TELEPRODUCTION TEST
VOLUME 1 NUMBER 7
VIDEO TEST SIGNALS
PART 1
For the next few issues, we’ll be taking a look at
come across consisting of standard sync,
the test and reference signals that are provided by
blanking, color burst and setup. See Figure 7-1. It
video generators for the purpose of adjusting and
provides, on a single cable, all that’s needed to
evaluating video components and systems. The
synchronize cameras, TBCs, SEGs, character
purpose will be to explain the uses of commonly
used test signals and illustrate the effects of
distortion on both the waveform and the picture.
generators and other components that are
capable of genlock operation.
Sources for the signals to be shown are Leader
Leader generators like the 410C, 408 and 411
generators such as Models 408 and 411.
normally provide a separate feed for black burst
Black Burst
from a BNC jack on the rear panel. Black burst is
While not a test signal in the usual sense, black
also available at the front panel jack of the 408 by
burst has become a major workhorse as a timing
selecting the RASTER pattern and switching off
reference. It’s the most simple video signal you’ll
the RED, GREEN and BLUE primaries.
Figure 7-1. Normal black burst.
Figure 7-2. How a asymmetrical signal resolves itself
around zero.
Figure 7-3. Effects of 100% white raster with DC
RESTORER switched off.
Figure 7-4. Window signal on underscanned monitor.
Brightness raised to show setup and blanking at raster
edges.
Black burst is also useful in gauging preservation
of the DC component of the video signal (or the
clamp circuits that are designed to restore the DC
component). Loss of the DC component removes
that part of the signal that changes very slowly as
when the camera is panned from bright to dark
scenes. Complete loss of the DC component
causes the signal to resolve or rearrange itself
around zero such that the total signal area above
zero equals the total signal area below zero.
Figure 7-2 illustrates this point in which a series
coupling capacitor blocks the DC component for a
simple signal that is similar in shape to the video
for a blank gray raster.
the video coupling circuits.
It was the tendency of TV sets in the 1950s to
crush blacks on bright scenes that caused
broadcasters to allow the use of setup to raise the
black floor of the signal.
The Window Signal
Also a very simple signal, the window is a 100%
white rectangle on a black background. See
Figure 7-4. Its width and height are half the active
width and height of the raster respectively. Thus,
in the absence of setup, it has an average picture
level (APL) of 50%.
The window is convenient for setting system
levels due to its simplicity and single IRE level. It
is also useful for setting FM deviation in VCRs
and is effective in calibrating video level meters
because of the 100 IRE peak and 50% APL
levels.
You can see this effect on the waveform monitor if
you switch off the DC RESTORER and set the
vertical position control to put blanking of the
black burst signal to zero IRE where it should be.
Refer back to Figure 7-1. Switching to a 100%
white raster results in the situation shown in
Figure 7-3. the signal has now rearranged itself
so that equal signal areas are above and below
the bias level set by the position control. Note that
sync and blanking have dropped off screen.
In terms of the TV picture, loss of the DC
component causes the dark part of the picture to
sink below visual extinction if a large part of the
picture is white (like a single skier on snow).
Similarly, a predominantly dark scene causes the
black level to rise and black becomes gray. To
gauge the effectiveness of DC preservation in a
TV set or monitor, use the window or color bar
signal and set brightness so that the black border
of the window signal or the black bar in the lower
right corner of the split field color bar signal is just
extinguished visually. Then switch to black burst.
The screen should go black. If it does not, all or
part of the DC component has been sacrificed in
Figure 7-5. Field rate tilt caused by a series capacitor in
the video feed.
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Figure 7-6. Monitor picture of Figure 7-5, DC restorer off.
Figure 7-7. H line rate tilt. Note effects at setup level.
A common use of the window is in evaluating
transient performance, particularly the effects of
low frequency distortion. Phase shift of low
frequency signals shows up as a tilt or sag in the
top or flat part of the waveform. Figure 7-5 shows
the effect of field rate tilt caused by phase shift at
frequencies below 60 Hz. The waveform monitor
is set for 2V display and the DC RESTO R E R
must be switched off to see this form of distortion.
If left on, the clamp will actually fix the waveform
by reclamping the signal on a line-by-line basis.
The effect in the picture on monitors without DC
restorers is a vertical shading from top to bottom.
See Figure 7-6. the top of the window is
somewhat brighter than the bottom, a condition
more easily seen by reducing monitor brightness.
This form of tilt can also be spotted in the 2H
display as a thickening of the line that represents
the white peak. This is because there are a
number of horizontal lines, each with a gradually
diminishing peak value due to the field rate tilt.
IRE units. It is expressed as a percentage. The
top of the waveform in Figure 7-7 droops about 8
IRE units for a tilt of 8%.
While low frequency distortion affects the flat or
“run” parts of the waveform, high frequency
distortion affects the signal transitions. A loss of
high frequency response slows the rise and fall
times as shown in Figure 7-9. the result is lack of
snap and a smear of leading and trailing edges.
Excess frequency response boosts high
frequency harmonics and results in overshoot at
the transitions. See Figure 7-10. The effect can
be an apparent improvement in picture resolution.
In fact, carefully controlled overshoot is the basis
for e n h a n c e m e n t or contour correction i n
cameras. But the effect can also cause picture
degradation depending on the period of the
overshoot or its amplitude. In systems where
Another form of tilt occurs when there is phase
shift at or near the horizontal line rate, 15.75 kHz.
Figure 7-7 shows a severe case of line rate tilt.
Note that the tilt continues in the opposite
direction in the black part of the waveform and
continues into the following line. The effect in the
picture is sharp horizontal streaks that extend to
either side of high contrast parts of the picture.
See Figure 7-8. Streaking is seen quite often in
broadcast picture and stands out dramatically in
high contrast images such as titles.
Tilt is evaluated using the waveform monitor by
setting the peak part of the waveform to 100 IRE
and measuring the sag over the window area in
Figure 7-8. Line rate tilt causes streaking that lasts from
one line into the next.
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Figure 7-9. Loss of high frequencies.
Figure 7-10. Overshoot and undershoot.
feedback is in use, a boost in high frequencies
can cause ringing at the transitions — several
damped-out cycles as shown in Figure 7-11. This
shows up as closely spaced “ghosts” in the
picture or a dirty looking blur in patterned parts of
the picture where picture element spacing is
close to the period of the ringing signal.
While the window is useful for evaluating low and
high frequency transient problems, other signals
such as the sin squared signal are better suited
for high frequency analysis. The sin squared
signal often accompanies the window signal or
bar signal.
The next issue will cover the use of the most
familiar test signal — color bars. We’ll look at full
field, EIA and SMPTE bars to see how they differ
and what their particular uses are.
Figure 7-11. Ringing.
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