Note: Descriptions are shown in the official language in which they were submitted.
~2S~
Measurerlent of subcarr~er to h~r~zontal_sync phase using a
~olar dis~lay
This is a division of copending Canadian Patent
Application Serial No. 477,368 which was filed on March
25, 1985.
This invention relates to the measurement of subcarrier
to horizontal sync (SC~H) phase using a polar display.
The background for this invention will be disclosed in
detail hereinbelow.
Summary of the Invention
According to an aspect of the present invention there
is provided an instrument for use in indicating SC/H phase
of a composite video signal, comprising a display device
for providing a polar display, a device for generating from
the reference subcarrier burst of the video signal a first
input signal for the display device such as to cause the
display device to display an indication at a predetermined
angular position of the polar display, phase determining
and indicating means for determining the time difference
between the sync point and that zero crossing of the
extended subcarrier wave which is closest to the sync
point and for generating from said time difference a
second input signal for the display device such as to
cause the display device to display an indication at a
position of the polar display that is angularly spaced
from a predetermined axis of the polar display by an angle
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given by the fraction having as its denominator the period
of the reference subcarrier wave and as its numerator the
product of said time difference and 360 degrees.
The present invention may be used to enable a vector-
scope to provide an indication of SC/H phase. In theconventional vectorscope, the reference subcarrier burst
is indicated by a vector on the 180 degree radius (-x
direction in Cartesian coordinates). In accordance with
the present invention, the time difference between the
sync point and the closest zero crossing of the extended
subcarrier wave is determined and the time difference is
converted to an angular measure within the subcarrier wave
cycle. The vectorscope can then be used to display an
indication in accordance with the angle corresponding to
the time difference, preferably in the form of a dot. In
the event that the datum radius from which the angle is
measured is the 180 degree radius, the dot is aligned with
the subcarrier vector when the subcarrier is in phase with
- horizontal sync. Thus, the present invention enables a
vectorscope to be used to provide an indication of SC/H
phase.
Brief DescriE~on of_ he Drawin~s
The present invention taken in conjunction with the
invention described in copending Canadian Patent
Application Serial No. 477,368 which was filed on March
25, 1985, will be described in detail hereinbelow with the
aid of the accompanying drawings, in which:
FIG. 1 illustrates diagrammatically the waveform of
the NTSC video signal during the horizontal blanking
interval,
FIG. 2 illustrates in block form the major components
of a conventional vectorscope for use in the NTSC system,
FIGq 3 illustrates in block form a vectorscope
embodying the present invention, adapted for use in the
NTSC system,
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FIG. 4 illustrates waveforms useful in understanding
operation of the FIG. 3 vectorscope,
FIG. 5 illustrates the display provided by the FIG. 3
vectorscope,
5FIG. 6 illustrates in block form a second vectorscope
embodying the present invention,
FIG. 7 illustrates the display provided by the FIG. 6
vectorscope,
FIG. 8 (appearing on the same sheet of drawings as
Figure 5) illustrates a waveform useful in understanding
operation of a vectorscope embodying the invention,
adapted for use in the PAL system,
FIG. 9 (appearing on the same sheet of drawings as
Figure 5) illustrates the display provided by the FIG. 3
vectorscope, adapted for use in the PAL system, and
FIGS. 10, 11 and 12 illustrate in block form
additional embodiments of the invention.
Detailed Description
It is well known that the composite color video
signals that are conventionally broadcast, for example
in the NTSC format, contain not only picture information
(luminance and chrominance components) but also timing
information (vertical sync pulses and horizontal sync
pulses) and other reference information (e.g. equalizing
pulses and color burst). As shown in FIG. 1, the
horizontal sync pulse 2 and burst 4 both occur in the
horizontal blanking interval, i.e., the interval between
the active line times of consecutive horizontal scan
lines. The horizontal sync pulse is a negative-going
pulse having an amplitude of 40 IRE units, the 50 percent
point 6 of the leading edge of the sync pulse regarded as
the horizontal sync point. Burst follows the horizontal
sync pulse in the horizontal blanking interval and
comprises a sinusoidal wave. The peak-to-peak amplitude
of the burst is 40 IRE units, and immediately before and
after the burst the signal is at blanking level (zero
IRE). The burst ideally has a sin-squared envelope, and
builds up from, and decays to, blanking level within one
or two cycles of the burst wave. In accordance with EIA
(Electronics Industries Associationl standard RS 170 A,
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the start of burst is defined by the zero-crossing
(positive or negative slope) that precedes the first half
cycle of subcarrier that is 50 percent or greater of the
burst amplitude, i.e., 40 IRE. The reference subcarrier
burst is used in the television receiver to control a
phase-locked oscillator which generates a continuous
wave at subcarrier frequency and is used to extract the
chrominance information from the composite video signal.
