9. Image
An example of alteration may be a DC component added to the signal, in the form of a DC offset repositioning the black level, since
this black level (“brightness”) will become crucial later on (clamping circuit) it will result in “black not being black”.
Another value that is subject to alteration is the amplitude of the signal, resulting in an altered “Gain” of the signal (“white level” or
contrast).
The alterations of the three color signals will happen independently i.e. the colors will end to be unbalanced, image 9-29
B
Black level
0.7V
Image 9-28
Black level
Δ
G
ΔΒ
Δ
R
R
G
B
Image 9-29
One can conclude here that a good color tracking can only be met by using th
ree previously (input) balanced
color signals
Analog Digital Conversion
The analog color signals must pass through an Analog/Digital convers
ion circuit prior to any digital processing in the PMP.
A typical ADC transforms the analog value into an 8 bit coded digital signal.
The graphic shows that when converting a signal containing a DC offset component the range of the converter is not optimally used.
Black level
Δ
R
ADC
0
255
i1 : superfluous information
i2 : video information
Image 9-30
One can conclude here that a good data conversion can only be met by using three previously (input) balanced
color signals
The objective of input balancing
The objective in input balancing is to “set” the same black level and the same white level for the three colors of a particular input
source.
Black level setting : brightness
White level setting : contrast
The same absolute black and white level for the three colors allows the same reference for Brightness and Contrast control of the
picture !
90
R5905158 HDF W SERIES 10/07/2012
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