TELEPRODUCTION TEST
VOLUME 1 NUMBER 11
VIDEO FREQUENCY RESPONSE
This issue returns to the use of video test signals
to show the test signals used to check video
frequency response in equipment and
transmission links.
Frequency response is gauged by plotting the
output voltage amplitude of a device or system
under test versus applied frequency. For example,
a device designed to handle the NTSC signal
should have uniform output over the range of zero
Hz (dc) to 4.2 MHz. A device with uniform output,
of let’s say 1V p-p, over this range is said to be
flat because a graph plotted of output voltage
versus frequency would appear as a straight line
as shown in Figure 11-1. From a technical point of
view, response is considered flat until the output
falls to the half-power value (3 dB down in
voltage) which means that output voltage falls to
0.707 of that measured at some reference
frequency (often mid-range). Thus Figure 11-1
shows a “flat” amplifier to 4.2 MHz because the
measured output falls to 70.7% at 4.2 MHz.
Available test signals use one of three methods to
gauge response. A point-by-point plot uses a CW
(continuous wave) test signal whose frequency
can be varied manually. The method is somewhat
laborious and is seldom used. The multiburst
signal plants six bursts of signals on each (or
selected) raster line for a sort of “spot check”
evaluation of response at selected key
frequencies. And the sweep technique varies
signal frequency over the designated range at the
field rate to actually draw the frequency response
on a scope or waveform monitor display.
Multiburst
Multiburst has long been used for a quick check
of facilities after a major cabling change has been
made. It is also extremely useful as an “inservice” test because the signal can be inserted
into a designated line in the vertical blanking
interval. This action permits tests to be conducted
without interfering with program material.
Multiburst is part of the FCC approved VITS
(Vertical Internal Test Signal) and is placed on line
17 of Field 1. These signals can be seen in
network broadcast signals. Figure 11-2 shows the
multiburst waveform from the Leader Model 411
using a waveform monitor set for 1H sweep. It
contains a calibration squarewave to the left of
the first burst to act as a calibrating reference.
The bursts are at 0.5, 1.0, 2.0, 3.0, 3.58 (subcarrier)
Figure 11-1. Frequency response is a graph of signal
voltage versus frequency.
Figure 11-2. Multiburst waveform.
Figure 11-3. Multiburst waveform, 2H display with FLAT
filter (left) and IRE filter (right).
Figure 11-4. Video sweep signal at the 1V sweep rate.
Note the presence of V sync and blanking.
and 4.2 MHz. Amplitude is 100 IRE (p-p) in this
case but 50% amplitude is preferred where
amplitude limiting in the device may distort the
waveform (and give an erroneous picture of flat
response).
Some
generators
provide
continuously variable amplitude. A practical
example of the use of multiburst is shown in
Figure 11-3 which shows the WFM with the IRE
filter activated. Note that response is down to 0
IRE at 3.58 MHz. Multiburst frequencies vary in
some cases. For example, the top burst in the
Model 411 is 4.2 MHz and in the Model 430 is 7
MHz. In Model 408, the last burst in the FIXed
mode is 4.2 MHz and in the VARiable mode can
be adjusted from 0.1 to 15 MHz with the
frequency indicated on the LCD screen.
Video Sweep
The sweep signal sweeps through the frequency
range of interest at a low repetition rate; for video
sweeps, the rate is synchronized to the vertical
field rate. Thus the sweep starts out at the lowest
frequency at the top of the picture and
progresses through the range to end at the
highest frequency at the bottom of the picture.
Although the field rate sweep gives a detailed
picture of frequency response, it must be
employed when a system is out of service. To
view the sweep signal, the scope or WFM should
be set up for 1V or 2V display. Figure 11-6 shows
a composite sweep signal from the 408 which
includes normal sync, blanking and burst using
the WFM set for a 1V sweep. Because you are
looking at an envelope of sinewaves whose
frequency varies continuously, the response
curve is represented by the outline of the top (or
bottom) edge of the sweep waveform. The sweep
frequency range starts below 0.1 MHz and
sweeps up to 5 MHz.
Frequency Markers
The response shown in Figure 11-5 is almost
meaningless unless you can identify frequencies
at various key points on the curve. The curve is
“marked” by a heterodyne process whereby fixedfrequency internal signals are compared with the
Figure 11-5. Flat frequency response with “drop-out”
markers.
Figure 11-6. 15 MHz sweep from Model 408 shows WFM
response.
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Figure 11-7. VCR playback shows substantially flat
response to 2 MHz with separate chroma response centered
at 3.58 MHz.
