Silicon Imaging , Inc. 2004 Page 24 of 39 Company Confidential
Auto Black Level Calibration
(Registers: 0x62)
The automatic black level calibration measures the average value of 256 pixels from two dark rows of the imager for
each of the four colors. The pixels are averaged as if they were light-sensitive and passed through the appropriate
color gain. This average is then digitally filtered over many frames. For each color, the new filtered average is
compared to minimum and a maximum acceptable level. If the average is lower than the minimum acceptable level,
the offset correction voltage for that color is increased. If it is above the maximum level, the level is decreased. The
upper threshold is automatically adjusted upwards whenever an upward shift in the black level from below the
minimum results in a new black level above the maximum. This prevents black level oscillation from below the
minimum to above the maximum. The lower threshold is increased with the maximum gain setting (out of all four
colors), according to Register settings. This prevents clipping of the black level. After changes to the sensor
configuration, large shifts in the black level calibration can result. To quickly adapt to this shift, a rapid sweep of the
black level during the dark-row readout is performed on the first frame after certain changes to the sensor registers.
Manual Black Level
(Registers: 0x60, 0x61,0x63 & 0x64)
The programmable analog offset stage corrects for analog offset that might be present in the analog signal. The
user would need to program Reg0x62 to enable the manual analog offset correction. The analog offset settings can
be independently adjusted for the colors of Green1, Green2, Red and Blue and are programmed through Reg0x60,
Reg0x61, Reg0x63 and Reg0x64 respectively.
Bit[8] of Reg0x60, Reg0x61, Reg0x63 and Reg0x64 determines the sign of the analog offset(these registers have
two’s complement representation). Bit[8] = 1 makes the analog correction negative instead of positive.
The lower 8 bits (Bit[7–0]) determine the absolute value of the analog offset to be corrected and Bit[8] determines the
sign of the correction. When Bit[8] is “1”, the sign of the correction is negative and vice versa. The analog value of
the correction relative to the analog gain stage can be determined from the following formula:
Analog offset = Bit[8–0] x 1 LSB
Note that the 1 LSB value in the formula is an estimate amount. It will deviate from 1 LSB with process variation.
Black Level Digital Control (
Register: 0x49)
Digital offset will be applied such that the average black level of a frame in a resulting image equals the value of this
register. This adjustment happens after black level calibration.