Although the NTSC frame is made up of 525 lines which
are scanned in two interlaced fields of 262.5 lines each,
the NTSC color signal requires a four field sequence. In
accordance with the definitions of the fields contained in
standard RS 170 A, the zero crossing of the extrapolated
color burst (the continuous wave at subcarrier frequency
and in phase with burst) must be coincident with the sync
point of the immediately preceding horizontal sync pulse
on even numbered lines, and the pattern of sync and burst
information for fields 1 and 3 is identical except for the
phase of burst. Thus, in field 1, the positive-going zero
crossing of the extrapolated color burst coincides with
the sync point on even numbered lines whereas in field 3
it is the negative-going zero crossing that coincides with
the sync point on even numbered lines. Standards such as
that set forth in RS 170 A are required in order to
facilitate matching between video signals from different
sources and also to facilitate operation of video signal
recording and processing equipment. Accordingly, in order
to identify the different fields of the four field color
sequence, and to adjust the subcarrier to horizontal sync
(SC/H) phase so as to achieve the desired coincidence
between the zero crossing point of the extrapolated color
burst and the sync point, it is necessary to be able to
measure the phase of the subcarrier burst relative to the
sync point.
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Several attempts have previously been made to measuee
SC/H phase. For example, using the Tektronix 1410 signal
generator, it is possible to generate a subcarrier in the
middle of an unused line. Since the leading edges of the
equalizing pulses are midway between sync pulses, a
measurement of subcarrier to horizontal sync phase can be
implied by comparing the subcarrier with the equalizing
pulse timing. Alternatively the 1410 signal generator
can generate a burst phased subcarrier during horizontal
blanking which replaces a sync pulse and which can be
compared with the remaining sync pulses. However, this
equipment is not always available to technicians who need
to make SC/H phase measurements. The GVG 3258 SC/H phase
meter provides a digital output of the phase difference
between subcarrier and horizontal sync, but this again
requires availability of dedicated equipment.
The vectorscope, which provides a polar display of
the amplitude and phase of signal components at subcarrier
frequency, is commonly used by video engineers and
technicians, but the coventional vectorscope cannot
be used to measure SC/H phase.
As used in this description and the appended claims,
the term "vectorscope" means an instrument having an input
terminal, a display surface, means for generating a visible
dot on the display surface, X and Y deflection means for
deflecting the position of the visible dot in mutually
perpendicular rectilinear directions, a subcarrier re-
generator connected to the input terminal for generating a
continuous wave signal at subcarrier frequency from, and
phase-locked to, the subcarrier burst of a video signal,
first and second demodulators having their outputs con-
nected to the X and Y deflection means respectively and
each having first and second inputs, means connecting the
,
~2 S 6~LS~6
output of the subcarrier regenerator to the first inputs
of the first and second demodulators with a quarter-period
relative phase difference, and a filter which passes the
subcarrier burst of the video signal and has an output
terminal for connection to the second inputs of the first
and second demodulators. The vectorscope provides a polar
display of the amplitude and phase of signal components at
subcarrier frequency.
FIG. 2 of the drawings illustrates in block form the
major components of a conventional vectorscope having a
CRT 10. The composite video input signal is applied by
way of an input terminal 12 to both a 3.8 MHz bandpass
filter 14 and a burst locked oscillator 16. The burst
locked oscillator 16 generates a continuous wave signal
lS. at subcarrier frequency (3.58 MHz) phase locked to burst.
The bandpass filter 14 passes components of the compo-
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site video signal that have a frequency of 3.58MHz, i. e., burst and the color components present
during the active line time of the video signal.
The output signal from the filter 14 is applied to
two demodulators 18 and 20, which may simply be
multipliers. The output of the oscillator 16 is
applied through a variable phase shifter 22
directly to the demodulator 18 and to the demodula-
tor 20 through a quarter period lof subcarrier
frequency) delay 24. The output of the demodulator
18 is applied to the %-deflection plates of the CRT
10. The output of the demodulator 20 is applied to
the Y-deflection plates of the CRT. It will thus
be understood that the vectorscope provides a dis-
play in polar coordinates of the amplitude andphase relative to burst of each of the color compo-
nents present in the composite video signal. By
using the phase shifter 22 to rotate the entire
display and align the vector representing burst
with a predetermined axis of the display, usually
the -X axis, a technician can determine whether the
subcarrier components present in a test signal
comply with prescribed standads defined by fixed
graticule markinqs. However, the conventional
vectorscope display yields no information concer-
ning SC/H phase.
In the case of the vectorscope shown in FIG.
3, the composite video signal is also used to
generate a signal representative of SC/H phase.
