PAL SYSTEM TELEVISION MEASUREMENT : DIFFERENTIAL GAIN

Differential Gain

DEFINITION
Differential gain, often referred to as "diff gain''', is present when chrominance gain is dependent on luminance level. These amplitude errors are a result of the system's inability to uniformly process the high-frequency
chrominance signal at all luminance levels.
Differential gain distortion is expressed in percent. Since both attenuation and peaking can occur in the same signal, it is important to specify whether the peak-to-peak amplitude difference or the peak deviation is being quoted. The reference for peak-to-peak results may be either the maximum chrominance amplitude or the amplitude of the chrominance packet at blanking level. Peak deviation measurements are generally referenced to the chrominance amplitude at blanking level.
PAL measurement standards generally refer to peak differential gain measurements. Two numbers are typically given to describe the amount of distortion:
the peak positive deviation and the peak negative deviation in chrominance amplitude from the amplitude at blanking level.
These numbers are expressed as a percentage of the blanking level chrominance amplitude.
Sometimes the larger of the two values is given as a single peak result.
Differential gain should be measured at different average picture levels and the worst error quoted.
PICTURE EFFECTS
When differential gain is present, colour saturation is not correctly reproduced. Differential gain is generally most noticeablein reds and yellows.
TEST SIGNALS
Differential gain is measured with a test signal that consists of uniform-amplitude chrominance superimposed on different luminance levels. A modulated staircase (5 or 10 step) or a modulated ramp is typically used.
Some generators, such as the Tektronix TG2000, offer a phase-alternate modulated ramp test signal. This signal can help detect distortions that have affected the U and V components differently. This is most likely to occur if the signal has been demodulated and the U and V components passed through separate processing
channels. If this signal is available, it may be desirable to repeat the measurement procedures outlined below for both signal vectors.

Figure 70. A modulated 5-step staircase test signal.

MEASUREMENT METHODS
Differential gain distortion can be quantified in a number of ways. Chrominance amplitudes  can be measured directly with a waveform monitor and large distortions can be seen on a vectorscope display. For precision
measurements, however, a vectorscope with a special differential gain mode or an automatic measurement set such as the VM700T is required.
Vectorscope Display.
In a vectorscope display, the dots corresponding to the various subcarr i e r packets will spread out in the
radial direction when differential gain is present. When using a ramp signal, the dot will become elongated in the horizontal direction. To make a measurement, first set the phase of the signal vector to the reference
position. Use the vectorscope variable gain control to bring the signal vector out to the graticule circle.
Vectorscope graticules generally have special differential phase and gain marks on the left-hand side to help quantify the distortion.
Peak-to-peak gain deviation can be read directly from the graticule. A peak reading is more difficult to obtain from this display since there is no convenient method of establishing which amplitude corresponds to the amplitude at blanking level.

Figure 71. A vectorscope display of a signal with 10% peak-to-peak differential gain. Differential phase distortion is also present

Waveform Monitor/Chrominance Filter.
Differential gain measurements can also be made with a waveform monitor. This process is facilitated by enabling the chrominance filter which passes only the high-frequency chrominance portion of the signal.
Peak-to-peak chrominance amplitudes can be easily compared in the resulting display.
To make a measurement, first normalize the peak-to-peak amplitude of the first chrominance packet (the one at blanking level) to 100 percent. Then measure the peak-to-peak amplitudes of the smallest and largest
packets. The positive and negative peak differential gain results are the differences between these two measurements and the blanking level amplitude.
Equations are given below.
Peak dG (Negative) = -100 [ [Vp p(Blanking) - Vp p(Smallest Packet)] / Vp p(Blanking) ] %
Peak dG (Positive) = +100 [ [Vp p(Blanking) - Vp p( L a rgest Packet)] / Vp p(Blanking) ] %
This measurement can also be made by using the 1781R voltage cursors in the RELATIVE mode.
Define the peak-to-peak amplitude of the blanking level packet as 100% and then move the cursors to measure peak-to-peak amplitude of the smallest and largest packets. Use the equations above to calculate results.

Figure 72. A chrominance filter display indicating about 6 % differential gain.

