ADV7152LS85 datasheet(26/32 Pages) AD

26 / 32 page

ADV7152

–26–

REV. B

APPENDIX 3

10-BIT DACS AND GAMMA CORRECTION

10-Bit DACs

10-bit RAM-DAC resolution allows for nonlinear video correc-

tion, in particular Gamma Correction. The ADV7152 allows for

an increase in color resolution from 24-bit to 30-bit effective

color without the necessity of a 30-bit deep frame buffer. In

true-color mode, for example, the part effectively operates as a

24-bit to 30-bit color look-up table.

Up to now we have assumed that there exists a linear relation-

ship between the actual RGB values input to a monitor and the

intensity produced on the screen. This, however, is not the case.

Half scale digital input (1000 0000) might correspond to only

20% output intensity on the CRT (Cathode Ray Tube). The in-

tensity (ICRT) produced on a CRT by an input value IIN is given

by:

ICRT = (IIN)χ

where

χ ranges from 2.0 to 2.8.

If the individual values of

χ for red, green and blue are known,

then so called “Gamma Correction” can be applied to each of

the three video input signals (IIN);

therefore:

IIN(corrected) = k(IIN)

(k = 1, normally)

Traditionally, there has been a tradeoff between implementing a

nonlinear graphics function, such as gamma correction, and

color dynamic range. The ADV7152 overcomes this by increas-

ing the individual color resolution of each of the red, green and

blue primary colors from 8 bits per color channel to 10 bits per

channel (24 bits to 30 bits).

The table highlights the loss of resolution when 8-bit data is

gamma-corrected to a value of 2.7 and quantized in a tradi-

tional 8-bit system. Note that there is no change in the 8-bit

quantized data for linear changes in the input data over much of

the transfer function. On the other hand, when quantized to 10

bits via the 10-bit RAMs and 10-bit DACs of the ADV7152, all

changes on the input 8-bit data are reflected in corresponding

changes in the 10-bit data.

The graph shows a typical gamma curve corresponding to a

gamma value of 2.7. This is programmed to the red, green and

blue RAMs of the color lookup table instead of the more tradi-

tional linear function. Different curves corresponding to any

particular gamma value can be independently programmed to

each of the red, green and blue RAMs.

Other applications of the 10-bit RAM-DAC include closed-loop

monitor color calibration.

Gamma Correction 8 Bits vs. 10 Bits

Gamma

Corrected

Quantized to

8-Bit Data

(2.7)

8 Bits

10 Bits

240

0.977797

250

1001

241

0.979304

250

1002

242

0.980807

251

1004

243

0.982306

251

1005

244

0.983801

25 1

1007

245

0.985292

252

1008

246

0.986780

252

1010

247

0.988264

252

1011

248

0.989744

253

1013

249

0.991220

253

1015

250

0.992693

254

1016

251

0.994161

254

1018

252

0.995626

254

1019

253

0.997088

255

1021

254

0.998546

255

1022

255

1.000000

255

1023

1.00

0.00

256

0.30

0.10

0.20

0.60

0.40

0.50

0.70

0.80

0.90

224

192

160

128

INPUT CODE – Decimal

GAMMA

CORRECTION

CURVE

LINEAR

RESPONSE

PRECEIVED

THE

EYE

CRT

RESPONSE

Gamma Correction Curve (Gamma Value = 2.7)

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