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.

[

]

H

V

K

V

Red

CM

−

=

2

(4)

[

]

V

2

2

−

= K

V

Green

CM

(5)

[

]

H

V

K

V

Blue

CM

+

=

2

(6)

where:

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

50

Ω

50

Ω

1k

Ω

1k

Ω

VSYNC

RECEIVED

GREEN VIDEO

50

Ω

50

Ω

RECEIVED

BLUE VIDEO

50

Ω

50

Ω

475

Ω

47pF

47pF

Figure 44. Extracting Sync Signals from Received Common-Mode Signal