Method and apparatus for measurement of component video signal characteristics using an oscilloscope

A method and apparatus for measuring characteristics of a component color video signal. In one embodiment the luminance component of a color video signal is applied to the vertical input of a cathode ray tube display device and a chrominance component is applied to the horizontal input. The resultant dots and transitions therebetween produced by the display trace provide an indication of component bandwidth, amplitude and relative time between components. All components can be compared simultaneously by alternately applying one chrominance component with the luminance component, and the other chrominance component with an inverted luminance component. In another embodiment one chrominance component is applied to the vertical input and the other is applied to the horizontal input. Delay circuits are provided for use with a time-division multiplexed video signal to compare time multiplexed components simultaneously. A graticule is provided for relating the display trace to a standard display.

BACKGROUND OF THE INVENTION 
This invention relates to component color video signal measurement methods 
and apparatus, particularly to measurements of spectral and temporal 
distortion, and to measurements of the characteristics of time-division 
multiplexed component video signals. 
In conventional color television systems the color characteristics of the 
video image, that is, the luminance, hue, and saturation, are oridinarily 
represented by three signal components. The components sometimes used are 
one luminance component and two distinct chrominance ("chroma") 
components. The two chroma components are typically synthesized from a 
weighted combination of red, green, and blue signal levels. 
The conventional method of transmission of television signals in the United 
States is based upon a method of frequency-division multiplexing adopted 
by the National Television System Committee (NTSC) in 1953. In that system 
signals representing the luminance and chroma components are 
frequency-division multiplexed and transmitted simultaneously. The chroma 
signals are shifted in phase 90 degrees from one another and thereafter 
used to modulate the same subcarrier, which is suppressed prior to adding 
the luminance signal with the product resulting from the subcarrier 
modulation. In demodulation the subcarrier must be regenerated. Hence, the 
relative phase and amplitudes of the chroma signals and the subcarrier are 
important. 
Modern technology has led to a trend toward the adoption of time-division 
multiplexed color television modulation. In these systems the luminance 
and chroma components are separated from one another in time and 
transmitted in sequence. For example, for each horizontal scanline a first 
time-compressed chroma component segment corresponding to that scan line 
is transmitted followed immediately by the transmission of a 
time-compressed segment of the second chroma component corresponding to 
the same scan line. The corresponding segment of the luminance component 
is transmitted immediately thereafter. The luminance segment also may be 
time-compressed. At the receiver, the first and second chroma segments, 
and possibly also the luminance segment, are time expanded, and the first 
and second chroma segments are delayed relative to the luminance segment 
in order to bring the three segments into time coincidence. Methods known 
as "time compressed color component" (TC3) and "multiplexed analogue 
component" (MAC) are variations of the foregoing scheme. 
In time-division multiplexed systems there is no color subcarrier, so there 
is no need to measure the relative amplitude and phase of chroma 
components in the traditional sense. However, the relative timing between 
chroma component segments, and between the luminance and chroma component 
segments is important, as it affects the accuracy of the transition from 
one color to another in the received image. The bandwidth of the signal 
channels for all of the components is still important because it affects 
image definition, and the relative signal levels of the components are 
important because they affect the hue and saturation of the image 
produced. 
A well known instrument for measuring the amplitude and phase 
characteristics of an NTSC frequency-division multiplexed signal is a 
vectorscope, for example, a Tektronix Model 520A NTSC vectorscope. As is 
commonly known in the art, such an instrument is essentially an 
oscilloscope having video signal decoding circuitry that provides to the 
horizontal and vertical deflection circuitry two signals representing the 
chroma components of the video signal. The oscilloscope trace is 
referenced to a polar coordinate system on the graticule, thereby 
providing a display indicative of the phase relative to subcarrier and 
amplitude of the color signals. Typically, positions are shown on a 
graticule over the display of the dots created by the trace representing 
selected saturated hues employed in the generation of a test pattern, 
ordinarily a color bar pattern. The trace transition between the dots is 
of little significance, since its shape is principally a function of the 
circuitry of the vectorscope itself. Thus, while such an instrument is 
valuable for measuring the relative phase and amplitude characteristics of 
chroma in an NTSC signal, it is not useful for measuring the spectral or 
transient characteristic of such a signal, nor is it useful for measuring 
the timing of a time-division multiplexed component color video signal. 
