PAL SYSTEM TELEVISION MEASUREMENT : CHROMINANCE NONLINEAR PHASE

Chrominance Nonlinear Phase

DEFINITION
Chrominance nonlinear phase distortion is present when chrominance phase is affected by chrominance amplitude.
These phase errors are a result of the system's inability to uniformly process all amplitudes of chrominance information. Chrominance nonlinear phase distortion is expressed in degrees of subcarrier phase. This
parameter should be measured at different average picture levels and the worst error quoted.
PICTURE EFFECTS
Like differential phase, the effects of chrominance nonlinear phase are averaged out in delayline PAL decoders. Hue shifts therefore cannot be detected in the picture.
TEST SIGNAL
A modulated pedestal signal, sometimes called a three level chrominance bar, is used to measure this distortion. This signal consists of a single phase, three level chrominance packet superimposed on a constant luminance level. A typical modulated pedestal signal will have a 350 mV luminance level and 140, 420, and 700 mV chrominance levels. This signal element is sometimes part of combination signals used as ITS.

Figure 83. A modulated pedestal test signal.

MEASUREMENT METHODS
Chrominance nonlinear phase is quantified by measuring the phase differences between the chrominance packets of the modulated pedestal signal.
Vectorscope.
Since phase information is required, a vectorscope is used to measure chrominance nonlinear phase. Examine the three dots (which correspond to the three chrominance levels) and measure the maximum
phase difference between the three signal vectors. This is easiest when the vectorscope variable gain is adjusted to bring the largest vector out to the graticule circle. When using a 1781R or a 521A Vectorscope, the calibrated phase shifter can be used to obtain a precise reading.

Figure 84. The 1781R vectorscope display showing a signal that suffers from chrominance nonlinear phase distortion.

VM700T Automatic Measurement.
Select CHROMINANCE NONLINEARITY in the VM700T MEASURE mode to obtain a display of this distortion. The chrominance nonlinear phase measurement is the middle graph in the display (see Figure
85). Measurement results are also available in the VM700T AUTO mode.

Figure 85. The VM700T Chrominance Nonlinearity display.

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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