ADV7330KST datasheet(43/76 Pages) AD

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REV. B

ADV7330

–43–

Gamma Correction

[Subaddress 24h–37h for HD, Subaddress 66h–79h for SD]

Gamma correction is available for SD and HD video. For each

standard there are 20 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, HD gamma

curve B at 2Eh–37h. SD gamma curve A is programmed at

Addresses 66h–6Fh, 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

wherever nonlinear processing is used.

Gamma correction uses the function

Signal

OUT

()

where

= 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 interpolation 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, 224. Locations 0, 16, 240, and 255 are

fixed and cannot be changed.

LOCATION

100

150

200

250

300

100

150

200

250

0.5

SIGNAL INPUT

SIGNAL OUTPUT

GAMMA CORRECTION BLOCK OUTPUT TO A RAMP INPUT

Figure 32. Signal Input (Ramp) and Signal Output

for Gamma 0.5

For the length of 16 to 240, the gamma correction curve has to

be calculated as follows:

where:

y = gamma corrected output

x = linear input signal

= gamma power factor

To program the gamma correction registers, the seven values for

y have to be calculated using the following formula:

−

()













×−

()+

−

()

240

where:

x(n–16) = Value for x along x-axis at points

n = 24, 32, 48, 64, 80, 96, 128, 160, 192 or 224

yn = Value for y along the y-axis, which has to be written into

the gamma correction register

For example:

y24 = [(8 / 224)

0.5

× 224] + 16 = 58*

y32 = [16 / 224)

0.5

× 224] + 16 = 76*

y48 = [(32 / 224)

0.5

× 224] + 16 = 101*

y64 = [(48 / 224)

0.5

× 224] + 16 =120*

y80 = [(64 / 224)

0.5

× 224] + 16 =136*

y96 = [(80 / 224)

0.5

× 224] + 16 = 150*

y128 = [(112 / 224)

0.5

× 224] + 16 = 174*

y160 = [(144 / 224)

0.5

× 224] + 16 = 195*

y192 = [(176 / 224)

0.5

× 224] + 16 = 214*

y224 = [(208 / 224)

0.5

× 224] + 16 = 232*

*rounded to the nearest integer

The gamma curves in Figure 32 and 33 are examples only; any

user defined curve is acceptable in the range of 16–240.

LOCATION

100

150

200

250

300

100

150

200

250

GAMMA CORRECTION BLOCK TO A RAMP INPUT FOR

VARIOUS GAMMA VALUES

0.3

0.5

1.5

1.8

SIGNAL

INPUT

Figure 33. Signal Input (Ramp) and Selectable

Gamma Output Curves

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