TELEPRODUCTION TEST
VOLUME 1 NUMBER 12
DIFFERENTIAL PHASE AND GAIN REVISITED
The basics of differential gain and phase
measurements were given in Volume 1 Number 9
(Video Test Signals, Part 3). But it’s time to take a
closer look and to try to clear up the differences
between the five types of modulated staircase
signals used in the measurement. the subject is of
particular important now because new FCC rules
covering cable operation will require the
measurement of “video” type distortions that
include differential phase and gain as well as
relative chroma delay.
The Basics
First, a quick review. The distortion known as
differential gain (DG) takes the form of an error in
the amplitude of the 3.58 MHz chroma signal as a
function of the value of the luminance level on
which it rides. The standard test signal is the
modulated stairstep or ramp. Figure 12-2 shows a
5-step modulated staircase. It consists of five
equal steps in luminance (Y) from blanking to
peak white. The Y steps are 20 IRE units each
from zero to 100 IRE. Added to each step is a
burst phase, -(B-Y), signal of 40 IRE units peakto-peak (same amplitude as burst). If there is no
DG distortion this signal will survive with all the
Figure 12-1. Video 5-step modulated staircase. Subcarrier
is at burst phase.
subcarrier packets at 20 IRE (p-p). This can be
seen on a waveform monitor when the CHROMA
filter is switched on. This removes the luminance
framework on which the chroma signal rides and
the signal appears as shown in Figure 12-2. Gain
has been reset so that the larger chroma packets,
the first three to the right of the burst, “fit” between
0 and 100 IRE. Centering is then reset to measure
the peak-to-peak amplitude of the smallest packet
(just before blanking). This turns out to be 94
IRE, yielding a DG reading of -6%. Situations
involving a lack of “headroom” that results in
compression of the top step cause a drop in color
saturation; the step to the far right on a picture
monitor shows a pale yellow-green as compared
with the rest.
Differential Phase (DP)
This form of distortion deals with an error in
subcarrier phase depending upon the Y level on
which the subcarrier rides. It shows up as a hue
shift. For example, a shift in phase of the signal on
the top step would shift the color from yellowgreen towards orange or more green. To measure
Figure 12-2. An example of DG distortion. Chroma filter is
on and gain has been reset so large subcarrier packets
span 100 IRE.
Figure 12-3. DG and DP on the vectorscope. Gain and
phase reset to put the burst vector on the circle. (Large,
fuzzy vector dots are caused by noise).
Figure 12-5. VITS signal for ANSI T1.502, 1988 (NTS-7)
with modulated staircase that reaches 110 IRE on chroma
peaks.
DP, a vectorscope is needed. Here DG shows up
as a change in vector amplitude (radius) and DP
shows up as a shift in vector phase, clockwise or
counter clockwise from the burst axis.
places all but one of the dots within that part of
the graticule used for DG/DP measurements.
Four of the dots fit within this target yielding a
spread in radius (gain) of more than 10% (2 major
divisions).
That part of the vectorscope at 9 o’clock is used
to gauge DG and DP. See Figure 12-3. Gain is
set to put the vector dot on the circle that is
marked for 360° phase. If there is no DG or DP, a
single vector dot will appear. If DG and DP exists,
two or more vector dots may appear. Figure 12-3
shows a particularly serious case of both DG and
DP. It was taken by passing the full modulated
staircase of Figure 12-1 through a VHF modulator
and demodulating the signal with a consumergrade tuner. Vector gain is calibrated by setting
gain and phase to place the burst vector dot on
the -(B-Y) axis at the phase-graduated circle. This
The angular spread for the group of four is about
8°. One dot, however, is outside the DG/DP
target. It represents the top step on the
modulated stairstep (which you could determine
by observing the waveform display). DG here is
greater than 20% and DP is almost 5°. This would
cause notable reduction in the saturation of the
right hand step in the picture but hue is also
shifted towards green.
Modulated Stair, FCC/VITS and NTC-7
The modulated staircase shown in Figure
12-1 is a standard video test signal with the
top luminance step at 100 IRE. But practical
Figure 12-4. VITS signal for Field 1, Line 18 is a half line of
modulated stairstep and a half line of pulse-bar. Note the
top staircase step. It does not exceed 100 IRE.
