PAL SYSTEM TELEVISION MEASUREMENT : LUMINANCE NONLINEARITY..

Luminance Nonlinearity

DEFINITION
Luminance nonlinearity, or differential luminance, is present when luminance gain is affected by luminance level. In other words, there is a nonlinear relationship between the input and output signals in the luminance
channel. This amplitude distortion is a result of the system's inability to uniformly process luminance information over the entire amplitude range.
The amount of luminance nonlinearity is expressed as a percentage.
Measurements are made by comparing the amplitudes of the individual steps in a staircase signal. The difference between the largest and smallest steps, expressed as a percentage of the largest step amplitude, is
the amount of luminance nonlinearity distortion. Measurements should be made at different average picture levels and the worst error quoted.
PICTURE EFFECTS
Luminance nonlinearity is not particularly noticeable in black and white pictures. However, if large amounts of distortion are present, a loss of detail may be seen in the shadows and highlights.
These effects correspond to crushing or clipping of the black and white information.
In colour pictures, luminance nonlinearity is often more noticeable. This is because colour saturation, to which the eye is more sensitive, is affected.
TEST SIGNALS
Luminance nonlinearity should be measured with a test signal that consists of uniformamplitude luminance steps.
Unmodulated 5 step or 10 step s t a i rcase signals are typically used.
If an unmodulated signal is not available, the measurement can also be made with a modulated staircase. This is generally not good practice, however, since both differential gain and luminance nonlinearity can have the
same net effect on the signal.

Figure 79. An unmodulated staircase signal.

MEASUREMENT METHODS
Luminance nonlinearities are quantified by comparing the step amplitudes of the test signal.
Since the steps were initially all of uniform height, any differences are a result of this distortion.
The waveform in Figure 80 exhibits luminance nonlinearity distortion. Note that the top step is shorter than the others.

Figure 80. An example of luminance nonlinearity distortion.

Waveform Display.
Luminance nonlinearity can be made with a waveform monitor by individually measuring each step in the
test signal. It is most convenient to use the variable gain to normalize the largest step to 100% (500 mV or 1 Volt) so percentage can be read directly from the graticule. Voltage cursors can also be used to measure the
steps. Although this method can yield accurate results, it is very time consuming and is not frequently used in practice.
Waveform Monitor — Differentiated Step Filter.
Some waveform monitors are equipped with a special
filter, usually called a “diff step” filter, for measurement of luminance nonlinearity. Since it provides an accurate and convenient method of evaluating this distortion, it is generally recommended practice to use Such a filter for this measurement.
External filters can be used if the waveform monitor is not equipped with the filter.
When the differentiated step filter is enabled, each step transition appears as a spike on the display. As the amplitude of each spike is proportional to the corresponding step height, the amount of distortion can be
determined by comparing the spike amplitudes.
Either the waveform monitor graticule or the voltage cursors can be used to measure the spikes. Use the variable gain to normalize the largest spike amplitude to 100% when using the graticule. The difference
between the largest and smallest spikes, expressed as a percentage of the largest, is the amount of
luminance nonlinearity.
The 1781R voltage cursors should be in the RELATIVE mode for this measurement.
Define the largest spike amplitude as 100%. Leave one cursor at the top of the largest spike and move the other cursor to the top of the smallest spike. The readout will indicate the amount of luminance nonlinearity
distortion (see Figure 81).

Figure 81. This photograph shows a 5 step staircase after it has been passed through a differentiated step filter. The 1781R voltage cursors indicate 9% luminance nonlinearity.

VM700T Automatic Measurement.
Select LUMINANCE NONLINEARITY in the VM700T MEASURE menu to obtain a display of this distortion. The VM700T uses an internal differentiated step filter to make this measurement. Measurement
results are also available in the AUTO mode.

Figure 82. The VM700T Luminance Nonlinearity 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