PAL SYSTEM TELEVISION MEASUREMENT : LONG TIME DISTORTION

Long Time Distortion

DEFINITION
Long time distortion is the low frequency transient resulting from a change in APL. This distortion usually appears as a very low frequency damped oscillation (see Figure 37). The affected signal components range in duration from 20 milliseconds to tens of seconds.
The peak overshoot that occurs as a result of an APL change, expressed as a percentage of the nominal luminance amplitude, is generally quoted as the amount of distortion. Settling time and occasionally the slope
(in percent per second) at the beginning of the phenomenon are also given.
PICTURE EFFECTS
Long time distortions are slow enough that they are often perceived as flicker in the picture.
TEST SIGNALS
Long time distortion is measured with a flat field test signal with variable APL. The signal should be "bounced'', or switched between 10% and 90% APL, at intervals no shorter than five times the settling time (see Figure 37).
MEASUREMENT METHODS
Long time distortions are measured by examining the damped low-frequency oscillation resulting from a change in APL.
Waveform Monitor
It is usually necessary to use a storage oscilloscope or a waveform monitor in the SLOW SWEEP mode to
measure long time distortion. A waveform photograph can be helpful in quantifying the distortion.
Once a stable display is obtained (or a photograph taken), measure overshoot and settling time (see Figure 37).
VM700T Automatic Measurement.
Select Bounce in the VM700T MEASURE mode to obtain a display of long time distortion (see Figure 38). Peak deviation and settling time are given at the bottom of the screen.

Figure 36. A flat field bounce signal.

Figure 37. Long time distortion measurement parameters.

Figure 38. The VM700T Bounce 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