Figure 11-10. Line rate sweep with markers at 0.5, 1, 2, 3,
3.58 and 4.5 MHz.
sweep frequency and the detection of a zero beat
at coincidence creates a pulse that effectively
chops a hole in the sweep output at the marker
frequency. These are called “drop-out” markers
because they appear on the waveform in the
same way that drop-outs appear in VCRs due to
missing oxide on the tape surface or dust that lifts
the tape off the head surface. The response is
marked in Figure 11-7 with fixed “drop-outs” at
0.5, 1.0, 2.0, 3.0, 3.58, 3.8 and 4.2 MHz. Model
408 also produces a wide band, 0.3-15 MHz
sweep. Figure 11-6 shows this sweep. This
sweep is marked at 2, 4, 6, 8, 10, 12 and 14
MHz. Note that response is substantially flat to 8
MHz and begins to trail off above that. This
represents the response of the waveform monitor
itself. Use of composite sweep which includes
normal sync, blanking, setup and burst permits
video output components such as VTRs and VCRs
Figure 11-8. An example of START-STOP sweep at to sweep
2 to 6 Hz to cover the CHROMA response of the WFM.
Figure 11-9. Non-composite sweep. Note the absence of
sync and blanking.
Figure 11 - 11. Appearance of line-rate sweep on the
picture monitor shows how markers are inserted.
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to be swept for frequency response. The VCR
needs sync to lock the scanner servo and some
VCRs automatically alter video bandwidth to
accommodate monochrome (no burst) and color
(burst present) operation. Figure 11-7 shows a
video sweep that has been recorded and then
played back on a VHS VCR in the color mode
(burst present). Note that there are really two
response curves, one for luminance (Y) signal
and another, centered at 3.58 MHz for the
chroma signal that is processed separately in the
VCR.
and appear superimposed on the waveform.
Figure 11-10 shows the line rate sweep signal
produced by Model 435B. It is viewed on the
WFM on the 1H sweep. The markers are at 0.5,
1, 2, 3, 3.58 and 4.5 MHz. Figure 11-11 shows
how the line rate sweep appears on the screen of
the picture monitor. You can see the raster lines
that carry the markers. Theoretically, the line rate
sweep cannot extend as deeply into the lower
frequencies. However, the low frequency limits
are very nearly the same.
The component/composite generator, Model
425A offers line rate sweeps for the components
Y, B-Y, R-Y, GBR as well as the NTSC composite
signal. For Y, B-Y, R-Y the sweep range is 0.2 to
5.6 MHz for Y (CH1) and 0.1 to 2.8 MHz for B-Y
and R-Y (CH2 and 3). Figure 11-12 shows a
parade display of Y, B-Y, R-Y (from left to right)
using channels 1 to 3 of the Model 5222
Component/Composite Waveform Monitor. This
allows both the wide band Y channel and the two
narrower band chroma channels to be gauged
separately.
Some sweep generators offer STA RT- S TO P
operation in which the low and high end CHROM
filter of the WFM has been selected and the start
and stop controls set to approximately 2 and 6
MHz respectively to sweep the restricted range of
the chroma bandpass filter. The markers are at 2,
3, 3.58, 3.8, 4.2 and 6 MHz. Leader Model 430
from which Figure 11-8 was made features
START-STOP sweep.
Composite Non-Composite
In some cases, the presence of standard sync,
blanking, setup and burst interferes with
response checks. This is so in circuits that do not
normally handle these signals. Examples are in
the preamps of cameras and the video head
preamps in VCRs. Here a non-composite sweep
is needed as shown in Figure 11-9. To view this
waveform, it is necessary to trigger the scope or
WFM externally and the sweep generator always
provides a trigger feed for this purpose. Model
430 offers non-composite sweep.
Line-Rate Sweep
Some generators such as Leader’s Model 435B
offer a line-rate sweep. This sweep starts at the
end of H blanking and finishes at the end of the
active line. Markers are added in separate lines
Figure 11-12. Parade display of Y, B-Y, R-Y line sweep.
LEADER VIDEO GENERATORS WITH VIDEO SWEEP OPERATION
MODEL
SWEEP WIDTH (MHz)
MARKERS (MHz)
408 Band 1
Band 2
0.1-5 Full
0.3-15 Full
0.5, 1, 2, 3, 3.68, 4.2
2, 4, 6, 8, 10, 12, 14
430
0.1-10 S.S.
0.1, l, 2, 3, 3.58, 3.8, 4.0, 4.2, 4.5, 5.5, 6,
7.5, 6, 7.5, 8.5, 10
0.5 Full
0.5, 1, 2, 3, 3.58, 4.5
0.2-5.6 Full
0.1-2.8 Full
0.1-2.8 Full
0.2-5.6 Full
0.2-5.6 Full
0.5, 1, 2, 3, 4, 5
0.25, 0.5, 1, 1.5, 2.0, 2.5
0.25. 0.5, 1, 1.5, 2.0, 2.5
0.5, 1, 2, 3, 4, 5
0.5, 1, 2, 3, 4, 5
LINE SWEEP
435B
425A
Y
B-Y
R-Y
GBR
Composite
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