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Theory of Operation
X64-AN Quad User's Manual
Contrast and Brightness Adjustment
Contrast and Brightness are controlled through the input gain of the ADC. The X64-AN Quad is
calibrated for standard video during manufacturing. The figure below shows the relationship between
input analog video (vertical axis) relative to the output digital data (horizontal axis).
Brightness controls the offset of the digitization line while contrast controls its gain (the slope of the
line). The vertical axis represents the voltage level of the incoming video signal (black level is 53mV,
white level is 714mV for RS170 video) while the horizontal axis shows the resulting pixel value. As
an example, using default brightness and contrast settings, an incoming video signal of 350mV will be
digitized to a value of 115. By increasing the brightness, the digitization line gets shifted down (with
no slope change). This creates a brighter digital image (same input voltage leads to a higher pixel
value). Likewise a decrease in brightness leads to a darker image (same input voltage leads to a lower
pixel value). Changing the brightness only affects the offset of the digitization line, not its slope.
Contrast controls the relationship between a change in input analog video to the change in digital pixel
value. Increasing contrast causes a greater difference in the digital pixel values for any constant
change in input video level. Likewise a decrease in contrast reduces the difference in the digital pixel
values for any constant change in input video level.
Default values of brightness and contrast use the full resolution of the X64-AN Quad ADC based on
the Sapera acquisition parameters Video Level Min/Max.
White level) 714 mV
(Black level) 53 mV
Video Voltage Level (NTSC)
Pixel Value
255
60%
170%
Contrast range
(default = 100%)
29%
-20%
Default Brightness/Contrast Line
350 mV
115
Example: 350 mV
video signal leads
to pixel value of 115
Brightness range
(default = 0%)
100%
0%
Figure 25: Brightness & Contrast Relationship
