Datasheet

Data Sheet ADV7390/ADV7391/ADV7392/ADV7393
Rev. J | Page 59 of 107
The reset value of the control registers is 0x00; that is, nominal
DAC current is output. Table 50 is an example of how the output
current of the DACs varies for a nominal 4.33 mA output current.
Table 50. DAC Gain Control
Subaddress 0x0B
DAC Current
(mA) % Gain Note
0100 0000 (0x40) 4.658 7.5000%
0011 1111 (0x3F) 4.653 7.3820%
0011 1110 (0x3E) 4.648 7.3640%
... ... ...
... ... ...
0000 0010 (0x02) 4.43 0.0360%
0000 0001 (0x01) 4.38 0.0180%
0000 0000 (0x00) 4.33 0.0000% Reset value,
nominal
1111 1111 (0xFF) 4.25 −0.0180%
1111 1110 (0xFE) 4.23 −0.0360%
...
...
...
... ... ...
1100 0010 (0xC2) 4.018 −7.3640%
1100 0001 (0xC1) 4.013 −7.3820%
1100 0000 (0xC0) 4.008 −7.5000%
GAMMA CORRECTION
Subaddress 0x44 to Subaddress 0x57 for ED/HD;
Subaddress 0xA6 to Subaddress 0xB9 for SD
Generally, gamma correction is applied to compensate for the
nonlinear relationship between signal input and output
brightness level (as perceived on a CRT). It can also be applied
wherever nonlinear processing is used.
Gamma correction uses the function
Signal
OUT
= (Signal
IN
)
γ
where γ is the gamma correction factor.
Gamma correction is available for SD and ED/HD video. For
both variations, there are twenty 8-bit registers. They are used
to program Gamma Correction Curve A and Gamma
Correction Curve B.
ED/HD gamma correction is enabled using Subaddress 0x35,
Bit 5. ED/HD Gamma Correction Curve A is programmed at
Subaddress 0x44 to Subaddress 0x4D, and ED/HD Gamma
Correction Curve B is programmed at Subaddress 0x4E to
Subaddress 0x57.
SD gamma correction is enabled using Subaddress 0x88, Bit 6.
SD Gamma Correction Curve A is programmed at Subaddress
0xA6 to Subaddress 0xAF, and SD Gamma Correction Curve B
is programmed at Subaddress 0xB0 to Subaddress 0xB9.
Gamma correction is performed on the luma data only. The
user can choose one of two correction curves, Curve A or
Curve B. Only one of these curves can be used at a time. For
ED/HD gamma correction, curve selection is controlled using
Subaddress 0x35, Bit 4. For SD gamma correction, curve
selection is controlled using Subaddress 0x88, Bit 7.
The shape of the gamma correction curve is controlled by
defining the curve response at 10 different locations along the
curve. By altering the response at these locations, the shape of
the gamma correction curve can be modified. Between these
points, linear interpolation is used to generate intermediate
values. Considering the curve to have a total length of 256
points, the 10 programmable locations are at the following
points: 24, 32, 48, 64, 80, 96, 128, 160, 192, and 224. The
following locations are fixed and cannot be changed: 0, 16, 240,
and 255.
From the curve locations, 16 to 240, the values at the
programmable locations and, therefore, the response of the
gamma correction curve, should be calculated to produce the
following result:
x
DESIRED
= (x
INPUT
)
γ
where:
x
DESIRED
is the desired gamma corrected output.
x
INPUT
is the linear input signal.
γ is the gamma correction factor.
To program the gamma correction registers, calculate the
10 programmable curve values using the following formula:
16
)16
240(
16240
16
+








−×






−
−
=γ
γ
n
n
where:
γ
n
is the value to be written into the gamma correction register
for point n on the gamma correction curve.
n = 24, 32, 48, 64, 80, 96, 128, 160, 192, or 224.
γ is the gamma correction factor.
For example, setting γ = 0.5 for all programmable curve data
points results in the following y
n
values:
y
24
= [(8/224)
0.5
× 224] + 16 = 58
y
32
= [(16/224)
0.5
× 224] + 16 = 76
y
48
= [(32/224)
0.5
× 224] + 16 = 101
y
64
= [(48/224)
0.5
× 224] + 16 = 120
y
80
= [(64/224)
0.5
× 224] + 16 = 136
y
96
= [(80/224)
0.5
× 224] + 16 = 150
y
128
= [(112/224)
0.5
× 224] + 16 = 174
y
160
= [(144/224)
0.5
× 224] + 16 = 195
y
192
= [(176/224)
0.5
× 224] + 16 = 214
y
224
= [(208/224)
0.5
× 224] + 16 = 232
where the sum of each equation is rounded to the nearest integer.
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