PAL SYSTEM TELEVISION MEASUREMENT : CHROMINANCE TO LUMINANCE GAIN AND DELAY

Chrominance-to-Luminance Gain and Delay
DEFINITION
Chrominance-to-luminance gain inequality (relative chrominance level) is a change in the gain ratio of the chrominance and luminance components of a video signal. The change is expressed in percent or dB with the number negative for low chrominance and positive for high chrominance. Chrominance-to-luminance delay inequality (relative chrominance time) is a change in the time relationship between the chrominance and luminance components of a video signal. The change is expressed in units of time, typically nanoseconds.
The number is positive for delayed chrominance and negative for advanced chrominance.
PICTURE EFFECTS
Gain errors most commonly appear as attenuation or peaking of the chrominance information.
This shows up in the picture as incorrect colour saturation. Delay distortion will cause colour smearing or bleeding, particularly at the edges of objects in the picture. It may also cause poor reproduction of sharp luminance transitions.
TEST SIGNALS
Chrominance-to-luminance gain and delay inequalities are measured with a 10T or 20T modulated sine-squared pulse. Many combination ITS signals include such a pulse. The frequency spectrum of a composite pulse includes energy at low frequencies and energy centered on the subcarrier frequency.
Selection of an appropriate pulse width is a trade-off between occupying the PAL chrominance bandwidth as fully as possible and obtaining a pulse with sufficient sensitivity to delay errors. The 10T pulse is more sensitive to delay errors than the 20T pulse, but does not occupy as much of the chrominance bandwidth. CCIR specifications generally recommend the use of 20T pulses while 10T pulses are commonly used in
the U.K.
A modulated bar is also sometimes used to measure chrominance- to-luminance gain inequalities.
MEASUREMENT METHODS
Conventional chrominance-toluminance gain and delay measurements are based on analysis of the baseline of a modulated sine-squared pulse. (See Appendix B for a definition of the time interval T.) This pulse is made up of a sine-squared luminance pulse and a chrominance packet with a sine-s q u a re d envelope (see Figure 21).

Figure 20. A combination signal that includes a 20T modulated pulse (CCIR Line 17).

Figure 21. The chrominance and luminance components of a modulated sine-squared pulse.

Modulated sine-squared pulses offer several advantages. First of all, they allow evaluation of both gain and delay differences with a single signal. A further advantage is that modulated sinesquared pulses eliminate the
need to separately establish a low-frequency amplitude reference with a white bar. Since a low-frequency reference pulse is present along with the highfrequency information, the amplitude of the pulse itself can be normalized.
The baseline of the modulated pulse is flat when chrominanceto-luminance gain and delay distortion is absent. Various types of gain and delay distortion affect the baseline in different ways. A single peak in the baseline
indicates the presence of gain errors only. Symmetrical positive and negative peaks indicate the presence of delay errors only. When both types of errors are present, the positive and n e gative peaks will have different
amplitudes and the zero crossing will not be at the centre of the pulse. Figure 22 shows the eff e c t s
of various types of distortion.
Waveform Monitor and Nomograph.
One method of quantifying chrominance-to-luminance inequalities involves measuring the peaks of the modulated pulse baseline distortion and applying these numbers to a nomograph. The nomograph converts the baseline measurements into gain and delay numbers.
To make a measurement, first normalize the pulse height to 100% (500 mV or 1000 mV is generally most convenient). The baseline distortion can be measured either by comparing the waveform to a graticule or by
using voltage cursors. Using a nomograph (see Figure 23), find the locations on the horizontal and vertical axes which correspond to the two measured distortion peaks. At the point where perpendicular lines
drawn from these two locations intersect, the gain and delay numbers may be read from the nomograph.

Figure 22. Effects of gain and delay inequalities on the modulated sinesquared pulse.

Figure 23. Chrominance-to-luminance gain and delay nomograph for a 20T pulse.

