Subcarrier to horizontal phase measurement and display for multiple video signals

A subcarrier to horizontal sync (SC/H) phase difference measurement and display device for multiple sources of composite video signals includes a sample clock generator providing a sampling clock at a fixed frequency, a digitizer providing numeric samples representing an amplitude of each of the sources at least during the interval between the horizontal sync pulse and the color burst of at least one horizontal line in each of the sources. The samples are stored in memory and a processor compares successive samples, while counting samples or monitoring elapsed time, to find the midpoint of the leading edge of the sync pulse and a zero crossing of the burst. The lapse of time (or number of samples) defines the SC/H phase difference and is displayed for all of the sources for direct graphic comparison. The digitizer, memory, processor and display are coupled over a data bus. A line counter responsive to horizontal syncs in each source, reset upon vertical retrace, triggers capture of the data at a selected horizontal line, which can be different for the respective sources, and the line numbers are displayed together with the graphic comparison.

BACKGROUND OF THE INVENTION 
1. Field of the Invention the Invention 
This invention relates to methods and apparatus for measuring and 
displaying the phase relationship between the subcarrier and the 
horizontal sync (the SC/H phase) of a video signal, and in particular to a 
high accuracy digital method and apparatus for measuring and displaying 
the SC/H phase of a plurality of video signals simultaneously. 
2. Prior Art 
Color video display apparatus have discrete phosphor areas disposed in a 
repetitive pattern of colors, the individual dots or segments for a 
particular color being activated by the electron beam as it scans over the 
screen in horizontal lines. The video color information encoded in the 
video signal is presented in timed coincidence with the scanning of the 
beam over the repetitive color patterns, and accordingly the phase 
relationship between the encoded color information and the sync pulses 
which trigger the scanning circuits of the display apparatus is an 
important consideration. Color information is synchronized to a subcarrier 
frequency defined by the color burst pulse which occurs shortly after the 
horizontal sync pulse for each horizontal line. Test equipment is 
available for measuring and displaying the phase relationship between the 
subcarrier and the horizontal sync pulse, commonly known as the SC/H 
phase, for use by video production personnel, broadcasters, etc. 
Known devices for SC/H phase measurement and display typically rely on 
variable analog devices for providing phase delays, ramp signals and the 
like. As a result, the only practical way to measure the SC/H phase of a 
plurality of video signals simultaneously has been to substantially 
duplicate the measurement and display apparatus for every signal which is 
to be measured. Multiple circuits, however, do not act identically due to 
variations in components and the like. The multiple circuits can be 
expected to provide at least slightly different results even for one 
signal. Moreover, the analog devices are subject to problems of accuracy 
and drift. 
According to one known method of SC/H phase measurement, one subcarrier 
signal is phase-locked to the horizontal sync and another subcarrier 
signal is phase-locked to the color burst. The SC/H phase difference is 
defined by the phase difference of these two subcarriers, and is 
displayed, for example, by using the signals as the X and Y inputs to an 
oscilloscope to provide a vectorscope type phase display. The phase 
difference is represented by the angular displacement of the displayed dot 
from a reference dot or line representing a zero phase angle. Prior art 
devices along these lines are disclosed in U. S. Pat. Nos. 
4,587,551--Penney; 4,788,585--Suzuki; and 4,694,324--Matney. Of course it 
is also possible to display the extent of phase difference between two 
signals as a numeric readout or as a bargraph. 
According to another known method, the time lapse between the midpoint of 
the leading edge of the horizontal sync pulse and a zero crossing of the 
color burst is measured. Alternatively the time lapse can be measured 
between the leading edge of the sync and the next zero crossing of a 
reference subcarrier regenerated from the color burst. This normally 
requires an analog circuit or a combination analog/digital circuit, for 
finding the midpoint of the leading edge and determining the time lapse. 
The time lapse can be translated into degrees of phase difference by 
relating the time to the period of the subcarrier, e.g., at the nominal 
3.579545 MHz for an NTSC subcarrier. The results can be displayed as a 
numeric readout, bargraph, etc. 
U. S. Pat. No. 4,758,890--Boyce discloses a circuit for comparing the 
subcarrier frequency to a horizontal sync pulse. The sync pulse is delayed 
by a variable delay circuit controlled in a feedback loop, and operable to 
delay the sync pulse to ensure that the leading edges of the sync pulse 
corresponds to a the zero crossing of the subcarrier. 
