PAL SYSTEM TELEVISION MEASUREMENT : TRANSIENT SYNC GAIN DISTORTION

Transient Sync Gain Distortion

DEFINITION
Transient sync gain distortion, also referred to as transient nonlinearity, is present when abrupt changes in APL temporarily affect sync amplitude. The amount of distortion is defined as the maximum transient departure in the amplitude of sync from the amplitude that existed before the change in APL. It is generally expressed as a percentage of the original amplitude, however, some standards specify the distortion as a percentage of the largest amplitude.
Measurement of this distortion requires an out-of-service test.
Both low-to-high and high-to-low APL changes should be evaluated.
PICTURE EFFECTS
Sudden switches between high APL and low APL pictures can cause transient brightness or saturation effects in the picture.
TEST SIGNAL
Transient gain distortion is measured with a flat field signal (black burst with pedestal).
A generator with a “bounce” feature can be used to make the APL transitions if the time interval between transitions is considerably longer than any transient effect.

Figure 92. A flat field bounce test signal.

MEASUREMENT METHODS
Transient gain changes are measured by abruptly changing APL and observing the transient effects on a waveform monitor.
Waveform Monitor. 
This distortion is easiest to evaluate with the test signal displayed on a waveform monitor with the differentiated step filter selected. (Recall that this filter produces spikes with amplitudes proportional to the step amplitudes). Be sure the DC restorer is turned off for this measurement.
Depending on the nature of the distortion, it may be possible to observe it when the waveform monitor is in the field sweep mode. Otherwise it will be necessary to use the 1781R SLOW SWEEP mode. (Some
1481 Waveform Monitors are equipped with the SLOW SWEEP option). A waveform photograph may make the measurement easier.
Adjust the waveform monitor variable gain to set the amplitude of the positive spike that corresponds to the trailing edge of sync equal to 100%. Switch between APL extremes, typically 12.5% and 87.5%. The resulting envelope of the sync spikes represents the transient distortion.
Measure the maximum departure from 100% to obtain the amount of transient sync nonlinearity.
The 1781R voltage cursors can also be used to make this measurement.
In the relative mode, define the positive sync spike as 100%. Then use the cursors to measure the largest deviation from that amplitude.


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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