PAL SYSTEM TELEVISION MEASUREMENT : FIELD TIME DISTORTION

Field Time Distortion

DEFINITION
Field time distortion causes field-rate tilt in video signals.
The affected signal components range in duration from 64 microseconds to 20 milliseconds.
The amount of distortion is generally expressed as a percentage of the amplitude at the centre of the line bar. K50 Hz measurements, which are discussed in the K FACTOR section of this booklet, provide
another method of describing field time distortions.
PICTURE EFFECTS
Field time linear distortion will cause top-to-bottom brightness inaccuracies in large picture details.
TEST SIGNALS
Field time distortion is measured with a field square wave. In this signal, each line in one half of the field is a 0-volt pedestal, while each line in the other half is a 700-millivolt pedestal. The signal usually includes normal
horizontal and vertical synchronization information.
MEASUREMENT METHODS
Field time distortions are quantified by measuring the amount of tilt in the top of the field bar (the 700 mV part of the field square wave signal). The maximum departure of the field bar top from the level at the centre of the field bar is generally quoted as the amount of distortion although peak-to-peak results are sometimes given. The measurement methods in this section are described in terms of peak results, but can readily be adapted for peak-to-peak measurements.
The centre of the line bar is usually used as the reference amplitude and the first and last 250 microseconds (about 4 lines) of the field bar should be ignored. Distortions in that region are not in the field time domain.

Figure 32. The field square wave test signal.

Waveform Monitor Graticule
The first step in making a field time distortion measurement is to normalize the gain. With the waveform monitor in a line-rate sweep mode, use the variable gain control to set the centre of the line bar to 100% (1000 mV or 500 mV). This can be done most accurately with the waveform monitor FAST DC restorer
on. The DC restorer will remove the effects of field time distortion from the waveform monitor display and reduce the vertical blurring seen in the line rate display. Now select a field-rate sweep and either the SLOW or OFF setting for the DC restorer.
Measure the peak positive and peak negative level change from the centre of the field bar excluding the first and last 4 lines. The larger of these two numbers, expressed as a percentage of the line bar amplitude, is
the amount of field time distortion (see Figure 33).
1781R Voltage Cursors.
The 1781R voltage cursors can be used in the RELATIVE mode to measure field time distortion. Select a
one-line or two-line sweep and define the centre of the line bar (relative to blanking) as 100%.
Remember to select the FAST DC restorer setting. Then select a field-rate sweep and set the DC restorer to SLOW or OFF. Place one cursor so that it intersects the top of the field bar in the middle. Use the other cursor to measure the peak positive and peak negative level deviation in the top of the bar ignoring the first and last 4 lines. The larger of the two numbers is the amount of field time distortion in percent.
VM700T Automatic Measurement.
Select TWO FIELD in the VM700T MEASURE mode to obtain a field time distortion result (see Figure 35). Field time distortion can also be measured in the AUTO mode.
NOTES
12. Externally Introduced Distortions. 
Externally introduced distortions such as mains hum are also considered field rate distortions. Be sure to turn the DC restorer OFF or select the SLOW clamp speed when measuring hum.

Figure 33. A 2-field waveform monitor display showing field time distortion.

Figure 34. The 1781R voltage cursors can be used to measure field time distortion.

Figure 35. The VM700T Two Field 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