PAL SYSTEM TELEVISION MEASUREMENT : SHORT TIME DISTORSION

Short Time Distortion

DEFINITION
Short time distortions cause amplitude changes, ringing, overshoot, and undershoot in fast rise times and 2T pulses.
The affected signal components range in duration from 0.100 microsecond to 1.0 microsecond.
For PAL systems, distortions in the short time domain are most often characterized by measuring K2T or Kpulse/bar. These measurements are described in the K Factor Ratings section of this booklet. Alternatively,
in a T rise time bar can be described in terms of the "percent SD'' method described in this section.
PICTURE EFFECTS
Short time distortions produce fuzzy vertical edges. Ringing can sometimes be interpreted as chrominance information (cross colour) causing colour artifacts near vertical edges.
TEST SIGNALS
Short time distortion can be measured with any signal that has a T rise time white bar. A T rise time bar has a 10%-to-90% rise time of nominally 100 nanoseconds (see Figure 27).
See Appendix B for a discussion of the time interval T.
It is very important a T rise time bar be used with the short time distortion graticule. Many common test signals have 2T rather than T rise times and are not suitable for this measurement. It should also be noted that
time signals will suffer signifi - cant distortion when passed through a TV transmitter as they contain spectral components that will be removed by the transmitter 5 or 6 MHz lowpass filter. Short time distortion measurements made on transmitted signals will therefore evaluate only those components in approximately the 200 nanosecond to 1 microsecond range.
MEASUREMENT METHODS
Measurements of the undershoot, overshoot, and ringing at the edge of a T rise bar are not generally quoted directly as a percent of the transition amplitude, but rather in terms of an amplitude weighting system that
yields results in ""percent SD''.
This weighting is necessary because the amount of distortion depends not only on the distortion amplitude but also on the time the distortion occurs with respect to the transition.
Although results can be calculated from the time and amplitude of the measured ringing lobes, special graticules, conversion tables, or nomographs are used in practice.

Figure 27. A T rise time bar has a 10% to 90% rise time of nominally 100 nanoseconds.

Waveform Monitor Graticule.
Graticules for measurement of short time distortion are not included in the 1781R. However, some organizations use custom graticules that indicate, for example, 2% and 5% SD limits.
The measurement procedure involves normalizing the gain and positioning the rising or falling edge of the bar in the graticule. The largest graticule limit touched by the waveform indicates the amount of distortion. Other values can be interpolated.
VM700 Automatic Measurement.
Select SHORT TIME DISTORTION in the VM700T MEASURE mode to obtain a SD result and a
tracking graticule (CCIR 421).
The user can also define custom graticules in this mode.
NOTES
10. Nonlinearities
If the device or system under measurement is free of nonlinear distortion, the rising and falling transitions will
exhibit symmetrical distortion. In the presence of nonlinearities, however, the transitions may be affected differently. It is prudent to measure, or at least inspect, both the positive and negative transitions.
11. Pulse-to-Bar Ratios. 
The amplitude ratio between a 2T pulse and a line bar is sometimes used as an indication of short time distortion. To make a pulse-to-bar measurement with a waveform monitor, first normalize the bar amplitude to 100%.
Now measure the pulse amplitude, in percent, to obtain pulseto- bar ratio reading. The 1781R's voltage cursors can be used in the RELATIVE mode to make measurements of this type.
A pulse-to-bar measurement can be obtained from the VM700T by selecting K FACTOR in the MEASURE mode. Both pulse-tobar ratio and Kpulse/bar results (see Note 17) are provided in this mode.

Figure 28. The VM700T Short Time Distortion 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