–15–

AD8051/AD8052/AD8054

REV. B

Sync Stripper

Synchronizing pulses are sometimes carried on video signals so

as not to require a separate channel to carry the synchronizing

information. However, for some functions, like A/D conversion,

it is not desirable to have the sync pulses on the video signal.

These pulses will reduce the dynamic range of the video signal

and do not provide any useful information for such a function.

A sync stripper will remove the synchronizing pulses from a

video signal while passing all the useful video information. Fig-

ure 47 shows a practical single supply circuit that uses only a

single AD8051. It is capable of directly driving a reverse termi-

nated video line.

AD8051

0.1 F

10 F

+

R1

1k

R2

1k

100

TO A/D

+0.8V

(OR 2

+3V OR +5V

GROUND

+0.4V

VIDEO WITH SYNC

GROUND

VIDEO WITHOUT SYNC

Figure 47. Sync Stripper

The video signal plus sync is applied to the noninverting input

with the proper termination. The amplifier gain is set equal to

two via the two 1 k

Ω resistors in the feedback circuit. A bias

voltage must be applied to R1 in order that the input signal has

the sync pulses stripped at the proper level.

The blanking level of the input video pulse is the desired place

to remove the sync information. This level is multiplied by two

by the amplifier. This level must be at ground at the output in

order for the sync stripping action to take place. Since the gain

of the amplifier from the input of R1 to the output is –1, a volt-

age equal to 2

× V

level come out at ground.

Single Supply Composite Video Line Driver

Many composite video signals have their blanking level at

ground and have video information that is both positive and

negative. Such signals require dual supply amplifiers to pass

them. However, by ac level shifting a single supply amplifier can

be used to pass these signals. The following complications may

arise from such techniques.

Signals of bounded peak-to-peak amplitude that vary in duty

cycle require larger dynamic swing capacity than their (bounded)

peak to peak amplitude after they are ac coupled. As a worst

case, the dynamic signal swing will approach twice the peak-

to-peak value. The two conditions that define the maximum

dynamic wing requirements are a signal that is mostly low, but

goes high with a duty cycle that is a small fraction of a percent.

The opposite condition defines the other extreme.

The worst case of composite video is not quite this demanding.

One bounding condition is a signal that is mostly black for an

entire frame, but has a white (full amplitude) minimum width

spike at least once in a frame.

The other extreme is for a full white video signal. The blanking

intervals and sync tips of such a signal will have negative-going

excursions is compliance with the composite video specifica-

tions. The combination of horizontal and vertical blanking inter-

vals limit such a signal to being at the highest (white) level for a

maximum of about 75% of the time.

As a result of the duty cycles between the two extremes pre-

sented above, a 1 V p-p composite video signal that is multiplied

by a gain of two requires about 3.2 V p-p of dynamic voltage

swing at the output for an op amp to pass a composite video

signal of arbitrary varying duty cycle without distortion.

Some circuits use a sync tip clamp to hold the sync tips at a

relatively constant level in order to lower the amount of dynamic

signal swing required. However, these circuits can have artifacts

like sync tip compression unless they are driven by a source with

a very low output impedance. The AD8051/AD8052/AD8054

have adequate signal swing when running on a single +5 V

supply to handle an ac coupled composite video signal.

The input to the circuit in Figure 48 is a standard composite

(1 V p-p) video signal that has the blanking level at ground. The

input network level shifts the video signal by means of ac cou-

pling. The noninverting input of the op amp is biased to half of

the supply voltage.

The feedback circuit provides unity gain for the dc biasing of the

input, and provides a gain of two for any signals that are in the

video bandwidth. The output is ac coupled and terminated to

drive the line.

The capacitor values were selected for providing minimum “tilt”

or field time distortion of the video signal. These values would

be required for video that is considered to be studio or broad-

cast quality. However, if a lower consumer grade of video,

sometimes referred to as “consumer video” is all that is desired,

the values and the cost of the capacitors can be reduced by as

much as a factor of five with minimum visible degradation in the

picture.

AD8051

1k

1k

+5V

IN

+

10 F

4.99k

220 F

+

1000 F

0.1 F

75

10 F

+

47 F

4.99k

75

75

COMPOSITE

VIDEO

0.1 F

10 F

+

Figure 48. Single Supply Composite Video Line Driver