PAL SYSTEM TELEVISION MEASUREMENT : LINEAR DISTORTIONS

II. LINEAR DISTORTIONS

Waveform distortions that are independent of signal amplitude a re re f e rred to as linear distort i o n s .
These distortions occur as a result of a system's inability to uniformly transfer amplitude and phase characteristics at all frequencies. When fast signal components such as transitions and high-frequency
differently than slower line-rate or field-rate information, linear distortions are probably present.
These distortions are most commonly caused by imperfect transfer characteristics of the equipment in the signal path.
However, linear distortions can also be externally introduced. Signals such as power line hum can couple into the video signal and manifest themselves as distortions.
One method of classifying linear distortions involves grouping them according to the duration of the signal components that are affected by the distortion. Four categories, each corresponding
to a familiar television time interval, have been identified. (The range of time intervals for each category may vary somewhat from definition to definition.) These categories are:
SHORT TIME (100 nanoseconds to 1 microsecond)
LINE TIME (1 microsecond to 64 microseconds)
FIELD TIME (64 microseconds to 20 milliseconds)
LONG TIME (greater than 20 milliseconds)
This classification is convenient because it allows easy correlation of the distortions with what is seen in the picture or in a waveform display. A single measurement for each category takes into account both amplitude
and phase distortions within that time range.
While the combination of these four categories covers the entire video spectrum, it is also useful to have methods of simultaneously evaluating response at all frequencies of interest.
Frequency response measurements look at amplitude versus frequency characteristics while group delay measurements examine phase versus frequency characteristics. Unlike the measurements classified by time
interval, frequency response and group delay measurements permit separation of amplitude distortions
from delay distortions.
In addition to these measurements, there is one specific case that needs to be examined in detail. The phase and amplitude relationships between the chrominance and luminance information in a signal are critical.
Chrominance-to-luminance gain and delay are therefore measured in order to quantify a system's ability to process chrominance and luminance in correct proportion and without relative time delays.
Sine-squared pulses and rise times are used extensively in the measurement of linear waveform distortions. It may be helpful to review the information in Appendix B which discusses the use of sine-squared pulses in television testing.

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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