As shown in FIG. 3, the composite video signal
is applied to a phase locked oscillator 28 which
generates a continuous wave 3.58 MHz signal. On
even numbered lines, a positive-going zero crossing
of the continuous wave signal coincides in time
with the sync point and on odd numbered lines a
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negative-going zero crossing of the continuous wave
coincides with the sync point. This phase reversal
of the continuous wave, which may be accomplished
by switching in a half-period delay on alternate
lines, compensates for the 180 degree change in the
phase relationship between sync and burst on conse-
cutive lines in the NTSC system, and consequently
the phase relationship between burst and the con-
tinuous wave signal does not change from line to
line.
The sync locked CW signal and the output of
the chroma filter 14 are applied to a switch 30,
which is controlled by a control logic circuit 32.
~he control logic 32 controls not only selection
between the filter 4 and the oscillator 28 but also
Z-axis blanking of the CRT 10 by a blanking circuit
34. The manner of operation of the control logic
32 when the vectorscope is in SC/H phase display
, mode is indicated in FIG. 4, in which the waveform
(a) represents the composite video signal, the
waveform ~b~ represents the state of the switch 30
and the waveform (c) represents the state of the
blanking circuit 34. When the vectorscope is in
. its normal display mode, the control logic 32
causes the switch 30 to select continuously the
filter 14, and the vectorscope functions in ~he
manner described with reference to FIG; 2. When
the vectorscope is operating in its SC/H phase
display mode, the control logic 32 causes the
switch to select the output of the filter 14 (wave-
form (b) low) only during sync and burst time, and
to select the sync locked CW (waveform (b) high)
during the remainder of the line time. The control
loqic 32 also controls the Z-axis blanking circuit
34 to blank the CRT 10 (waveform (c) low) during
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the switches between the filter 34 and the oscilla-
tor 28 and to unblank the CRT (waveform ~c) high)
for a portion of the time for which the filter 14
is selected, so as to provide the center dot 40 and
S burst vector 42 on the display, as shown in FIG. 5.
~he CRT is also unblanked for a portion of the
active line time, during which the sync locked CW
is selected, to display a vector representing the
phase and amplitude of the sync locked CW on those
lines. The duration of the latter unblanking is
varia~l~ to control the intensity of the display of
the sync locked CW vector relative to the burst
vector and center dot. Preferably, the unblanking
time is chosen so that only the outer extremity of
the sync locked CW vector is visible, and therefore
the sync locked CW vector is indicated by a dot 44.
The amplitude of the sync locked CW is greater than
that of burst, and therefore the sync locked CW
vector extends beyond the burst vector and the dot
44 indicating sync locked CW is visually distin-
guishable from the burst vector even when it is
disposed at the same angular position of the polar
display. If the sync locked CW is in phase with
burst, the dot 44 is at the same angular position
of the polar display as the burst vector 42.
If the phase of the CW signal provided by the
oscillator were not reversed on consecutive lines,
the change in phase relationship between sync and
burst on consecutive lines would result in display
of two sync locked CW vectors (or dots) for odd
` numbered lines and even numbered lines respective-
ly, and it might be difficult to determine which
vector ~or dot) represents the sync locked CW for
even numbered lines. An alternative way of
avoiding this problem would be to proqram the con-
trol logic 32 so that the CRT remains blankedduring the entire active line time for odd numbered
lines.
In the production of a television transmission
using several video signal sources, it is necessary
in order to avoid unacceptable signal degradation
upon switching from a first source to a second
source to ensure that the correct color frame rela-
tionship exists between the signals from the two
sources. This can be done by ensuring that the
subcarrier burst of each signal is in phase with
sync of that signal, and that the bursts of the
respective signals are in phase with each other.
The vectorscope shown in FIG. 6 can be used to
examine simultaneously the SC/H phase of two video
signals.
In the case of FIG. 6, the signal being trans-
mitted Ithe reference signal~ is applied through an
, input terminal 46 to the burst locked oscillator 16
and to a first sync locked oscillator 28a. The
signal that is to be selected (the selected signal)
is applied through an input terminal 48 to a second
sync locked oscillator 28b and to the bandpass
filter 14. The outputs of the filter 14 and the
oscillators 28 are applied to a switch 30' which
selects among these outputs under control of the
control logic 32'. The control logic causes the
switch to select the output of the filter 14 only
during burst and sync time of the selected video
signal, the output of the oscillator 28a during
; line 1 of field 1 of the reference signal, and the
output of the oscillator 28b at other times. The
CRT 10 is blanked during switches between the
oscillators 28a and 28b and the filter 14, and is
unblanked for a portlon of the time for which the
,- .
125~196
11
filter 14 is selected. Accordingly, the CRT dis-
plays a sync dot 44a at an angular position repre-
senting SC/H phase of the reference signal and a
vector 42b and a sync dot 44b whose relative angu-
lar positions represent SC/H phase of the selectedvideo signal. The CW output signals provided by
sync locked oscillators 28a and 28b are of dif-
ferent amplitudes, and therefore the two sync dots
can be readily distinguished based on radial dis-
tance from the center dot.