U. S. Pat. Nos. 4,603,346--Melling, Jr.; 4,680,620--Baker et al; and 
4,792,845--Judge are additional examples of phase comparators and 
displays. U. S. Pat. No. 4,470,064--Michener discloses a circuit operable 
to capture and digitize the value of a burst-locked subcarrier and a 
quadrature subcarrier as a means of acquiring data necessary to calculate 
the SC/H phase. Additionally, the Tektronix model VM-700 Video Measurement 
Set calculates SC/H phase from a digitized signal. The VM-700 and the 
Michener device employ digital techniques, and Michener is switchable 
between two video inputs. Nevertheless, the devices can only deal with one 
signal at a time. 
If a transmitted signal at one SC/H phase is suddenly substituted with a 
signal at a different SC/H phase, the result is a flash of poor color 
purity and/or a variation in intensity. It would be desirable to provide 
an accurate and drift free measurement apparatus for SC/H phase 
measurement that can measure and display multiple video sources 
simultaneously, i.e., without the need for the operator to switch between 
video sources for one-at-a-time measurements. Such a system would be 
particularly helpful as a production and broadcast tool in conjunction 
with video switchers coupled to multiple video sources such as a plurality 
of prerecorded sources and the like, assisting in the merging of multiple 
signal sources into a program. 
The present invention provides a digital circuit operable to accomplish 
multiple source SC/H phase measurements, and provides a means for 
simultaneous display of the results in a preferably graphic format. In 
this manner the phase conditions of a plurality of sources can be 
monitored. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide accurate and drift free 
measurement of the subcarrier to horizontal sync phase relationship for 
each of a plurality of video signals, and to display the results 
simultaneously, for graphic and numeric comparison. 
It is a further object to minimize the complexity and expense of SC/H phase 
measurement equipment monitoring multiple video sources. 
It is also an object of the invention to employ a digital processor for 
direct measurement of SC/H phase difference for multiple sources, the 
results being displayed in direct comparison format. 
These and other objects are accomplished by a subcarrier to horizontal sync 
(SC/H) phase difference measurement and display device for multiple 
sources of composite video signals including a sample clock generator 
providing a sampling clock at a fixed frequency, a digitizer providing 
numeric samples representing an amplitude of each of the sources at least 
during the interval between the horizontal sync pulse and the color burst 
of at least one horizontal line in each of the sources. The samples are 
stored in memory and a processor compares successive samples, while 
counting samples or monitoring elapsed time, to find the midpoint of the 
leading edge of the sync pulse and a zero crossing of the burst. The lapse 
of time (o number of samples) defines the SC/H phase difference and is 
converted to a phase angle with reference to the subcarrier frequency, and 
displayed for all of the sources for direct graphic comparison. The 
digitizer, memory, processor and display are coupled over a data bus. 
Preferably a high frequency sample clock captures the data in a line store 
memory for numerical analysis by a processor operating asynchronously with 
the sample clock. A line counter responsive to horizontal syncs in each 
source, reset upon vertical retrace, triggers capture of a plurality of 
data samples at a selected horizontal line. Preferably the same line is 
captured in each of the sources. The line number is displayed together 
with the graphic comparison. A display memory and display controller 
convert the data for display on a CRT. The SC/H measurement and display 
functions are preferably one of a number of display options incorporated 
in a test apparatus according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the block diagram of FIG. 1, the apparatus of the invention is 
coupled to a plurality of video sources 22, the SC/H phase differences of 
which are to be measured, and displayed graphically via display means 56 
to produce an image as shown in FIG. 2. The SC/H phase difference is 
defined as the time lapse between the midpoint 64 of the leading edge 58 
of the horizontal sync pulse, shown in FIG. 3, and the rising zero 
crossing 68 of the burst pulse 66, measured in degrees of phase at the 
burst or video subcarrier frequency. 
Each of the composite video inputs 22 is coupled to an input buffer 
amplifier 24, and the outputs of the buffer amplifiers are coupled to a 
four-to-one multiplexer 26, operable under control of CPU 44 to select one 
of the four signals for coupling onto the output of multiplexer 26. The 
CPU cycles repetitively through the inputs, and the data displayed for 
each channel represents the last time its SC/H phase was measured. 
The selected video signal is adjusted a to DC offset and scaled for 
amplitude by level adjust circuit 28, in order to employ the full span of 
the analog to digital converter 32 which will sample the video signal. 