Accordingly, it would be desirable to have a method and apparatus for 
measuring the spectral, timing, and amplitude characteristics of any type 
of component color video signal, and specifically for the measuring of a 
time-division multiplexed signal. 
SUMMARY OF THE INVENTION 
The present invention provides a novel method and apparatus for measuring 
the characteristics of a component color video signal by means of a visual 
display. 
In one embodiment of the invention, the chroma components of a component 
color video signal are compared to the luminance component. An electronic 
visual display apparatus having one input corresponding to the abscissa of 
a Cartesian coordinate system and another input corresponding to the 
ordinate, such as a cathode ray tube display, is employed, the decoded 
luminance component being applied to the ordinate input and the decoded 
chrominance component being applied to the abscissa input of the display. 
Considering those two signals as orthogonal vectors having directions 
along the ordinate and abscissa, respectively, the display shows a 
representation of the vector sum of those two signals. The resultant trace 
permits measurements of the relative bandwidth of the chrominance and 
luminance signals, time delays between the chrominance and luminance 
signals, and amplitude variations in the signals. Superposition of a 
graticule having a standard display reference on the face of the display 
apparatus allows these characteristics to be related to a standard, and 
quantified. Such a display is useful for bandwidth, time delay, and 
amplitude measurements in a decoded time-division multiplexed system, and 
bandwidth and amplitude measurements in a decoded frequency-division 
multiplexed system. 
A first chroma component signal is provided to the abscissa while the 
luminance component signal is applied to the ordinate, are a second chroma 
component signal is provided to the abscissa while the luminance component 
signal is inverted and applied to the ordinate, and so forth. As a result 
the first two quadrants of the display compare the first chrominance 
component to the luminance component, and the last two quadrants of the 
display compare the second chroma component to the luminance component. 
In the case of a time division multiplexed signal, time delay circuitry is 
provided so that the chroma component signals are shifted in time so as to 
occur, absent time delay distortion, simultaneously with corresponding 
portions of the luminance component signal. 
Accordingly, it is a principle objective of the present invention to 
provide a novel method and apparatus for measuring the characteristics of 
a component color video signal. 
It is another objective of the present invention to provide a visual 
display method and apparatus for measuring the relative spectral 
characteristics of a component color video signal. 
It is a further objective of the present invention to provide a visual 
display method and apparatus for measuring the amplitude characteristics 
of a component color video signal. 
It is another object of the present invention to provide a method and 
apparatus for measuring time delay distortion in a time-division 
multiplexed component color video signal. 
The foregoing and other objectives, features, and advantages of the 
invention will be more readily understood upon consideration of the 
following detailed description of the invention, taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the display of the present invention is based upon a 
Cartesian coordinate system wherein the amplitude of one signal is 
represented by a point on the abscissa 46 and the amplitude of the other 
signal is represented by a point on the ordinate 48. Those two signals can 
be represented as vectors in the directions of the abscissa and ordinate, 
respectively. The vector 50 corresponds to an instantaneous chroma 
component signal, while the vector 52 corresponds to an instantaneous 
luminance component signal. In the foregoing example dot 54 represents the 
vector sum 56 of the aforementioned chroma and luminance component 
signals. 
As a practical matter, the display is produced by an electronic visual 
display device such as a cathode ray tube display device 22, having a 
horizontal (x) input corresponding to the abscissa and a vertical (y) 
input corresponding to the ordinate, as shown in FIG. 2. The display is 
constructed so as to compare both chroma components to the luminance 
component. This is accomplished by alternately applying the luminance 
component signal to the vertical input while a first chroma component 
signal is applied to the horizontal input, then applying an inverted 
luminance component signal to the vertical input while the second chroma 
component signal is applied to the horizontal input, so that the first two 
quadrants of the display represent the first chroma component compared to 
the luminance component and the last two quadrants of the display 
represent the second chroma component compared to the luminance component, 
as shown in FIG. 1. This is because the minimum amplitude of the luminance 
signal is assumed to be zero, while the chroma signals take on both 
positive and negative values. 
