PAL SYSTEM TELEVISION MEASUREMENT : CHROMINANCE NONLINEAR GAIN

Chrominance Nonlinear Gain

DEFINITION
Chrominance nonlinear gain distortion is present when chrominance gain is affected by chrominance amplitude. These amplitude errors are a result of the system's inability to uniformly process all amplitudes of
chrominance information.
Chrominance nonlinear gain distortion is the amplitude deviation expressed as a percentage of the nominal amplitude. This measurement is made on the lowest and highest chrominance levels with the middle level
normalized to its nominal value.
The larger of the two resulting numbers is generally taken as the overall result.
This distortion should be measured at different average picture levels and the worst distortion should be quoted.
PICTURE EFFECTS
Chrominance nonlinear gain is often seen as attenuation of relatively high amplitude chrominance signals. It will appear in the TV picture as incorrect colour saturation.
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 86. A modulated pedestal test signal.

MEASUREMENT METHODS
Chrominance nonlinear gain distortion is quantified by measuring how much the amplitudes of  the chrominance packets deviate from their nominal values.
Waveform Monitor.
The waveform monitor graticule should be used for this measurement. First use the waveform monitor variable gain to normalize the middle subcarrier packet to its prescribed value of 420 mV. The amount of chrominance nonlinear gain distortion is the largest deviation from nominal value for the other two packets expressed as a percentage of the nominal amplitude of the affected packet.

Figure 87. This signal exhibits chrominance nonlinear gain distortion. Note that the amplitude of the largest packet is reduced.

VM700T Automatic Measurement.
Select CHROMINANCE NONLINEARITY in the VM700T MEASURE menu to make this Measurement. Chrominance nonlinear gain is shown on the upper graph. This parameter can also be measured in the VM700T AUTO mode.

Figure 88. The VM700T Chrominance Nonlinearity display.

NOTES
23. Chroma Filter.
It is sometimes recommended that waveform monitor chroma filter be enabled when measuring chrominance
nonlinear gain. While the chroma filter will make the display more symmetrical, the same results should be obtained either way since it is the peak-to-peak amplitudes being measured.
A possible exception is a case where chrominance harmonic distortion is present. The chrominance filter can remove the effects of harmonic distortion which are likely to be different for each chrominance level.


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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