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The G input also arrives via 75Ω coax and adds its
contribution to the Y signal via a 549Ω resistor R9, which
is tied to the inverting input of amplifier A2. There is also
an 86.6Ω termination resistor R12, which yields a 75Ω
termination when considered in parallel with R9. Using
superposition, it is straightforward to determine the
output of amplifier A2. Although inverted, it sums the R,
G and B signals in the standard proportions of 0.3R,
0.59G and 0.11B that are used to create the Y signal.
Amplifier B1 then inverts and amplifies the signal by 2,
resulting in the Y output.
Buffered Color-Difference to RGB Matrix
The LT1399 can be used to create buffered RGB outputs
from color-difference signals (Figure 9). The R output is
a back-terminated 75Ω signal created using resistor R5
and LT1399 amplifier A1 configured for a gain of +2 via
324Ω resistors R3 and R4. The noninverting input of
amplifier A1 is connected via 1k resistors R1 and R2 to
the Y and R-Y inputs respectively, resulting in cancella-
tion of the Y signal at the amplifier input. The remaining
R signal is then amplified by A1.
The B output is also a back-terminated 75Ω signal
created using resistor R16 and amplifier A3 configured
for a gain of +2 via 324Ω resistors R14 and R15. The
noninverting input of amplifier A3 is connected via 1k
resistors R12 and R13 to the Y and B-Y inputs respec-
tively, resulting in cancellation of the Y signal at the
amplifier input. The remaining B signal is then amplified
by A3.
The G output is the most complicated of the three. It is a
weighted sum of the Y, R-Y and B-Y inputs. The Y input
is attenuated via resistors R6 and R7 such that amplifier
A2’s noninverting input sees 0.83Y. Using superposition,
we can calculate the positive gain of A2 by assuming that
R8 and R9 are grounded. This results in a gain of 2.41 and
a contribution at the output of A2 of 2Y. The R-Y input is
amplified by A2 with the gain set by resistors R8 and R10,
giving an amplification of –1.02. This results in a contri-
bution at the output of A2 of 1.02Y – 1.02R. The B-Y input
is amplified by A2 with the gain set by resistors R9 and
R10, giving an amplification of –0.37. This results in a
contribution at the output of A2 of 0.37Y – 0.37B.
If we now sum the three contributions at the output of A2,
we get:
A2
OUT
= 3.40Y – 1.02R – 0.37B
It is important to remember though that Y is a weighted
sum of R, G and B such that:
Y = 0.3R + 0.59G + 0.11B
If we substitute for Y at the output of A2 we then get:
A2
OUT
= (1.02R – 1.02R) + 2G + (0.37B – 0.37B)
= 2G
The back-termination resistor R11 then halves the output
of A2 resulting in the G output.
–
+
A2
1/3 LT1399
R7
1k
B-Y
R-Y
Y
R10
324Ω
R11
75Ω
R6
205Ω
R2
1k
R1
1k
R8
316Ω
R9
845Ω
–
+
A3
1/3 LT1399
R14
324Ω
B
G
R16
75Ω
R12
1k
R13
1k
R15
324Ω
ALL RESISTORS 1%
V
S
= ±5V
–
+
A1
1/3 LT1399
R3
324Ω
R
R5
75Ω
R4
324Ω
1398/99 F09
Figure 9. Buffered Color-Difference to RGB Matrix