The CRT does not display a vector representingphase of the reference color burst. On initial
set-up, the reference signal may be connected to
both terminals 46 and 48, in which case a vector
indicated 42a and representing the phase of
reference burst will be displayed, and by adjusting
the phase shifter 22 the vector 42a may be aligned
with a predetermined radius, e. g., the 180 degree
radius, of the polar display. Thereafter, changes
in the phase of reference burst will cause the
entire display to rotate, whereas changes in the
angular position of only the sync dot 44a represent
changes in reference SC/H phase.
A vectorscope embodying the invention and
adapted for use in the PAL system comprises essen-
tially the same functional elements as are shown in
FIG. 3 or 6. However, in order to accomodate the
25 Hz offset that exists between burst and sync in
the PAL system, the controls performed by the con-
trol logic 32 or 32' are somewhat different. Thus,if the control logic 32 of the FIG. 3 vectorscope
carried out only the controls indicated by the
waveforms shown in FIG. 4, the sync dot would
describe a complete circle, because SC/H phase is
different for every line of each field. In accor-
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12
dance with the PAL standard, SC/H phase is definedon line 1 o~ field 1. In the PAL version of the
FIG. 3 vectorscope, the control logic 32 is usd to
blank the sync dot for a ~ew lines before and after
line 1, as indicated in FIG. 8. The resulting
display is shown in FIG. 9, and it will be seen
that the part circle formed by the unblanked sync
dot on either side of the blanked portions aid in
locatinq the dot 44' representing the sync locked
CW vector for line 1. The resulting gaps in the
circle form a coarse display of SC/H phase which is
usable from a distance. For reasons that are well
understood by persons skilled in the art, two burst
vectors 42' are shown in FIG. 9. Similarly, in the
case of the PAL version of the FIG. 6 vectorscope,
the control logic 32' blanks the sync dot for a few
lines before and after line 1 of the reference
signal.
It will be appreciated that the invention is
not restricted to the particular instruments that
have been described with reference to FIGS. 3 and
6, since variations may be made therein without
departing from the scope of the invention as
defined in the appended claims, and equivalents
thereof. For example whereas in the case of FIG. 3
the measure of sync timing relative to burst is
obtained by generating the sync locked CW, and this
CW signal is used directly as an input to the
vectorscope and is processed through the vector-
scope's conventional functional elements to providethe desired display, other means of generating a
signal representative of the phase angle correspon-
ding to the time difference between the sync point
and the closest zero crossing of the extended
subcarrier wave may be used. For example, as shown
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in FIG. 10 it would be possible to use a sync detector 46
to generate a signal at the sync point and to use this
signal to control a sampler 48 for sampling the extended
subcarrier wave. The amplitude of the extended subcarrier
S wave at the sample point is dependent upon SC/H phase, and
may be used to address a look-up device 50 that generates
signals for application to the X and Y plates of the CRT.
Alternatively, a burst pulse generator 52 may be used to
generate a sampling pulse at the first positive-going zero
crossing point of burst, and this pulse may be used to
control a sampler 54 for sampling a sync locked continuous
wave provided by an oscillator 55 (FIG. 11). Again, the
amplitude of the wave at the sample point is representative
of the SC/H phase and may be used to address a look-up
device 56 that generates X and Y signals for the CRT.
Still further, a sync detector 58 could be used to
generate a first pulse and the next positive-going zero
crossing point of the burst could be used by a burst pulse
generator 60 to generate a second pulse, and a simple time
measuring circuit 62 could be used to determined the delay
between the pulses (FIG. 12). This time difference could
translated through a look-up device 64 into a measure of
SC/H phase. In addition, if it were desired to compare
two video signals, e.g., an input signal and a reference
signal for color framing purposes, the vectorscope could
be constructed with two sync locked oscillators fed by the
two signals respectively. In
this case, the outputs of the two oscillators would be
applied to the switch 30 (FIG. 3), which would time
multiplex these outputs and the output of the filter
into the display to enable comparison of the timing
~'2S6~96
of the sync points of the two video signals to each
other and to burst. Moreover, whereas in the case
- of FIG. 6 the vectorscope displays the subcarrier
frequency components of only the selected video
S signal, if the vectorscope were used in a produc-
tion facility that had only a small number of
signal sources correct color framing of two video
signals (the reference signal and the selected
signal) may be achieved by comparing the two video
signals directly. In this case, a second bandpass
filter wo~ld be associated with the input terminal
46 and the switch 30' would select among four
possible signals (two bursts and two sync locked CW
signals). It would then be possible to provide a
technician with information that would permit the
selected video signal to be brought into the
correct color frame relationship with the reference
signal.
. .
