adv7152 Analog Devices, Inc., adv7152 Datasheet - Page 26

• Manufacturer Part Number: adv7152
• Description: Cmos 220 Mhz True-color Graphics Triple 10-bit Video Ram-dac
• Manufacturer: Analog Devices, Inc.

ADV7152
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 (I
by:
where ranges from 2.0 to 2.8.
If the individual values of for red, green and blue are known,
then so called “Gamma Correction” can be applied to each of
the three video input signals (I
therefore:
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.
CRT
) produced on a CRT by an input value I
I
IN(corrected)
= k(I
I
CRT
IN
IN
= (I
)
);
1/
IN
)
10-BIT DACS AND GAMMA CORRECTION
(k = 1, normally)
IN
is given
APPENDIX 3
–26–
8-Bit Data
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
Gamma Correction Curve (Gamma Value = 2.7)
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 8 Bits vs. 10 Bits
Gamma
Corrected
(2.7)
0.977797
0.979304
0.980807
0.982306
0.983801
0.985292
0.986780
0.988264
0.989744
0.991220
0.992693
0.994161
0.995626
0.997088
0.998546
1.000000
32
64
INPUT CODE – Decimal
96
Quantized to
8 Bits
250
250
251
251
25 1
252
252
252
253
253
254
254
254
255
255
255
128
160
192
Quantized to
10 Bits
1001
1002
1004
1005
1007
1008
1010
1011
1013
1015
1016
1018
1019
1021
1022
1023
224
REV. B
256