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Fig. 5-2 Conceptual diagram of gain and offset levels
(5-3)
Level setting
The level setting is mostly divided into the
following two groups:
●
Gain setting
This is to set the gain (amplification ratio)
of the preamplifier between the CCD
imaging device inside the camera to A/D
converter.
●
Offset setting
This is to set the offset of the preamplifier
between the CCD imaging device inside
the camera to A/D converter.
→
Refer to the next section (Section 6) for
the specific setting method.
(Note) As for the offset setting, it is recommended to use the factory default setting except for a special case.
(Note) Follow the procedure (gain setting
→
offset setting) if fine tuning of the offset value is required.
(5-4) MGC gain setting value (Manual gain control)
●
Variable gain amplifier and integrated gain
The image signal output from CCD is amplified inside the camera
through the following fixed gain amplifier and then through the
variable gain amplifier before being input into the A/D converter.
The left block chart shows this flow.
(Note) The gain value (dB) described here is the one based on the CCD output (0 dB) as the baseline.
The equipment is controlled by giving 0 to 255 as the MGC setting value.
The correlation between this setting value and the MGC gain (Preamplifier
gain) is shown in the right graph.
(Note) When the CCD element receives excessive light with a low
gain value of the amplifier, the signals of the nonlinear section of the
CCD element and the preamplifier are output at the high brightness
area due to the restriction of the dynamic range of the CCD light
receiving element.
In this state, unnatural image (Note below) may appear in the
neighborhood of the saturating signal area of the image due to the
characteristic of the nonlinear section.
This phenomenon, which is associated with the saturation characteristic
of the CCD element, is not a failure arising from the camera.
→
To eliminate this phenomenon, reduce the amount of light by stopping
down the lens and newly set a higher gain value. Then, the output signal
from the CCD element at the saturating area will be appropriately saturated
into a white level for the output.
(Note)The above described unnatural image represents the following states:
●
Black and white look inverted at the saturating area.
●
The outline of the saturating area is blurred.
●
The saturating area slightly shifts upward or downward.
●
The brightness value of the saturating area does not reach 1023. (when in 10 bit output)
Ti me
T ime
Ti me
１ ０ ２ ３
０
０
１ ０ ２ ３
０
In cr ea se in O FF SET
In cr eas e in GA IN
I mag e si gna l
１ ０ ２ ３
（B efo re s et tin g ad ju stm en t ）
Si gn al le ve l
S ig nal l ev el
Si gn al l eve l
Ｃ Ｃ Ｄ
P r ea m pl if i er
Ａ／Ｄ
ＧＡＩＮ
V a ri a bl e g ai n a mp l if i er
5.1 dB
～
32.6 dB
I nt eg rat ed g ai n
MGC setting
Fi xe d g a in a m pl i fi e r
1d B(Fi xed)
6. 1dB
～
33.6dB
42
36
30
24
18
12
6
0
64
128
192
(255)
MGC GAIN CODE
M
G
C
G
A
IN
(d
B
)