AD8051/AD8052/AD8054
Sync Stripper
goes high with a duty cycle that is a small fraction of a percent.
The opposite condition defines the other extreme.
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.
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.
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.
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.
VIDEO WITHOUT SYNC
VIDEO WITH SYNC
V
GROUND
+0.4V
BLANK
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.
GROUND
+3V OR +5V
+
0.1F
10F
100⍀
V
IN
TO A/D
AD8051
R2
1k⍀
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.
R1
1k⍀
+0.8V
)
BLANK
Figure 47. Sync Stripper
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 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 kΩ 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 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.
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 × VBLANK must be applied to make the blanking
level come out at ground.
+5V
4.99k⍀
Single Supply Composite Video Line Driver
+
10F
4.99k⍀
+
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.
0.1F
10F
COMPOSITE
VIDEO
47F
+
R
BT
IN
1000F
75⍀
R
+
T
V
OUT
10F
AD8051
75⍀
R
L
75⍀
0.1F
R
F
1k⍀
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
R
1k⍀
G
220F
Figure 48. Single Supply Composite Video Line Driver
REV. B
–15–
ADI [ ADI ]
