![](http://datasheet.mmic.net.cn/310000/ADV7300A_datasheet_16243960/ADV7300A_44.png)
REV. A
–44–
ADV7300A/ADV7301A
Gamma Correction
[Subaddresses 21h–37h for HD;
Subaddresses 66h–79h for SD]
Gamma correction is available for SD and HD video. For each
standard there are twenty 8-bit wide registers. They are used to
program the Gamma Correction Curves A and B. HD Gamma
Curve A is programmed at Addresses 24h–2Dh and HD
Gamma Curve B at 2Eh–37h. SD Gamma Curve A is pro-
grammed at Addresses 66h–6Fh, and SD Gamma Curve B at
Addresses 70h–79h.
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:
=
(
where equals the 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 10 predefined
locations. In changing the values at these locations, the gamma
curve can be modified. Between these points, linear interpola-
tion is used to generate intermediate values. Considering the
curve to have a total length of 256 points, the 10 locations are
at: 24, 32, 48, 64, 80, 96, 128, 160, 192, and 224. Locations 0,
16, 240, and 255 are fixed and cannot be changed.
For the length of 16 to 240, the gamma correction curve must
be calculated as:
Signal
Signal
OUT
IN
)
γ
y
x
=
γ
where
y
= gamma corrected output,
x
= linear input signal, and
= the gamma power factor.
To program the gamma correction registers, the values for
y
must be calculated using the formula:
y
x
n
n
=
×
(
)
+
(
)
16
240
16
240
16
16
γ
where
x
(n–16)
= the value for x along the x-axis at points
n
= 24,
32, 48, 64, 80, 96, 128, 160, 192, or 224;
y
n
= the value for y
along the y-axis, which has to be written into the Gamma Cor-
rection Register.
Example:
y
y
y
y
24
0 5
.
32
0 5
.
48
0 5
.
64
0 5
.
8
224
224
16
58
16
224
224
16
76
32
224
224
16
101
48
224
224
16
120
=
×
+
+
=
=
×
+
=
=
×
+
=
=
×
+
=
*
*
*
*
*
*
*
y
y
y
y
80
0 5
.
96
0 5
.
128
0 5
.
160
0 5
.
64
224
224
16
136
80
224
224
16
150
112
224
224
16
174
144
224
224
16
=
×
+
=
=
×
=
=
×
+
+
=
=
×
+
=
=
×
+
=
=
×
=
195
176
224
224
16
214
208
224
224
16
232
192
y
0 5
.
224
0 5
.
*
*
*
y
The gamma curves shown in Figures 61 and 62 are examples.
Any user-defined curve is acceptable in the range of 16–240.
LOCATION
00
50
100
150
200
250
300
50
100
150
200
250
0.5
SIGNAL INPUT
SIGNAL OUTPUT
G
Figure 61. Signal Input (Ramp) and Signal Output
for Gamma 0.5
*
Rounded to the nearest integer