MSWU-81E 07/08/2013 43
8.Image
8.7 Input Balance

Unbalanced color signals

When transporting signals, there is alway s a risk of deterioration of the information containe d in the signals.
In case of information contained in the amplit ude of the signals which is the case of data color sign als (R, G, B),image 9-29 ,
we are quite sure that the amplitude of these color signa ls is subject to alterations.
An example of alteration may be a DC component add ed to the signal, in the form of a DC offset repositioni ng the black level,
since this black level (“brightness”) will becom e crucial later on (clamping circuit) it will result in “black not bei ng black”.
Another value that is subject to alteration is th e amplitude of the signal, resulting in an altered “Gain” of t he signal (“white
level” or contrast).
The alterations of the three color signals will hap pen independently i.e. the colors will end to be unb alanced, image 9-30

B

Black level
0.7V
Image9-29
Black level
∆
G
∆Β
∆
R
R
GB
One can conclude here that a good color track ing can only be met by using three previously (input)
balanced color signals

Analog Digital Conversion

The analog color signals must pass throug h an Analog/Digital conversion circuit prior to a ny digital processing in the PMP. A
typical ADC transforms the analo g value into an 8 bit coded digital signal.
The graphic shows that when converting a signal conta ining a DC offset component the range of the conver ter is not optimally
used.
Black level
∆
R
ADC
0
255
i1 : superfluous information
i2 : video information
One can conclude here that a good data conversion can o nly be met by using three previously (input)
balanced color signals

The objective of input balancing

The objective in input balancing is to “set” t he same black level and the same white level for the th ree colors of a particular
input source.