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ADV7194
–43–
REV. A
GAMMA CORRECTION REGISTERS 0–13 (GAMMA
CORRECTION 0–13)
(Address (SR5–SR0) = 26H–32H)
The Gamma Correction Registers are fourteen 8-bit-wide regis-
ters. They are used to program the gamma correction Curves A
and B.
Generally gamma correction is applied to compensate for the
nonlinear relationship between signal input and brightness level
output (as perceived on the CRT). It can also be applied wher-
ever nonlinear processing is used.
Gamma correction uses the function:
SignalOUT = (SignalIN)γ
where
γ = gamma power factor
Gamma correction is performed on the luma data only. The
user has the choice to use two different curves, Curve A or
Curve B. At any one time only one of these curves can be used.
The response of the curve is programmed at seven predened
locations. In changing the values at these locations the gamma
curve can be modied. Between these points linear interpolation
is used to generate intermediate values. Considering the curve
to have a total length of 256 points, the seven locations are at:
32, 64, 96, 128, 160, 192, and 224.
Location 0, 16, 240 and 255 are xed and can not be changed.
For the length of 16 to 240 the gamma correction curve has to
be calculated as below:
y = xγ
where
y = gamma corrected output
x = linear input signal
γ = gamma power factor
To program the gamma correction registers, the seven values for
y have to be calculated using the following formula:
yn = [x(n–16)/(240–16)]γ
× (240–16) + 16
where
x(n-16) = Value for x along x-axis at points n = 32, 64, 96, 128,
160, 192 or 224
yn
= Value for y along the y-axis, which has to be written
into the gamma correction register
Example:
DNR27
DNR26
DNR25
DNR24
DNR23
DNR22
DNR21
DNR20
BLOCK OFFSET CONTROL
DNR DNR DNR DNR
27
26
25
24
0000
0 PIXEL OFFSET
0001
1 PIXEL OFFSET
0010
2 PIXEL OFFSET
1101
13 PIXEL OFFSET
1110
14 PIXEL OFFSET
1111
15 PIXEL OFFSET
DNR INPUT SELECT CONTROL
DNR DNR DNR
22
21
20
0
1
FILTER A
0
1
0
FILTER B
0
1
FILTER C
1
0
FILTER D
0
DNR MODE
1
DNR
SHARPNESS
MODE
DNR23
DNR MODE
CONTROL
Figure 90. DNR Register 2
y32 = [(16/224)0.5
× 224] + 16 = 76*
y64 = [(48/224)0.5
× 224] + 16 =120*
y96 = [(80/224)0.5
× 224] + 16 = 150*
y128 = [(112/224)0.5
× 224] + 16 = 174*
*Rounded to the nearest integer.
The above will result in a gamma curve shown below, assuming
a ramp signal as an input.
250
200
150
100
50
0
300
250
200
150
100
50
300
SIGNAL OUTPUT
SIGNAL INPUT
0.5
GAMMA CORRECTION BLOCK OUTPUT
TO A RAMP INPUT
GAMMA-CORRECTED
AMPLITUDE
0
50
100
150
200
250
LOCATION
Figure 91. Signal Input (Ramp) and Signal Output for
Gamma 0.5
250
200
150
100
50
0
300
SIGNAL OUTPUTS
SIGNAL
INPUT
0.5
GAMMA CORRECTION BLOCK OUTPUT
TO A RAMP INPUT FOR VARIOUS GAMMA VALUES
GAMMA-CORRECTED
AMPLITUDE
0
50
100
150
200
250
LOCATION
0.3
1.5
1.8
Figure 92. Signal Input (Ramp) and Selectable Gamma
Output Curves
The gamma curves shown above are examples only, any user
dened curve is acceptable in the range of 16–240.