Additionally, sync stripper 30 extracts the horizontal sync pulses, and 
routes them to line rate controller 38. Line rate controller 38 counts the 
horizontal sync pulses, thus counting the lines of video, and resets the 
count upon the occurrence of a vertical sync. The line rate controller 
triggers the line storage controller 40 when the line count equals a 
predetermined number, i.e., the video line at which the SC/H phase is to 
be measured. Typically, for NTSC video signals, the SC/H phase difference 
is measured for line number ten of field one of the video signal. It is 
also possible to measure the SC/H phase difference at a different line, 
using an input means (not shown) coupled to the line rate controller 38. 
Normally it is desirable to measure the SC/H phase for all the sources 22 
at the same line in their respective fields. When the line count equals 
the predetermined number, the "capture" signal coupled to the line storage 
controller 40 is activated, and a line of video is sampled and stored. 
Analog to digital converter 32 is coupled to a sampling rate clock 36 
through line storage controller 40. The sampling rate is substantially 
higher than the video subcarrier frequency (about 3.58 MHz), for example 
20 MHz, thereby providing a sample every 50 nS during the video line. Line 
storage memory 34 is coupled to the output of the A/D converter 32, and 
loads sample data under control of a memory clock signal from the line 
storage controller 40, and at an address incremented by the line storage 
controller, such that the samples are stored successively. 
Whereas the SC/H phase measurement according to the invention is preferably 
one of the functions of a video test apparatus which accomplishes 
additional functions such as display of a selected line of video, the 
entire video line is preferably sampled. For the purpose of SC/H phase 
alone, however, it is only necessary to obtain a sufficient number of 
samples to define the leading edge of the horizontal sync pulse and a zero 
crossing of the burst at the subcarrier frequency. 
The CPU or processor 44 executes a program stored in CPU ROM 48, using 
memory in CPU RAM 46, both coupled to the CPU via address/data bus 42, 
which is also coupled to the line storage memory 34. The CPU 44 is 
programmed to effect the mathematical operations needed to convert the 
sample data into an SC/H phase difference. The CPU operates independently 
and asynchronously of sampling via line storage controller 40, and in the 
preferred embodiment is not coupled for direct access to line storage 
memory 34. Accordingly, one function of the line storage memory is to 
transfer the sample data at least for the SC/H phase measurement from the 
line storage memory to the RAM 46 coupled to the CPU via the address/data 
bus 42. This can be accomplished on a regular basis or at the conclusion 
of the recording of the video line. 
The CPU examines the successive samples to determine the 50% point of the 
leading edge of the horizontal sync, and a zero crossing of the subcarrier 
burst. A search for the leading edge of the horizontal sync pulse is 
performed by first calculating the derivative of the signal (i.e., the 
difference from sample to sample) in the area of samples where the leading 
edge of sync is expected to occur, and determining the sample for which 
the derivative is most negative. That sample can be referred to as sample 
P. Whereas the sync pulse is nominally sinusoidal in shape, the derivative 
is at a negative maximum at the midpoint, normally at a time in the 
sampling interval immediately before or after the instant sample P was 
captured, but also possibly coincident with the sample. Starting from 
sample P, assumed to be the closest sample to the 50% point, several 
samples before and several samples after are examined. The earlier samples 
define the level of the "front porch" area 60 of the sync pulse as shown 
in FIG. 3. The later samples define the sync tip or floor level 62. The 
precise 50% level of the leading edge is then calculated as the average of 
these two levels. 
The calculated 50% level of the leading edge is compared to the levels of 
the samples adjacent sample P, at which the derivative was most negative, 
in order to determine whether the precise 50% level occurred before or 
after the instant at which sample P was collected. The exact time of the 
50% level ("t.sub.50 ") is then interpolated to a time in the interval 
between sample P and the previous or subsequent sample, i.e., to a precise 
point in time between samples. This interpolation is based on the position 
of the calculated 50% level in the range defined by the difference between 
the level of sample P and the next or previous sample. The interpolation 
is performed by fitting a sinusoidal curve (the nominal shape of the sync 
edge) to P and the next or previous sample. Then the time of the 50% point 
is calculated by the processor. Of course if the calculated 50% level is 
equal to the level of sample P, the 50% point is deemed to have occurred 
at the time of sample P. 
