ADV7160 Datasheet, PDF (37/44 Pages) Analog Devices – 96-Bit, 220 MHz True-Color Video RAM-DAC

ADV7160/ADV7162
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 ADV7160/ADV7162
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 op-
erates 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 intensity (I_CRT) produced on a CRT by an input value I_IN is
given by:
I_CRT = (I_IN)^c
where c ranges from 2.0 to 2.8.
If the individual values of c for red, green and blue are known,
then so called “Gamma Correction” can be applied to each of
the three video input signals (I_IN); therefore:
I_{IN(corrected)} = k(I_IN)^1/c
Traditionally, there has been a trade-off between implementing
a nonlinear graphics function, such as gamma correction, and
color dynamic range. The ADV7160/ADV7162 overcomes this
by increasing 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 traditional
8-bit system. Note that there is no change in the 8-bit quan-
tized 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 ADV7160/
ADV7162, 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 look-up 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
8-Bit Data (2.7)
Quantized to
8 Bits
240
0.977797
250
241
0.979304
250
242
0.980807
251
243
0.982306
251
244
0.983801
251
245
0.985292
252
246
0.986780
252
247
0.988264
252
248
0.989744
253
249
0.991220
253
250
0.992693
254
251
0.994161
254
252
0.995626
254
253
0.997088
255
254
0.998546
255
255
1.000000
255
Quantized to
10 Bits
1001
1002
1004
1005
1007
1008
1010
1011
1013
1015
1016
1018
1019
1021
1022
1023
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
GAMMA ^CORRECTION
LINEAR ^RESPONSE
CURVE
RECIEVED
BY
THE
EYE
CRT ^RESPONSE
32
64
96 128 160 192 224 256
INPUT CODE – DECIMAL
Gamma Correction Curve (Gamma Value = 2.7)
REV. 0
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