ADV7342 datasheet(57/88 Pages) AD

57 / 88 page

ADV7342/ADV7343

Rev. 0 | Page 57 of 88

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 has a total length of 256 points,

the 10 programmable locations are at points 24, 32, 48, 64, 80,

96, 128, 160, 192, and 224. Locations 0, 16, 240, and 255 are

fixed and cannot be changed.

From 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:

xDESIRED

= (xINPUT)γ

where:

xDESIRED

is the desired gamma corrected output.

xINPUT

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:

)

240

(

240

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⎟

⎠

⎞

⎜

⎝

⎛

−

where:

γn

is the value to be written into the gamma correction register

for point n on the gamma correction curve.

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

is the gamma correction factor.

For example, setting γ = 0.5 for all programmable curve data

points results in the following yn values:

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

where the sum of each equation is rounded to the nearest integer.

The gamma curves in Figure 70 and Figure 71 are examples only;

any user-defined curve in the range from 16 to 240 is acceptable.

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 70. Signal Input (Ramp) and Signal Output for Gamma 0.5

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

SIG

L I

Figure 71. Signal Input (Ramp) and Selectable Output Curves

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