Figure 12-6. 10% APL: one line of modulated staircase (10
step in this case) and four lines of black.
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Figure 12-7. 90% APL: One line of modulated staircase and
four lines of 100% white.
Figure 12-8. Menu for setting the 408 for 80% luminance.
d i fficulties arise with this signal in situations
where “headroom” is necessarily limited. For
example, in broadcast TV video signals are
limited to 100 IRE units which corresponds to
12.5% modulation at the transmitter. The 12.5%
modulation is held as a minimum to prevent
“intercarrier buzz” in TV sets. (You may have
heard the buzz when someone wearing a white
shirt appears on camera.) For this reason the
modulated staircase signal used where an
amplitude modulator is in use (TV or cable) holds
the total signal within 100 IRE. Figure 12-4 shows
one of the VITS (Vertical Interval Test Signals)
identified as FCC Composite Radiated Signal
(radiated meaning it went through an amplitude
modulator) for VITS Field 1, Line 18. Note that
the signal consists of half a line of modulated
staircase and half a line of pulse bar (two test
signals on one line). But look carefully. The top
luminance step is 80 IRE and 20 IRE chroma
peak just reaches up to 100 IRE. So this test
signal is the logical choice where amplitude
modulation takes place. If you push the standard
video staircase of Figure 12-1 and amplitude
modulate it, the far right bar will look almost white
(no saturation) on a TV set.
subcarrier reaches up to 110 IRE. This signal is
intended to test network transmission links
(satellite and such) which have a greater
headroom.
10% and 90% APL
So far we’ve identified 3 types of modulated
staircase; video (full field), FCC composite
radiated and NTC-7. The next two refer to video
modulated staircase signals in which the average
picture level (APL) is at two different levels:10%
predominantly dark) and 90% predominantly
bright. This checks for variations in DG and DP
as the dc level of the signal varies. To make a
10% APL, the signal is formed by alternating one
line of staircase (which has a 50% APL) and four
lines of black (zero %). Averaging these we get
50% divided by 5 or 10% APL. Figure 12-6 shows
a 10% APL modulated stairstep produced as one
Another VITS test signal was developed by the
Network Transmission Committee (NTC) in
Report #7. This signal is now called out in
standard EIA/TIA-250C and ANSI T 1.502, 1988.
It is very similar to the FCC composite radiated
signal except that the staircase and pulse-bar
signals are reversed in time sequence. See
Figure 12-5. Note also that the top Y step in the
modulated staircase is at 90 IRE and the peak
Figure 12-9. Modulated staircase similar to FCC
“Composite Radiated” with subcarrier peaks reaching
100 IRE.
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Figure 12-10. 408 with luminance set to 90% to obtain an
NTC-7 type of modulated staircase.
Figure 12-11. Vector DG/DP using the same setup as
Figure 12-3 except that luminance has been reduced to
80% (FCC radiated type).
of the test signals from Leader’s Model 408
generator. For a 90% APL, the signal is changed
to one line of modulated staircase and four lines
of 100 IRE (450 divided by 5 or 90% APL). The
408 produces this signal as well. See Figure 12-7.
For rigorous testing, DG/DP should be measured
at 10%, 50% and 90% APL.
To set up the FCC radiated type, use the program
or manual mode to set the Y level to 80%. See
Figures 12-8 and 12-9. Note that the chroma
peaks reach up to 100 IRE. For NTC-7 type
modulated stair, dial in the 90% value for
luminance. See Figure 12-10. Note that the
subcarrier peaks reach up to 110 IRE. Figure 1211 shows the situation that produced the DP
display of Figure 12-3 but this time the luminance
value of the staircase has been reduced to 80
IRE (like FCC composite radiated). The situation
is still bad but note that the single vector outside
the DG/DP target has joined the rest of the group
yielding an overall DG of about 8% and DP
somewhat greater than 6°.
Using the 408 to Get Standard,
FCC and NTC-7 Test signals
The 408 produces a full field modulated staircase
at 50%, 10% and 90%. And due to its ability to
program luminance and chrominance levels, it
can also be made to produce full field versions of
the FCC and NTSC-7 modulated staircase
signals.
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