser diode.
When the CNT0 signal goes "H", the IC801 makes the sample hold circuit enter the hold mode. In this mode, the
IC801 converts the laser intensity controlled by the APC to voltage and stores it in the hold condenser (C803).

3. Horizontal Synchronization Control

The CPU controls the horizontal synchronization based on the HORIZONTAL SYNCHRONOUS INPUT signal (/
BDI) sent from the BD sensor in the laser scanner.
The CPU turns CNT0 and CNT1 signals to "L" and "H" respectively, making the laser diode emit a laser beam at the
specified timing based on the /BDI signal.
The laser beam passes through the collimator lens and cylindrical lens, and strikes the four-faced scanning mirror.
Then, the laser beam reflects off the scanning mirror, passes through the BD lens on the scanning light path, and enters
the BD sensor.
Detecting the laser beam, the BD sensor generates the HORIZONTAL SYNCHRONOUS signal
(/BD) based on the /BDI signal and then sends it to the interface controller.
4. Image Masking Control
The CPU performs the image masking control in order to prevent the laser diode from emitting in the non-image area.
In this control, the CPU controls the period of the laser diode emission by conducting the CNT0 and CNT1 signals
that are sent to the IC801.
In the non-image area, the CPU sets the CNT0 and CNT1 signals "H" as to implement the image masking. The
IC801 prohibits the laser diode from emitting regardless of the video signals (VDO, /VDO) sent from the interface
controller.
In the image masking area, the CPU sets the CNT0 signal "H" and CNT1 signal "L." as to stop the image masking.
The IC801 turns the laser ON and OFF according to the VDO, /VDO signals sent from the interface controller.
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