Datasheet

AD8145
Rev. A | Page 20 of 24
SYNC PULSE EXTRACTION USING COMPARATORS
The AD8134 encoding equations are given in Equation 4,
Equation 5, and Equation 6.
The AD8145 is particularly useful in KVM applications. KVM
networks transmit and receive computer video signals that typically
comprise red, green, and blue (RGB) video signals and separate
horizontal and vertical sync signals. Because the sync signals are
separate and not embedded in the color signals, it is advantageous
to transmit them using a simple scheme that encodes them among
the three common-mode voltages of the RGB signals. The AD8134
triple differential driver is a natural complement to the AD8145
and performs the sync pulse encoding with the necessary
circuitry on-chip.
[
]
HV
K
VRed
CM
−=
2
(4)
[
]
V2
2
−=
K
VGreen
CM
(5)
[
]
HV
K
VBlue
CM
+=
2
(6)
where:
Red V
CM
, Green V
CM
, and Blue V
CM
are the transmitted common-
mode voltages of the respective color signals.
K is an adjustable gain constant that is set by the AD8134.
V and H are the vertical and horizontal sync pulses, defined
with a weight of −1 when the pulses are in their low states and a
weight of +1 when they are in their high states.
The AD8134 data sheet contains further details regarding the
encoding scheme. Figure 44 illustrates how the AD8145 comparators
can be used to extract the horizontal and vertical sync pulses that
are encoded on the RGB common-mode voltages by the AD8134.
RECEIVED
RED VIDEO
HSYNC
RED CMV
GREEN CMV
BLUE CMV
50Ω
50Ω
1kΩ
1kΩ
VSYNC
RECEIVED
GREEN VIDEO
50Ω
50Ω
RECEIVED
BLUE VIDEO
50Ω
50Ω
R
S
R
S
475Ω
47pF
47pF
06307-045
Figure 44. Extracting Sync Signals from Received Common-Mode Signal