When making measurements in this manner, it is important to know whether the signal is a 10T or a 20T pulse. The same nomograph can be used for both but a correction factor must be applied. The nomograph in
Figure 23 is for a 20T pulse and the result must be divided by two when using a 10T pulse.
1781R Semi-Automatic Procedure.
The CHROMA/LUMA selection in the 1781R MEASURE menu eliminates the need for a nomograph.
The on-screen readout guides the user through cursor measurements of the various parameters required to obtain a number from a nomograph.
After all parameters have been entered, the instrument calculates the results (see Figure 24).
The accuracy and resolution of this method are roughly equivalent to using the graticule and a nomograph.
Waveform Monitor Graticule Approximations. 
When a system is free of significant nonlinearity and delay distortion is within certain limits, chrominance-toluminance gain inequalities can be measured directly by comparing the height of the modulated pulse to the white bar. This method and the nomograph will yield identical results when there is no delay distortion. It is
generally considered a valid approximation for signals with delay distortion in the 100 to 200 nanosecond range and is accurate to within a few percent for signals with several hundred nanoseconds of delay.
This measurement is made by normalizing the white bar amplitude to 100% and then measuring the amplitude difference between the modulated pulse top and the white bar. This difference number, times two, is the amount of chrominance-toluminance gain distortion in percent. Note that when the pulse top is higher or lower than the bar, the bottom of the pulse is displaced from the baseline by the same amount. Thus the
peak-to-peak difference between the modulated pulse and the bar is actually twice the difference between their peak values, hence the factor of two.
The lines at the centre of the baseline on the 1781R and 1481 external graticules can be used to estimate chrominance-toluminance delay errors. This method yields valid results only if gain errors are negligible (the
baseline distortion should appear symmetrical). To use these graticule marks, first use the variable gain to normalize the modulated pulse height to 700 mV. Then centre the pulse on the two graticule lines which
cross in the centre of the baseline (see Figure 25). The graticule lines indicate 200 nanoseconds of delay for a 20T pulse and 100 nanoseconds for a 10T pulse. With X5 vertical gain selected (in addition to the variable
gain required to normalize the pulse), the lines indicate 40 nanoseconds of delay for the 20T pulse and 20 nanoseconds for the 10T pulse.

Figure 24. Results obtained with the CHROMA/LUMA selection in the 1781R MEASURE mode.

Figure 25. The 1781R graticule indicates that this signal has approximately 200 nanoseconds of chrominance-to-luminance delay.
VM700T Automatic Measurement.
Chrominance-to-luminance gain and delay errors can be measured by selecting CHROM/LUM GAIN DELAY in the VM700T MEASURE mode. Numeric results are given in this mode and both parameters are simultaneously plotted on the graph (see Figure 26). Delay is plotted on the X axis and gain inequality on the Y axis. These measurements are also available in the VM700T AUTO mode.
Calibrated Delay Fixture.
Another method of measuring these distortions involves use of a calibrated delay fixture. The fixture allows incremental adjustment of the delay until there is only one peak in the baseline indicating all delay errors have been nulled out. The delay value can then be read from the fixture and gain measured from the graticule.
This method can be highly accurate but requires the use of specialized equipment.
NOTES
9. Harmonic Distortion.
If harmonic distortion is present, there may be multiple aberrations in the baseline rather than one or two
clearly distinguishable peaks. In this case, nomograph measurement techniques are indeterminate.
The VM700T, however, is capable of removing the effects of harmonic distortion and will yield valid results. Minor discrepancies between the results of the two methods may be attributable to the presence of small
amounts of harmonic distortion as well as to the higher inherent resolution of the VM700T method.

Figure 26. The Chrom/Lum Gain Delay display in the VM700T MEASURE mode.

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Table of Contents

Preface	3
Good Measurement Practices	4
EQUIPMENT REQUIREMENTS	4
CALIBRATION	6
INSTRUMENT CONFIGURATION	6
DEMODULATED RF SIGNALS	8
TERMINATION	8
DEFINITION OF THE PAL TELEVISION STANDARD	8
PERFORMANCE GOALS	8
Waveform Distortions and Measurement Methods	9
I VIDEO AMPLITUDE AND TIME MEASUREMENTS	9
Amplitude Measurements	10
Time Measurements	12
SCH Phase	15
II LINEAR DISTORTIONS	18
Chrominance-to-Luminance Gain and Delay	19
Short Time Distortion	24
Line Time Distortion	26
Field Time Distortion	28
Long Time Distortion	30
Frequency Response	31
Group Delay	36
K Factor Ratings	38
III NONLINEAR DISTORTIONS	41
Differential Phase	42
Differential Gain	46
Luminance Nonlinearity	50
Chrominance Nonlinear P h a s e	5 2
Chrominance Nonlinear Gain	5 3
Chrominance-to-Luminance Intermodulation	54
Transient Sync Gain Distortion	55
Steady-State (Static) Sync Gain Distortion	56
IV NOISE MEASUREMENTS	57
Signal-to-Noise Ratio	58
V TRANSMITTER MEASUREMENTS	6 0
ICPM	61
Depth of Modulation	63
GLOSSARY OF TELEVISION TERMS	64
APPENDICES
APPENDIX A - PAL COLOUR BARS	67
APPENDIX B - SINE-SQUARED PULSES