B-Y Sweep
Some vectorscopes are equipped with a special mode for making accurate differential gain measurements. Since differential gain affects the B-Y (U axis) signal (see Figure 73), a line-rate sweep of demodulated
B-Y information can be used to measure the amount of distortion.
Errors manifest themselves as tilt or level changes across the line. Like the R-Y display used to measure differential phase, this display provides greater resolution and allows determination of how the distortion varies over a line. In the 1781R, both “single trace” and “double trace” versions of this display are available.
Both are accessed by selecting DIFF GAIN in the MEASURE menu.

Figure 73. Differential gain distortion affects the B-Y (U) signal.

Single Trace Method.
The single trace differential gain display is familiar to users of the 521A vectorscope and it is also available
in the 1781R by selecting SINGLE in the DIFF GAIN menu. The amount of distortion is quantified by comparing the demodulated waveform to a vertical graticule scale.
The phase shifter should be used to set the signal vector to the reference (9 o'clock) position prior to making this measurement.
Adjust the vectorscope variable gain control so the signal vector extends to the edge of the graticule circle. Make sure the 1781R's w a v e f o rm gain is in the calibrated (1 volt full scale) setting.
In the 1781R, the differential gain display appears on the waveform screen. Compare the waveform to the vertical scale on the graticule and measure the largest deviation between the part of the signal that  corresponds to blanking-level chrominance and the largest and smallest packets. One major graticule
division (100 mV) is equal to one percent.

Figure 74. A single trace DIFF GAIN display indicating a distortion of about 3%.

Double Trace Method.
The double trace method in the 1781R provides a highly accurate way of measuring the amount of tilt or
level shift in a one-line sweep of the B-Y information. This method is very similar to the differential phase double trace method described earlier, the difference being a calibrated gain control rather than a calibrated
phase control is used to null the traces.
Select DOUBLE in the 1781R DIFF GAIN menu to make this measurement. Use the phase shifter to set the signal vector to the reference phase position.
The vectorscope variable gain must be adjusted so the signal vector reaches the graticule circle. The 1781R waveform monitor gain setting is not critical in this mode (see Note 21).
Now refer to the waveform (right-hand) display, and start the measurement procedure by using the large knob to overlay the blanking level portions of the inverted and non-inverted waveforms. Press REF SET to set the readout to 0.00 percent (see Figure 75). Now use the large knob to bring together the largest positive and/or negative excursions. The readout now indicates the amount of differential gain distortion (see Figure 76).

F i g u re 75. The 1781R double trace DIFF GAIN display with the readout zeroed .

Figure 76. The 1781R double trace DIFF GAIN display showing measurementresults. With attenuation only, peak and peak-to-peak results are the same.

VM700T Automatic Measurement.
To make an automatic measurement of differential gain with the VM700T, select DGDP in the MEASURE mode. Both differential phase and differential gain are shown on the same display (the upper graph is differential gain). Measurements results are also available in the AUTO mode.

Figure 77. The VM700T DG DP display.

NOTES
20. Demodulated “B-Y” Signal.
It
should be noted that in instruments such as the 521A Vectorscope and the 1781R, the displayed signal is not simply the B-Y demodulator output of the vectorscope. Rather, an envelope (square law) detector scheme is used. The demodulated signal is derived by multiplying the signal by itself rather than by a constant-phase CW subcarrier as in a synchronous demodulator. The primary advantage of this method is that in the presence of both differential phase and differential gain, synchronous detection yields a phase-dependent term, but square law detection does not. Thus the presence of differential phase does not affect the  ifferential
gain result.
21. 1781R Waveform and Vector Gains.
When using the single trace mode, the vector gain must be set to the graticule circle and the waveform gain must be in the calibrated position. The graticule is only calibrated to 1 percent per division under these conditions.
In the double mode display, more waveform vertical gain (X5 or VAR) may be introduced for greater resolution. However, correct results will be obtained only when the vectorscope gain is set to the graticule circle.
22. Simultaneous Display of DP and DG.
It is sometimes useful to have a display that shows both differential phase and differential gain, particularly when adjusting equipment for minimum distortion. A display which shows a one-line sweep of differential phase on the left and a one-line sweep of differential gain on the right can be accessed by selecting DP & DG in the 1781R MEASURE menu (see Figure 78). As noted above, the VM700T DG DP display also shows both distortions simultaneously.

Figure 78. The 1781R DP & DG 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