A search for the rising zero crossing of the color burst is performed by 
examining the derivative of the signal (i.e., sample to sample difference) 
in the area where the color burst is expected to have occurred. The sample 
(referred to as sample M) having the most positive derivative is assumed 
to be the sample closest to the rising zero crossing of the burst, again 
due to the sinusoidal nature of the signal. A digital low-pass function is 
then applied to several samples neighboring sample M to filter out the 
high frequency burst portion of the signal and to define the "back porch" 
level 70 as shown in FIG. 3, namely the low frequency component of the 
signal during the burst. In the same manner as with the 50% point of the 
horizontal sync, the back porch level is compared to the samples 
immediately adjacent sample M to determine whether the zero crossing 
(actually the back porch level crossing) occurred before or after the 
instant of sample M. The proportionate point of the zero crossing level in 
the range between the level of sample M and this next or previous sample 
are then related to the time the samples were captured, thus interpolating 
the precise time ("t.sub.z ") of the zero crossing. 
The difference between times t.sub.50 and t.sub.z is then calculated to 
yield a difference in units of time. However, the SC/H phase difference is 
needed in degrees of phase of the subcarrier frequency. The time 
difference (t.sub.z -t.sub.50) is converted to a number of cycles at the 
frequency of the subcarrier (e.g., 3.579545 MHz for NTSC, or a period of 
about 280 nS), resulting in an integer plus a fractional number of cycles. 
The result is rounded to an integer and subtracted from the 
integer-plus-fraction number, providing a fraction between -0.5 and +0.5. 
This fraction is multiplied times 360, to give the phase difference in 
number of degrees, ranging between -180 degrees and +180 degrees. 
Preferably, the results of the computation are stored by the processor in a 
cycling FIFO memory, comprising one FIFO for each of the plural video 
signals at the input. The entries in the FIFO for each source are averaged 
with the other entries in the FIFO for that source, thus reducing the 
effects of noise and random errors. The results are displayed both 
graphically and numerically on the display. 
According to the preferred embodiment the display device has a pixel-type 
display memory 50 having a field of pixels which are read out repetitively 
under control of a display controller 52. The variable portions of the 
pixel display field, namely the moving markers 78, the numeric displays 80 
and the line number 82, are generated from the results of calculations by 
the CPU, transmitted to the display memory 50 over the address/data bus 
42. The invariable portions of the display, including the graduation lines 
74 and the descriptive labels, need only be loaded once by the CPU into 
the display memory 50, preferably according to a routine stored in the CPU 
ROM and executed upon initialization of the SC/H mode of operation of the 
device. The display memory can be used simply to provide light/dark image 
data, each bit in the memory representing a pixel which will be one of two 
intensities. Alternatively, a more complex, e.g., color display can be 
provided, for example with the marker elements 78 shown in a distinct 
color or intensity. 
In the preferred embodiment the pixel data in the display memory is read 
out repetitively as intensity data, and at the same time the X and Y 
deflection inputs to the display means are cycled through the 
corresponding X and Y positions. For example, the display means can be a 
CRT with deflection drive circuits and a kinescope driver, the outputs of 
which are modulated by the analog levels produced by D/A converters 54 
coupled to the data outputs of the display memory 50, which is addressed 
and clocked by display controller 52 for reading data in or out. 
The invention provides a highly accurate means for measuring the SC/H phase 
difference of a plurality of source signals simultaneously, permitting a 
direct graphic and numeric comparison of the SC/H phase for all the 
sources. In this manner the operator can readily set up a switcher or the 
like with video sources having SC/H phase differences which are close 
together, such that switching from one source to another has a minimal 
effect on color purity and the like as perceived by a viewer. 
A number of variations on the invention are possible. The particular nature 
of the graphic display can of course be varied in its particulars of data 
presentation, color, etc. The span of the scales 74 can be made variable 
under user control or automatically by the CPU in order to set the 
graduation lines and upper and lower limits wide enough to accommodate the 
greatest SC/H phase difference while using as much as possible of the 
available scale area. These and other variations are believed to be within 
the scope of the invention defined herein. 
The invention has been discussed in connection with preferred exemplary 
embodiments, and such variations on the preferred embodiments will now 
become apparent to persons skilled in the art. Whereas the invention is 
intended to encompass the disclosed embodiments and a range of variations 
in accordance herewith, reference should be made to the appended claims 
and their reasonable equivalents in order to assess the scope of the 
invention in which exclusive rights are claimed.