PAL SYSTEM TELEVISION MEASUREMENT : CHROMINANCE TO LUMINANCE INTERMODULATION

Chrominance-to-Luminance Intermodulation

DEFINITION
Chrominance-to-luminance intermodulation, also known as crosstalk or cross-modulation, is present when luminance amplitude is affected by superimposed chrominance. The luminance change may be caused by clipping of high-amplitude chrominance peaks, quadrature distortion, or various crosstalk and intermodulation effects.
The deviation in the pedestal level may be expressed:
• As a percentage of the pedestal level
• As a percentage of the measured white bar amplitude
• As a percentage of 700 millivolts
These definitions will yield different measurement results under some conditions so it is important to standardize on a single method of making intermodulation measurements.
PICTURE EFFECTS
When intermodulation distortion is present, colour saturation will not be accurately represented in affected pictures.
TEST SIGNALS
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 89. This combination ITS contains the Modulated Pedestal signal element (CCIR Line 331).

MEASUREMENT METHODS
Chrominance-to-luminance intermodulation is quantified by measuring the effects that chrominance packets of different amplitudes have on the luminance level that they are superimposed on. This process is
facilitated by removing the chrominance information from the display with a waveform monitor filter.
Waveform Monitor.
The chrominance information can be filtered off with either the luminance or lowpass filter in the 1781R. The Y display of the 521A Vectorscope also works well.
Details of the measurement method will depend on the method chosen to express the amount of distortion. In general, the appropriate part of the signal must be normalized using the waveform monitor variable gain
control. Then measure the largest level shift in the top of the luminance pedestal.
The 1781R voltage cursors can be used in the relative mode to make this measurement. In Figure 90, the level shift is 8.5% of the pedestal level.

Figure 90. A chrominance-to-luminance intermodulation distortion of 8.5% referenced to the pedestal level.

VM700T Automatic Measurement.
Select CHROMINANCE NONLINEARITY in the VM700T MEASURE menu to measure chrominance-to-luminance intermodulation. This parameter is shown on the lower graph.
Measurement results are also available in the VM700T AUTO mode.

Figure 91. 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