PAL SYSTEM TELEVISION MEASUREMENT : LINE TIME DISTORTION

Line Time Distortion

DEFINITION
Line time distortion causes tilt in line-rate signal components such as white bars. The affected signal components range in duration from 1.0 microsecond to 64 microseconds. The amount of distortion is expressed as a percentage of the line bar amplitude at the centre of the bar.
Distortions in the line time domain can also be quantified by measuring Kbar as discussed in the K FACTOR Ratings section of this booklet.
PICTURE EFFECTS
In large picture detail, this distortion produces brightness variations between the left and right sides of the screen. Horizontal streaking and smearing may also be apparent.
TEST SIGNAL
Line time distortion is measured with a signal that includes a 10 microsecond or 25 microsecond white bar. Rise time of the bar is not critical for this measure m e n t .
MEASUREMENT METHODS
Line time distortion is quantified by measuring the amount of tilt in the top of the line bar. For PAL systems, the maximum departure of the bar top from the level at the centre of the line bar is most often quoted as the
amount of distortion. In some cases the peak-to-peak level variation is given, particularly when a 10 microsecond bar is used.
The measurement methods in this section are described in terms of peak results but can readily be adapted for peak-topeak measurements.
In either case, the tilt is expressed as a percentage of the level at the centre of the bar. The first and last microsecond of the bar should be ignored as errors near the transition are in the short time domain.
Waveform Monitor Graticule.
The graticule on a waveform monitor can be used to quantify this distortion.
Measure the maximum deviation from the centre of the bar and express that number as a percentage of the level at bar centre. It is generally most convenient to use the variable gain to normalize the centre of the
bar to 500 or 1000 mV.
Deviations in the top of the bar can then be read directly from the graticule in percent.
Remember to ignore the first and last microsecond.

Figure 29. Pulse and bar signal.

1781R Voltage Cursors. 
Waveform monitor voltage cursors in the RELATIVE mode can be used to measure line time distortion.
Define the amplitude difference between blanking level and the bar centre as 100%. Leave one cursor at the bar centre and move the other cursor to measure the peak positive and peak negative deviations in the top of
the bar. The largest of these numbers (ignore the sign) is the amount of line time distortion.
The 1781R time cursors are convenient for locating the appropriate time interval in the centre of the bar. Set the time separation to the bar time (usually 10 or 25 microseconds) minus 2 micro-seconds. Put the time
cursors in the TRACK mode, and move the two cursors together until they are centered on the bar (see Figure 30).
VM700T Automatic Measurement.
Select BAR LINE TIME in the VM700T MEASURE menu to obtain a line time distortion result (see Figure 31). Line time distortion can also be measured in the AUTO mode.

 Figure 30. The 1781R voltage and time cursors can facilitate line time distortion measurements.

Figure 31. The VM700T Bar Line Time display.

Lihat juga

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