The aforedescribed display can be used effectively for frequency-division 
multiplexed as well as time-division multiplexed video signals. In FIG. 2 
the video signal input is represented by one or more input channels 
58a-58n. Where a composite frequency-division multiplexed or time-division 
multiplexed video signal is used, only one input channel is necessary. 
Additional channels may be desired for other types of systems. In the 
first case, a decoder 60 converts the video signal input into three 
components, that is, the chroma 1 component 62, the chroma 2 component 64, 
and the luminance component 66. Inversion of the luminance component 
signal is accomplishd by an inverter 70. Selection between the two chroma 
component signals is accomplished periodically by an electronic switch 68, 
and selection between the luminance component signal and the inverted 
luminance component signal is accomplished by an electronic switch 72, the 
two electronic switches being synchronized. In the case of a timedivision 
multiplexed video signal, in which the chroma components (and possibly 
also the luminance component) are time-compressed, the decoder 60 time 
expands the chroma components (and the luminance component if 
appropriate). Delay circuits 74 and 76 are provided for the chroma 1 and 
chroma 2 signals, respectively, in order for portions of those two signals 
and of the luminance signal corresponding to the same portion of the video 
image to occur simultaneously, absent time delay distortion. 
In FIG. 3(a), an idealized luminance component signal 78 and an idealized 
chrominance component signal 80 are shown in the time domain at 82. The 
image that would result in the display of the type shown in FIG. 1 is 
shown at 84. For a color bar test pattern 30, the color 32 would 
correspond to dot 86 and the color 34 would correspond to dot 88. Since 
the bandwidth of the luminance signal is ordinarily wider than the 
bandwidth of a chroma signal, an S-shape transitional trace ordinarily 
results. However, if a change in the bandwidth occurs, the shape of the 
transitional trace would change as well. Thus, the bandwidth 
characteristics can be measured by the shape of this transitional trace. 
A condition where a chroma component signal is delayed with respect to the 
luminance component signal is shown at 90 of FIG. 3(b), and the resultant 
image is shown at 92. A condition where the chroma component signal is 
advanced with respect to the luminance component signal is shown at 94 of 
FIG. 3(c), and the resultant image at 96 thereof. It can be seen that a 
time delay also results in a distinct change in the shape of the 
transitional trace, that is, a shortening or lengthening of the ends of 
the S-shaped curve. 
Referring again to FIG. 1, the positions that the dots, representing 
distinct colors of a color bar test pattern, should assume on the display 
can be identified on a graticule by an appropriate symbol, such as boxes 
90. As shown in FIG. 4, where the luminance level is out of proportion to 
the chrominance level in a display of the type shown in FIG. 1, when the 
dots do not fall within their corresponding box, an amplitude error is 
indicated. 
An appropriate graticule can be provided by conventional means, such as 
etching reference lines and symbols onto a glass plate. The plate is then 
placed over the face of the display apparatus, for example, over the face 
of a CRT. Alternatively, a graticule could be generated using the trace of 
the display device. 
In use of the aforedescribed apparatus one would transmit an appropriate 
test pattern, that is, a color bar test pattern, over the video system and 
apply the received signal to the appropriate apparatus of FIG. 6. 
Measurement of the characteristics of the signal is accomplished by 
observing the resultant display trace, particularly by relating the shape 
and position of the trace to the symbols on the graticule. 
It may be advantageous to employ smaller width color bars than have 
heretofore been used in an NTSC color bar test pattern to reduce the time 
that the display trace spends at the color dots relative to the 
transitional traces and thereby increase the relative intensity of the 
transitional traces, since the information contained in the transition 
traces is of great importance in the use of this method and apparatus. 
The terms and expressions which have been employed in the foregoing 
specification are used therein as terms of description and not of 
limitation, and there is no intention of the use of such terms and 
expressions of exluding equivalents of the features shown and described or 
portions thereof, it being recognized that the scope of the invention is 
defined and limited only by the claims which follow.